dietary protein and amino acids

4 Dietary Proteinand Amino Acids

EFFECTS OF PROTEIN ON DIETARY INTAKE AND APPETITE

Studies have been rather inconclusive regarding the effect of protein on dietary intake and appetite.Until recently the effect of dietary protein on the level of free essential amino acids in brain and onserotonin levels has been the focus of these studies.1–7 In a review of these studies8 the authorsconcluded that analysis of evidence of associations among dietary protein content, brain amino acidand serotonin concentrations, and protein self-selection by rats suggests that:

1. Protein intake is not regulated precisely, although rats will select between low- and high-protein diets to obtain an adequate, but not excessive, amount of protein.

2. Associations between brain-serotonin concentration and protein intake are weak, althoughconsumption of single meals of protein-deficient diets will elevate brain-serotonin concen-tration.

3. Nature of signals that drive rats to avoid diets containing inadequate or excessive amountsof protein remains obscure.

4. Whole brain amino acid and serotonin concentrations are quite stable over the usual rangeof protein intakes, owing to competition among amino acids for uptake across the blood–brain barrier and effective metabolic regulation of blood amino acid concentrations.

5. Protein intake and preference are not in themselves regulated, but what appears to beregulation of intake and preference is a reflection of the responses of systems for control ofplasma amino acid concentrations.

6. Relative stability of the average protein intake of groups of self-selecting rats (which givesthe appearance of regulation) results from averaging the variable behavioral responses—learned aversions and preferences—of rats to the variety of sensory cues arising from dietsthat differ in protein content.

Studies in humans have shown that high-protein versions of the same food systems show moresensory-specific satiety and decrease hunger more than lower-protein versions.9

CLASSIFICATION OF PROTEINS

Depending on the composition, proteins may be divided into two main categories: (1) simple and (2)conjugated. These two main categories can be further subdivided.

The following text lists the various categories of proteins along with their distinguishingcharacteristics.

SIMPLE PROTEINS

Simple proteins consist of only amino acids or their derivatives and include:

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Albuminoids (scleroproteins): These proteins are insoluble in water, highly resistant to enzymaticdigestion, and some become gelatinous upon boiling in water or dilute acids or bases. Examplesinclude collagen, elastin, and keratin. They are found mainly in supporting tissues and are some-times referred to as fibrous protein.

Albumins: These proteins are readily soluble in water and coagulate upon heating. They are presentin egg, milk, and serum.

Globulins: These proteins have low solubility in water but their solubility increases with the additionof neutral salts. They coagulate upon heating. They are abundant in nature. Examples are serumglobulins, muscle globulins, and numerous plant globulins.

Glutelins: These proteins are insoluble in water but soluble in dilute acids or bases. They areabundant in cereal grains. Examples are wheat glutenin, rice oryzenin, and barley hordenin.

Prolamines: These proteins are insoluble in water, absolute alcohol or neutral solvents, but solublein 80% ethanol. On hydrolysis they give large quantities of proline and ammonia. Examples are zeinin corn and gliadin in wheat.

Wheat, because of its gluten content, occupies a unique position in food. Gluten is a mixture oftwo proteins, gliadin and glutenin; these two, when mixed with water, give the characteristicstickiness which enables the molecules, present in wheat flour, to be bound together by moderateheat with the production of dough fromwhich bread is baked. Rye has a small content of gluten and so(with difficulty) can be made into a loaf. Oats, barley, maize, millets, and rice cannot be made intobread. The grains may be eaten after boiling or their flour made into porridge, bannocks, tortillas, etc.

Protamines and histones: These are basic polypeptides, soluble in water, but not coagulated by heat.Large amounts are found in male fish roes and also in cellular nucleoproteins. Protamines have apractical use in the commercial production of delayed-action insulins.

CONJUGATED PROTEINS

Conjugated proteins are joined to various nonprotein substances and include:

Chromoproteins: Combination of a protein and a pigmented (colored) substance. A commonexample is hemoglobin-hematin and protein.

Lecithoproteins: Combination of protein and lecithin. They are found in fiber of clotted blood andvitellin of egg.

Lipoproteins: Water-soluble combination of fat and protein. They act as a vehicle for the transport offat in the blood. They all contain triglycerides, cholesterol, and phospholipids in varying propor-tions. For example, low-density lipoprotein (LDL; ‘‘bad cholesterol’’) is the major carrier ofcholesterol in the bloodstream.

Metalloproteins: Proteins that are complexed with metals. One example is transferrin, a metallo-protein that can bind with copper, iron, and zinc. Various enzymes contain minerals.

Mucoproteins or glycoproteins: Contain carbohydrates such as mannose and galactose. Examplesare mucin from the mucus secretion, and some hormones such as the human chorionic gonadotropinfound in the urine of pregnant females.

Nucleoproteins: Combination of simple proteins and nucleic acids. Present in germs of seeds andglandular tissue.

Phosphoproteins: Compounds containing protein and phosphorus in a form other than phospholipidor nucleic acid. Examples are casein in milk and ovovitellin in eggs.

Possibly a third category, derived proteins, may be added to the two discussed earlier. Essen-tially derived proteins are the products of digestion. They are fragments of various sizes. Fromlargest to smallest, in terms of the number of amino acids, derived proteins are proteoses, peptones,polypeptides, and peptides.


140 Amino Acids and Proteins for the Athlete: The Anabolic Edge


Proteins may also be classified according to their structure, an important property for theirability to function biologically. Some proteins are round to ellipsoidal and are called globularproteins. These are found in the tissue fluids of animals and plants in which they readily disperse,either in true solutions or colloidal suspension. Enzymes, protein hormones, hemoglobin, andglobulins, including caseinogen in milk, albumin in egg white, and albumins and globulins ofblood, are all globular proteins. These proteins, while not easily digestible, are high-quality proteinsand contain a good proportion of the essential amino acids.

Other proteins form long chains bound together in a parallel fashion, and are called fibrousproteins. These consist of long coiled or folded chains of amino acids bound together by peptidelinkages. They are the proteins of connective tissue and elastic tissue including collagen, elastin, andkeratin, and are found in the protective and supportive tissues of animals such as skin, hair, andtendons. Many of the fibrous proteins are not very digestible, and if they are, they are usually poor-quality proteins.

While animal proteins can be divided into fibrous and globular, plant proteins are not so easilyclassified but broadly speaking most are glutelins or prolamines.

FUNCTIONS OF PROTEINS

Each distinctive protein performs a specific function in the body. In general, proteins may beclassified as performing four basic functions.

GROWTH

The formation of new tissues requires the synthesis of protein. Such conditions occur during periodsof growth—from infancy to adulthood, in pregnancy and lactation (each liter of human milkcontains about 12 g of protein), and during adaptation to exercise. Wound healing requires thesynthesis of new proteins, as do burns, fractures, and hemorrhage.

MAINTENANCE

Protein in all humans, regardless of age, is continually being degraded and resynthesized—a processcalled protein turnover. Blood cells must be replaced every 120 days, while cells which line theintestine are renewed every 1.5 days. Moreover, as discussed in detail later on, protein is lost fromthe body in the perspiration, hair, fingernails, skin, urine, and feces. This constant turnover and lossof protein requires that the body has a pool of amino acids upon which it can draw to replace losses.This pool is replenished by dietary protein.

REGULATORY

Proteins in the cells and body fluids provide regulatory functions. Hemoglobin, an iron-bearingprotein carries oxygen to the tissues.Water balance and osmotic pressure are regulated by the proteinsof the blood plasma. Furthermore, the proteins act as buffers controlling the acid–base balance of thebody, especially in the intracellular fluids. Many of the hormones which regulate body processes areproteins; for example, insulin, glucagon, GH, and the digestive-tract hormones. Enzymes, thecatalysts for almost all chemical reactions of the body, are proteins. Antibodies, which protectthe body from infectious diseases, are proteins. The blood-clotting mechanism of the body isdependent upon proteins.

Certain amino acids derived from dietary protein or synthesized in the body also performimportant regulatory functions. For example, arginine participates in the formation of the finalmetabolic product of nitrogen metabolism—urea—in the liver and glycine participates in thesynthesis of purines, porphyrins, creatine, and glyoxylic acid.


Dietary Protein and Amino Acids 141


ENERGY

The catabolism of amino acids yields energy, about 4 kcal=g of protein. In order for amino acidsto be used for energy, the amino group (NH2) must first be removed by a process knownas deamination (see later). The carbon skeleton remaining following deamination—a-keto acidresidues—may be either converted to glucose (glycogenic pathway) or metabolized in fat pathways(ketogenic pathway). When energy consumption in the form of carbohydrates and fats is low, theenergy needs of the body take priority and dietary and tissue proteins will be utilized at the expenseof the building or repair processes of the body.

In the body, amino acids may primarily be used for protein synthesis or energy production,depending upon:

1. Protein quality2. Caloric level of the diet3. Stage of development, including growth, pregnancy, and lactation4. Prior nutritional status5. Stress factors such as fever, injury, and immobilization

COAGULATION AND DENATURATION

Many water-soluble proteins coagulate when subjected to heat at about 1008C or above, as occurs inthe normal process of cooking. A familiar example would be the change of the white of an egg onboiling. Once a protein has undergone this change, its specific properties (e.g., enzymic, hormonal,or immunological) are permanently destroyed.

Proteins also undergo denaturation in which they become less soluble in water. This occurswhen they are exposed to a variety of agents such as moderate heat, ultraviolet light, or alcohol andmild acids or alkalis. Proteins are most easily denatured at their isoelectric point, that is, theparticular pH at which the electric charges on their NH2 and COOH groups precisely balance;this varies from one protein to another. The exact nature of the denaturation process is obscurealthough it involves some disorganization of the specific arrangement of the component aminoacids. To a certain extent, it is reversible once normal conditions are restored, but most enzymes andallergens lose their specific properties once denatured.

PROTEIN DIGESTION, ABSORPTION, AND METABOLISM

Protein digestion is the mechanical, chemical, and enzymatic breakdown of the protein in food intosmaller units. Digestion involves several stages including the mechanical extraction of the proteinfrom the food, denaturation of the protein, and hydrolysis of the peptide bonds. Protein ismechanically extracted from the food in the process of mastication and by the action of the stomach.The low pH of the stomach plays a role in denaturation of the extracted protein, thus making it moreaccessible to the proteolytic enzymes of the gut. The initial increase in gastric volume and delay ingastric emptying that occurs with macronutrient intake10 helps to ensure effective extraction anddenaturation of the protein prior to its entry in the duodenum.

In the small intestine, enzymes from the pancreas and the small intestine split dietary proteininto peptides (small groups of bonded amino acids) and individual amino acids. Protein must bedigested to amino acids or di- and tripeptides before absorption can occur, although at times largerpeptides can be absorbed. In the first few days of life and in certain disease states, polypeptides andundigested protein may be absorbed.11

Measurements of the net absorption of amino acids after a meal containing 15 g of milk proteinshow that it is 70%–80% complete in 3 h.




There is some evidence that hydrolyzed-protein fragments (i.e., peptides) cross the smallintestine and reach peripheral tissue via the systemic circulation.12 Dietary peptides can havespecific actions locally, on the GI tract, or at more distant sites having an influence on physiologicalprocesses. These bioactive peptides can alter cellular metabolism and may act as vasoregulators,growth factors, releasing hormones, or neurotransmitters. The concept of dietary bioactive peptidesoffers an explanation for varying effects of diet on physiologic responses. Current experimentalevidence indicates that diets that possess the capability of producing luminal peptides are superiorto diets lacking this capacity. More research needs to be done to determine the anabolic andergogenic effects of dietary bioactive peptides before they can be used effectively as nutritionalsupplements.

For the early stages of the digestion of protein, four types of enzymes are important, pepsins(secreted by serous cells in the gastric gland of the stomach), trypsins, elastases, and thechymotrypsins (all secreted by the acinar cells of the pancreas). The product of the action ofthese proteolytic enzymes is a series of peptides of various sizes. These are degraded further bythe action of several peptidases (exopeptidases) that remove terminal amino acids, includingcarboxypeptidases A and B which hydrolyze amino acids sequentially from the carboxyl end ofpeptides. Aminopeptidases, which are secreted by the absorptive cells of the small intestine,hydrolyze amino acids sequentially from the amino end of peptides.

In addition, dipeptidases, which are structurally associated with the brush border of theabsorptive cells, hydrolyze dipeptides into their component amino acids. The extent to whichthese act on substrates in the lumen, within the membrane of the cells or within the cell itself, isnot known.

Protein digestion in the intestine results in the hydrolysis of both ingested and endogenousproteins, in the form of digestive enzymes, other secreted proteins, and desquamated epithelial cells.Between 50 and 70 g of endogenous protein is digested daily almost equal to the average dailyamount ingested.

The final stages of the hydrolysis of peptides to dipeptides and amino acids, and their absorp-tion, occur in the jejunum and ileum. The transport of amino acids and di- and tripeptides from thelumen into the cell is an active process (more on this topic will be covered later when discussingprotein and amino acid supplements).

In general, proteins from animals are more completely and rapidly absorbed than vegetableproteins, possibly because of the cellulose covering in plant cells and the increased fiber associatedwith vegetable proteins.

Until the early 1950s, the absorption products of protein digestion were widely thought to befree amino acids for which there were several discernable transport mechanisms. There is now goodevidence that small peptides (mainly di- and tripeptides), but to a much lesser extent, and up toseveral amino acid residues are absorbed from the gut lumen at least as rapidly as free aminoacids.13–17

The mammalian Hþ-peptide symporter PEPT1 expressed in intestinal epithelial cellsmediates the absorption of small peptides following the digestion of dietary proteins.18,19 However,only a small proportion of these peptides taken up by the enterocytes appear in the systemiccirculation.

Many conditions, including dietary protein levels as well as certain free amino acids andpeptides, are able to change PEPT1 expression in the small intestine and its maximal transportactivity.20–27 In rats fed increasing quantities of dietary protein for 3 days, the abundance of PEPT1in the brush border membrane increased almost proportionally to the protein intake followed by aconcomitant increase in peptide transport activity.28

There is also a growing scientific interest in intestinal peptide transport in pharmacology, withPEPT1 as a prime target for efficient oral delivery of drugs.29,30




REQUIREMENT FOR DIETARY PROTEIN—AMINO ACID NEEDS

The requirement for dietary protein consists of two components:

1. Nutritionally essential amino acids (isoleucine, leucine, lysine, methionine, phenylalanine,threonine, tryptophan, and valine) under all conditions, and for conditionally essentialamino acids (arginine, cysteine, glutamine, glycine, histidine, proline, taurine, and tyro-sine) under specific physiological and pathological conditions.

2. Nonspecific nitrogen for the synthesis of the nutritionally dispensable amino acids (asparticacid, asparagine, glutamic acid, alanine, and serine) and other physiologically importantnitrogen-containing compounds such as nucleic acids, creatine, and porphyrins.

With respect to the first component, it is usually accepted that the nutritive values of various food-protein sources are to a large extent determined by the concentration and availability of theindividual indispensable amino acids. Hence, the efficiency with which a given source of foodprotein is utilized in support of an adequate state of nutritional health depends both on thephysiological requirements for the indispensable amino acids and total nitrogen and on the concen-tration of specific amino acids in the source of interest.

QUALITY OF PROTEINS

All proteins are made up of varying numbers of amino acids attached together in a specific sequenceand having a specific architecture. The sequence of the amino acid and form of the proteindifferentiates one protein from another, and give the protein special physiological and biologicalproperties.

The quantity and quality of protein in the diet is important in determining the effects on proteinmetabolism. Increasing protein intake leads to increased amino acid levels systemically, which inturn leads to increased protein synthesis, decreased endogenous protein breakdown, and increasedamino acid oxidation.31

The quality of dietary protein is also important in determining changes in protein metabolism.Certain proteins are considered to be biologically more effective. An example would be thedifference between soy and milk proteins. Consumption of a soy-protein meal, as compared tocasein, results in lower protein synthesis and higher oxidation of amino acids, as seen by the greaterurea production than consumption of a casein-protein meal.32 Another study found that the ingestionof soy protein resulted in a lower whole-body retention of dietary nitrogen than did milk protein.33

In a recent review, milk-protein supplementation was found to be more effective than soy-protein supplementation in stimulating amino acid uptake and net protein deposition in skeletalmuscle after resistance exercise.34 In another review, milk proteins were found to be more effectivein stimulating amino acid uptake and net protein deposition in skeletal muscle after resistanceexercise than hydrolyzed soy proteins.35

The findings revealed that evenwhen balanced quantities of total protein and energy are consumed,milk proteins are more effective in stimulating amino acid uptake and net protein deposition in skeletalmuscle after resistance exercise than are hydrolyzed soy proteins. Importantly, the finding of increasedamino acid uptake was independent of the differences in amino acid composition of the two proteins.The authors proposed that the improved net protein deposition with milk-protein consumption is alsonot due to differences in amino acid composition, but is due to a different pattern of amino acid deliveryassociated with milk versus hydrolyzed soy proteins. These findings suggest that, in healthy humansubjects, casein has a higher biological value (BV) than soy protein.

It has been shown that proteins found in whole foods may impact on protein metabolismdifferently because of the presence of other macronutrients. In a recent study, milk ingestion wasfound to stimulate net uptake of phenylalanine and threonine representing net muscle protein




synthesis following resistance exercise.34 These results of the study suggest that whole milk mayhave increased utilization of available amino acids for protein synthesis. This is likely becausecarbohydrates and fats alter the rate of absorption of the amino acids, and the hormonal response,and subsequently the fate of the amino acids absorbed. For example, the addition of sucroseto milk proteins increased whole-body nitrogen retention, but primarily in splanchnic tissues,whereas addition of fat to milk proteins resulted in greater dietary nitrogen retention in peripheraltissues.36

The protein source seems also to be important for weight loss and body composition. A numberof studies in the last decade, in both animals37,38 and humans,39–41 have suggested that supplemen-tation with soy protein was effective and better than other proteins for weight loss.

However, recently that notion has changed. A recent study has found that soy does not enhanceweight loss.42 And another recent study evaluating the weight-loss efficacy, body-compositionchanges, and other effects of either three soy shakes or three casein shakes daily as part of a16-week, energy-restricted diet for obese women found no difference between the two proteins.43

SLOW AND FAST DIETARY PROTEINS

We all know that there are differences in carbohydrates—high glycemic, low glycemic, simplesugars, starches, etc. And we know that different carbohydrates are absorbed in the gut and appearin the blood at different rates depending on various factors—for example, simple sugars areabsorbed quickly and more complex ones, depending on how quickly they can be broken down,are absorbed more slowly. This makes up the basis for the glycemic index of not only foods but alsowhole meals since the presence of protein and fat with the carbohydrates usually slows down theabsorption over the whole digestive process. Fast and slow carbohydrates have different metaboliceffects on the hormones and on various metabolic processes.

Now we also have slow (e.g., whey and soy) and fast (e.g., casein) dietary proteins. The speedof absorption of dietary amino acids by the gut varies according to the type of ingested dietaryprotein and the presence of other macronutrients. The speed of absorption can affect postprandial(after meals) protein synthesis, breakdown, and deposition.44,45

It has been shown that the postprandial amino acid levels differ a lot depending on the mode ofadministration of a dietary protein; a single protein meal results in an acute but transient peak ofamino acids whereas the same amount of the same protein given in a continuous manner, whichmimics a slow absorption, induces a smaller but prolonged increase.

Since amino acids are potent modulators of protein synthesis, breakdown, and oxidation,different patterns of postprandial amino acidemia (the level of amino acids in the blood) mightwell result in different postprandial protein kinetics and gain. Therefore, the speed of absorption bythe gut of amino acids derived from dietary proteins will have different effects on whole-bodyprotein synthesis, breakdown, and oxidation, which in turn control protein deposition.

For example, one study looked at both casein- and whey-protein absorption and the subsequentmetabolic effects.46 In this study two labeled milk proteins, casein and whey protein, of differentphysicochemical properties were ingested as one single meal by healthy adults, and postprandialwhole-body leucine kinetics was assessed. Whey protein induced a dramatic but short increase ofplasma amino acids. Casein induced a prolonged plateau of moderate hyperaminoacidemia, prob-ably because of a slow gastric emptying. Whole-body protein breakdown was inhibited by 34%after casein ingestion but not after whey-protein ingestion. Postprandial protein synthesis wasstimulated by 68% with the whey-protein meal and to a lesser extent (þ31%) with the casein meal.

Under the conditions of this study, that is a single protein meal with no energy added, twodietary proteins were shown to have different metabolic fates and uses. After whey-proteiningestion, the plasma appearance of dietary amino acids is fast, high, and transient. This aminoacid pattern is associated with an increased protein synthesis and oxidation and no change in proteinbreakdown. By contrast, the plasma appearance of dietary amino acids after a casein meal is slower,




lower, and prolonged with a different whole-body metabolic response: protein synthesis slightlyincreases, oxidation is moderately stimulated, but protein breakdown is markedly inhibited.

This study demonstrates that dietary amino acid absorption is faster with whey protein than withcasein. It is very likely that a slower gastric emptying was mostly responsible for the slowerappearance of amino acids into the plasma. Indeed, casein clots in the stomach whereas wheyprotein is rapidly emptied from the stomach into the duodenum. The results of the study demonstratethat amino acids derived from casein are indeed slowly released from the gut and that slow and fastproteins differently modulate postprandial changes of whole-body protein synthesis, breakdown,oxidation, and deposition.

After whey-protein ingestion, large amounts of dietary amino acids flood the small body pool ina short time, resulting in a dramatic increase in amino acid concentrations. This is probablyresponsible for the stimulation of protein synthesis. This dramatic stimulation of protein synthesisand absence of protein breakdown inhibition is quite different from the pattern observed with classicfeeding studies and with the use of only one protein source.

In conclusion, the study demonstrated that the speed of amino acid absorption after proteiningestion has a major impact on the postprandial metabolic response to a single protein meal. Theslowly absorbed casein promotes postprandial protein deposition by an inhibition of proteinbreakdown without excessive increase in amino acid concentration. By contrast, a fast dietaryprotein stimulates protein synthesis but also oxidation. This impact of amino acid absorption speedon protein metabolism is true when proteins are given alone, but as for carbohydrate, this might beblunted in more complex meals that could affect gastric emptying (lipids) and=or insulin response(carbohydrate).

In light of the fact that both hyperaminoacidemia47–49 and resistance exercise50–54 independ-ently stimulate muscle protein synthesis, a recent study (by Wilkinson et al. 2007) looked at howdifferent proteins differ in their ability to support muscle protein accretion.55

The study investigated the effect of oral ingestion of either fluid nonfat milk or an isonitrogen-ous and isoenergetic macronutrient-matched soy-protein beverage on whole-body and muscle-protein turnover after an acute bout of resistance exercise in trained men. The authors hypothesizedthat the ingestion of milk protein would stimulate muscle anabolism to a greater degree than wouldthe ingestion of soy protein, because of the differences in postprandial aminoacidemia, whencompared whey against casein.

In this study arterial–venous amino acid balance and muscle fractional synthesis rates weremeasured in young men who consumed fluid milk or a soy-protein beverage in a crossover designafter a bout of resistance exercise.

The primary finding of the current study was that intact dietary proteins, as against portions ofintact proteins such as concentrates or isolates of whey, soy, or casein, can support an anabolicenvironment for muscle protein accretion.

Two other studies carried out to date found that the ingestion of whole proteins after resistanceexercise can support positive muscle protein balance.56,57 However, the Wilkinson study was thefirst to show that the source of intact dietary protein (i.e., milk compared with soy) is important fordetermining the degree of postexercise anabolism.

The study55 also found a significantly greater uptake of amino acids across the leg and a greaterrate of muscle protein synthesis in the 3 h after exercise with the milk-protein consumption ascompared to soy-protein ingestion. Thus milk protein promoted a more sustained net positiveprotein balance after resistance exercise than did soy protein.

The authors concluded that since the milk and soy proteins provided equal amounts of essentialamino acids, and that the level of essential amino acids drive protein synthesis,58 it is likely thatdifferences in the delivery of and patterns of change in amino acids are responsible for theobserved differences in net amino acid balance and rates of muscle protein synthesis. Because ofdifferences in digestion rates, milk proteins may provide a slower pattern of amino acid deliveryto the muscle than soy protein.




Ingestion of soy protein results in a rapid rise and fall in blood amino acid concentrations,whereas milk-protein ingestion produces a more moderate rise and a sustained elevation.59

Interestingly, these increases in anabolic processes were seen without any concurrent increasesin whole-body protein oxidation. Part of the explanation for this lack of increase is that the testmeals consumed by participants in this study had 30% of total energy from fat, which would likelyhave slowed digestion and, thus, the rate of appearance of amino acids into general circulation.Also, the dose of protein used (7.5 g indispensable amino acids) did not stimulate amino acidoxidation.

Previous studies that examined the effect of ingestion of similar quantities of crystalline aminoacids on muscle protein turnover have shown that increases in net protein balance with the ingestionof 40 g crystalline indispensable amino acids (8.3 g leucine)60 were similar in magnitude to that seenwith the ingestion of only 6 g crystalline amino acids (2.2 g leucine).61 These data suggest that,when large quantities of amino acids are ingested, amino acids are likely to be directed todeamination and oxidation.

The authors of this study55 proposed that the digestion rate and, therefore, the ensuinghyperaminoacidemia that differed between the milk and soy groups after exercise is what affectedthe net uptake of amino acids in the exercised leg.

However, regardless of their conclusions, because there are variations between the proteins, it isstill possible that the differences in amino acid composition between the two proteins had someeffect on protein accretion. For example, the analysis of the proteins in this study found that thecontent of methionine in the soy protein (1.4%) was lower than that in the milk protein (2.6%).

EFFECTS OF DIETARY PROTEIN ON PROTEIN METABOLISM

Protein metabolism is affected by several factors including energy, protein and macronutrientintake, and exercise.62,63 An increase in dietary protein results in an increase in nitrogen retention64

and an increase in amino acid oxidation, as well as increases in the rate of protein turnover.65,66

As mentioned earlier, the speed of absorption can also modulate protein metabolism bymodulating protein synthesis, protein catabolism, and amino acid oxidation. The speed of absorp-tion is a function of the state of the GI system (previous dietary intake and presence or absence ofany pathology), the type of dietary protein (free amino acids, hydrolysates, and whey protein arerapidly absorbed in comparison to say casein—see discussion under fast and slow dietary pro-teins),67 and the presence of other dietary macronutrients.68

Both high-protein diet and rapid absorption of amino acids tend to increase amino acidoxidation46 as well as gluconeogenesis.69 Both processes underlie the importance of dietary proteinand amino acids to overall energy metabolism.

PROTEIN QUALITY—AMINO ACID REQUIREMENTS

Early studies on protein quality demonstrated that some proteins supported growth and evensurvival better than others, and that a certain quantity of dietary protein was needed. This earlywork on the effects of feeding proteins of known amino acid composition, or diets containingspecified pure amino acids, on the rate of growth or nitrogen balance of an experimental animalresulted in proteins being classified as complete, partially incomplete, and incomplete and also led tothe concept of essential or indispensable and nonessential or dispensable amino acids. It was alsofound that the amino acid requirements vary according to the species and age of an animal, and aredetermined in the last analysis by the amino acid composition of the tissue proteins formed duringgrowth.70

Not all proteins are of equal nutritional value; this reflects their differing amino acid content.Although most proteins contain most of the 22 or so amino acids, these are present in widelydiffering proportions. Complete proteins contain all the essential amino acids in sufficient amounts




to maintain life and support growth in that animal. Partially incomplete proteins can maintain life,but cannot support growth. Incomplete proteins cannot maintain life or support growth. Since plantproteins may be deficient in one or more essential amino acids and meat contains all the essentialamino acids, the chances of developing a deficiency are greater for vegetarians than for meat eaters(see later). A lack of essential amino acids in the diet results in a variety of adverse effects thatdepend on the degree and length of deficiency. Dairy products, meat, fish, eggs, and poultry areexamples of foods that contain complete proteins.

Essential amino acids are those which cannot be synthesized by the body or that cannotbe synthesized at a sufficient rate to supply the normal requirements for protein biosynthesis.71

These amino acids must therefore be present in the diet. The nonessential amino acids can besynthesized at a sufficient rate (provided, of course, that the supplies of amino nitrogen and carbonprecursors are adequate). Thus an amino acid is nonessential if its carbon skeleton can be formedin the body, and an amino group can be transferred to it from some available donor compound, aprocess called transamination (see earlier discussion). For example, the body can make as muchalanine as it needs quickly and easily because the carbon chain is pyruvate, a common metabolicproduct to which the amino group from another common nonessential amino acid can be added.

The use of the word essential, however, does not mean that the other nonessential amino acidsare not equally as essential for formation of the proteins but only that the others are not essential inthe diet because they can be synthesized in the body. For protein synthesis to take place, all theamino acids required must be available. If the diet lacks one or more of these essential amino acids,the body’s ability to synthesize new protein is adversely affected.72

There are no RDAs for any of the individual amino acids. Instead, since protein is made up ofamino acids, RDAs are expressed in terms of amino acid patterns in protein intake.

Studies have shown that the currently accepted 1985 criteria for amino acid requirement patternare not capable of maintaining body amino acid homeostasis or balance.73,74 Other diets, however,such as the Massachusetts Institute of Technology (MIT) requirement pattern (MIT diet), or the egg-protein pattern (egg diet) are capable of maintaining amino acid homeostasis. Thus, it is hypothe-sized that current international estimations of essential amino acid requirements are far too low andmust be modified in light of present research.75

While the presence of essential amino acids is critical to protein synthesis, there is someevidence that lack of the nonessential amino acids can result in lower plasma levels of theseamino acids,76 which may ultimately compromise protein synthesis in situations where there israpid growth.

During periods of active growth, or tissue repair, more nitrogen is ingested than excreted(positive nitrogen balance). Conversely, in malnutrition or starvation, less nitrogen is ingestedthan excreted (negative nitrogen balance). To determine whether an amino acid is essential, it isomitted from the diet while all the other amino acids are included. If the omission results in negativenitrogen balance, the amino acid is deemed essential. In the absence of this single amino acid, thebody has been unable to synthesize certain proteins so that the nitrogen that would have been usedin this synthesis is excreted. By this criterion, the following amino acids are essential in humans:isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

However, not all the other amino acids can be considered nonessential at all times. In somecases of accelerated growth and other conditions, such as the nutritional support of the catabolicpatient,77 certain amino acids may become essential because the body cannot synthesize enoughamino acid for that particular situation or the amino acid becomes rate limiting for protein synthesis.These amino acids are often referred to as conditionally essential.

For example, histidine is known to be required for normal growth in a child and is necessary forproper growth in the rat. It seems likely that histidine is also essential in human adults under certainconditions and under conditions of dietary deficiency may be a limiting factor for growth.78 Taurinemay also be conditionally essential. The last 20 years have seen the status of taurine change from anend product of methionine and cysteine metabolism and substance conjugated to bile acids to that of




an important, and sometimes essential, nutrient. It is now added to most synthetic human-infantformulas and pediatric-parenteral solutions throughout the world.79 Proline and glutamine are otherexamples of amino acids that may be conditionally essential. In one study, the authors suggest thatsome nonessential amino acids such as proline and glutamate-glutamine may become limitingduring lactation because of their unique contributions to milk-protein synthesis.80

Moreover, there are other special considerations for certain of the nonessential amino acids.Although humans can synthesize most of the nonessential amino acids from glucose and ammonia,tyrosine synthesis requires the availability of phenylalanine, and cysteine requires the availability ofmethionine. Both phenylalanine and methionine are essential amino acids, and if they are availablein the diet at or below minimal requirement levels, tyrosine and cysteine can become essential aminoacids in that the lack of precursor amino acids decreases the ability of the body to produce theseamino acids and they then become rate limiting for protein synthesis (see later).

For example, in some patients with liver disease, the hepatic conversion of phenylalanine totyrosine, and methionine to cystine, is inadequate, and unless sufficient cystine=tyrosine is adminis-tered,81 repletion of lean tissue (net protein synthesis) will be substantially limited and bodyfunction will be impaired.

However, while obtaining adequate amounts of the essential amino acids is important, theproportion of nonessential amino acid nitrogen has an influence on the essential amino acidrequirements.82,83 However, while increasing the amount of dietary nonessential amino acids mayincrease protein synthesis by increasing the level of dietary protein, it may not influence the overallrequirements for the essential amino acids.84 On the other hand, if the ratio of essential amino acidsto the total nitrogen in a food is too high, essential amino acids will be used as sources of nitrogenfor the nonessential amino acids.

Not only completeness but also digestibility are issues of concern with respect to protein quality.It is well known that not all protein sources are utilized equally well and that the bioavailability ofingested protein varies according to the source of protein. A protein is digestible if a high proportionof its amino acids reach the body’s cells so that they can synthesize the proteins they require. Thisindicates that the nitrogen in the protein is utilized with little waste. High-quality proteins are bothcomplete and highly digestible, meaning that smaller quantities need be ingested than would be thecase for proteins of lower quality.

Egg protein is usually the standard against which other sources are often compared since it isutilized for growth in animals with a 85%–90% efficiency (BV). Plant proteins, because they aremore difficult to digest and have a lower essential amino acid content are utilized for growth only50%–80% as well as egg protein.

The quality or type of protein may also affect the absorption or utilization of other nutrients. In astudy, the influence of different protein sources on zinc absorption was evaluated.85 The results ofthis study demonstrated that zinc absorption is inhibited by certain protein sources, such as bovineserum albumin and soybean-protein isolate, while other proteins have little or no effect.

VEGETARIAN DIETS

The word ‘‘vegetarian’’ is derived from the Latin vegetus, meaning whole, sound, fresh, lively. Theoriginal definition of vegetarian was ‘‘with or without eggs or dairy products’’ and that definition isstill used today by the Vegetarian Society. However, most vegetarians in India exclude eggs fromtheir diet, as did those in the classical Mediterranean lands such as Pythagoras. In fact prior to 1847,nonmeat eaters were generally known as ‘‘Pythagoreans’’ or adherents of the ‘‘PythagoreanSystem,’’ after the ancient Greek vegetarian Pythagoras.

More and more people are becoming vegetarians. For some of them becoming a vegetarianis a statement to promote animal rights or to save the environment while for others it is simply achoice they make for various other reasons including health or just fitting in with alternativelifestyles.




In general, people have distorted ideas on just who a vegetarian is. The word vegetariansometimes evokes visions of hippies with peace signs painted on various parts of their bodiescarrying signs and demonstrating against the killing of animals. That caricature, like most, is untrue.Vegetarians are people in all walks of life who, for moral and other reasons, simply do not eat redmeat and, to some extent, other animal foods.

TYPES OF VEGETARIANS

Vegetarians generally abstain from eating red meat and usually forgo eating poultry, fish, and otherseafood. Depending on the type of vegetarian, the forbidden foods may also include any food ofanimal origin. While there are many different kinds of vegetarians, the following two basic types arethe most common:

1. Lacto–ovo vegetarian: The most common form of vegetarianism avoids consuming animalflesh but allows dairy products and eggs with a diet of vegetables, fruits, nuts, seeds,legumes, and grains. Vegetarians may also avoid milk products but consume eggs (lactovegetarian) or vice versa (ovo vegetarian).

2. Vegan: This type of vegetarian excludes all animal products including diary, eggs, andeven honey. Their diet is derived exclusively from plants and is based on grains, veget-ables, legumes, fruits, seeds, and nuts. Their entire protein intake is in the form of plantprotein. Some vegans also refuse to eat yeast products. Also, the strict vegan avoids anyproducts derived from animals such as leather, wool, fur, down, silk, ivory, and pearl.Additionally, cosmetics and household items that contain animal ingredients or that aretested on animals are avoided.

Dietary vegans are people who follow a vegan diet, but do not necessarily try and exclude nonfooduses of animals.

While these two types are the most common, there are many other types and classifications forvegetarian eating. For example, a fruitarian is even stricter and avoids any plant products exceptthose parts of the plant that are cast off or dropped from the plant and that do not involve thedestruction of the plant itself. For example, fruits can be picked without killing the plant, but notvegetables such as celery and carrots.

There are also a number of confusing terms used for different kinds of vegetarians. Pescatarianis a vegetarian, usually a lacto–ovo, who also eats fish. A semi-vegetarian eats less meat than theaverage person does. A pseudo-vegetarian claims to be a vegetarian, but is not. For example, theymay eat less meat than the average person may or they may eat dairy foods, eggs, chicken, and fishbut no other animal flesh. This term is often used by true vegetarians to describe semi-vegetariansand pescatarians.

In addition to these somewhat confusing terms, there are several others that are also in use thatdescribe variable eating patterns. For example, a vegetable consumer means anyone who consumesvegetables, but may not necessarily be a vegetarian. An ‘‘herbivore’’ means eating grass and=orplants but again may not necessarily qualify as a vegetarian. A ‘‘plant-eater’’ mainly eats plants, butmay not necessarily qualify as a vegetarian.

As the term implies, nonmeat eater does not eat meat. Most commonly used definitions do notconsider fish, fowl, or seafood to be ‘‘meat.’’ Animal fats and oils, bone meal, and skin are notconsidered meat. The term ‘‘kosher’’ is derived from a complex set of Jewish dietary laws but doesnot imply vegan or lacto–ovo vegetarian.

NUTRITIONAL QUALITY OF VEGETARIAN DIETS

While the nutritional quality of the foods eaten by lacto–ovo vegetarians is quite high and almost onpar nutritionally with that eaten by those that eat red meat, the more restrictive the vegetarian diet,




the more difficult it is to get the nutrients you need. In cases of vegans, the diet must be verycarefully planned so that the athlete can get his or her full quota of high-quality protein and othermacro- and micronutrients. As the diet becomes much more restrictive (e.g., in fruitarians) itbecomes impossible to meet the nutritional demands of high-level athletic activity.

On the other hand, if appropriately planned, vegan diets, though restrictive, can provideadequate nutrition even for competitive athlete including those in the power sports such as body-building, power-lifting, and Olympic lifting.

One of the main concerns for vegetarian athletes is that they obtain enough high-quality proteinin their diets to meet their increased protein needs. However, this can be accomplished if theychoose wisely among the various plant proteins. Protein deficiencies may be more of a problem withvegans than with lacto–ovo vegetarians who eat eggs, milk, yogurt, cheese, and other dairy foods,since these foods are excellent sources of high-quality protein.

The key for strict vegetarians, or vegans who consume no milk, eggs, or other animal proteins,is to eat a variety of grains that have complementary amino acids. Beans and rice are an example ofmixing legumes (peas and beans) and grains. Also, tofu is an excellent addition to a vegetarian diet.Tofu is a high-quality plant protein that contains all essential amino acids and offers the bonus ofphytochemicals that protect against heart disease and cancer.

VEGETARIAN FOOD GUIDE—BASIC FOUR FOOD GROUPS

It is not difficult for most people to become vegetarians. The vegetarian foods make up much of theU.S. Department of Agriculture and Department of Health and Human Services’ Food GuidePyramid, which recommends 6–11 daily servings of bread, cereal, rice, and pasta. Daily intakesadvised for other foods are three to five servings of vegetables; two to four servings of fruits; two tothree servings of milk, yogurt, and cheese; and two to three servings of meat, poultry, fish, drybeans, eggs, and nuts. The guide advises using fats, oils, and sweets sparingly. Looking at the foodpyramid it is easy to see that an ovo–lacto vegetarian and even a vegan (except for the dairyservings) can easily fulfill each category in the food pyramid.

For vegans the four food groups of meat, dairy, grains, and fruits=vegetables are replaced by thefour food groups of grains, legumes, vegetables, and fruits.

Let us examine these groups in more detail:

Whole grains: This group includes bread, rice, pasta, hot or cold cereal, corn, millet, barley, bulgur,buckwheat groats, and tortillas. Build a meal based on a dish of whole grains as these foods are richin fiber and other complex carbohydrates, as well as protein, B vitamins, and zinc.

Vegetables: Vegetables contain many nutrients such as vitamin C, b-carotene, riboflavin and othervitamins, iron, calcium, and fiber. Dark-green, leafy vegetables such as broccoli, collards, kale,mustard and turnip greens, chicory, or bok choy are especially good sources of these importantnutrients. Dark-yellow and orange vegetables such as carrots, winter squash, sweet potatoes,and pumpkins provide extra b-carotene. Include generous portions of a variety of vegetables inyour diet.

Legumes: Beans, peas, and lentils are all good sources of fiber, protein, iron, calcium, zinc, and Bvitamins. This group also includes chickpeas, beans, soy milk, tofu, tempeh, and textured vegetableprotein.

Fruit: Fruits are rich in fiber, vitamin C, and b-carotene. Be sure to include at least one serving eachof fruits that are high in vitamin C—citrus fruits, melons, and strawberries are all good choices.Choose whole fruit over juices, which do not contain as much healthy fiber. Juices may bepreferable for those who need to take in a lot of calories but who are getting enough fiber.

For lacto–ovo vegetarians, the incorporation of egg or milk products can make a world ofdifference as far as accessibility to first-quality protein and fewer problems with most micronutrientintakes.




SUPPLYING REQUIRED NUTRIENTS

Vegetarians who eat no meat, fish, poultry, or dairy foods face the greatest risk of nutritionaldeficiency. On the other hand, no matter which diet you choose it is important to plan that diet sothat it not only matches your vegetarian needs but also provides you with the nutrients you need toremain healthy.

Nutrients most likely to be lacking and some nonanimal sources of these nutrients are presentedin Table 4.1.

NUTRITIONAL CONSIDERATIONS OF VEGETARIANS

Special Nutrient Needs of Vegetarians

While choosing to be a vegetarian is a personal decision, it is important to know that the vegetarianway of eating can cause some macro- and micronutrient deficiencies. As such, it is vital to realizethat the decision to become a vegetarian is also accompanied by a need to learn how to eat the rightfoods to ensure you are getting your healthy share of all the necessary nutrients. You have to knowenough about creating a diet so you do not end up with any deficiencies. Just how careful you haveto be depends on the type of vegetarian diet you have adopted.

Being a vegetarian does not guarantee improved health. Depending on the type of vegetariandiet, you can make use of the various protein supplements and may need additional vitamins andminerals. For instance, if you abstain from dairy products you may need to supplement your dietwith calcium, magnesium, and vitamin D.

Several studies both on vegetarian men and women have shown that compared to nonvegetar-ians, they ingested less calcium, iron, zinc, and vitamin B12.

86,87 Most subjects in these studies ateless than half the RDA for B12, a vitamin crucial for healthy red blood cells and nerve fibers. Sincevitamin B12 is found only in animal products, such as red meat, fish, shellfish, eggs, and milk, strictvegetarians (or ‘‘vegans’’) must increase their consumption of foods, such as soy milk, that arefortified with this vitamin.

French researchers caution that a strict vegan diet may lead to deficiency of important vitaminsthat are critical to eyesight. In a case report,88 a French patient lost most of his eyesight resultingfrom following a strict vegan diet. The patient did not supplement with vitamins, leading to B12

deficiency. This vitamin is important in maintaining the health of nerves, including the optic nervethat transmits signals from the eye to the brain. The researcher also found the patient had below-normal levels of vitamins B1, B12, A, C, D, E, and zinc and selenium.

TABLE 4.1Possible Nutrient Deficiencies and Nonanimal-Based Food Sources

Nutrients Food Sources

Vitamin B12 Fortified soy milk and cereals

Vitamin D Fortified margarine and sunshineCalcium Tofu, broccoli, seeds, nuts, kale, bok choy, legumes, greens, calcium-enriched

grain products, and lime-processed tortillas

Iron Legumes, tofu, green leafy vegetables, dried fruit, whole grains, and iron-fortifiedcereals and breads, especially whole wheat

Zinc Whole grains, whole-wheat bread, legumes, nuts, and tofu




VEGETARIAN ATHLETES

Since vegetarian athletes need more of some of the critical nutrients, they have to be especiallycareful to make sure that their diets fulfill these needs. Thus, as vegetarian styles of eating becomemore popular among athletes, the risk of poorly planned diets leading to nutrient insufficiencies anddeficiencies increases. Inadequate dietary intakes of iron and zinc have been observed in athleteswho have eliminated meat from their diets. Marginal iron or zinc status may adversely affect, whilefrank iron or zinc deficiency definitely decreases exercise performance.

For athletes, while it is possible to obtain all essential nutrients by eating a completely plant-based diet, it requires critical planning and execution.89 Athletes must also learn that it is not enoughto just cut meats out of the diet. These foods have many essential nutrients that are more difficult toget elsewhere. Realistically, however, it may be difficult for the vegetarian athletes, especiallyvegans, to get the nutrients they need. Also because vegan diets are typically high in fiber, it may bedifficult to take in enough food to satisfy energy requirements.

Additionally, there are special considerations regarding age and gender of the vegetarian athlete.Amenorrhea may be more common among vegetarian than nonvegetarian female athletes.90 Effortsto maintain normal menstrual cycles might include increasing energy and fat intake, reducing fiber,and reducing strenuous training. When vegetarian female athletes are properly nourished, especiallywith adequate calories, their menstrual-cycle function should be normal compared with that ofmatched nonvegetarian women.

Vegetarian diets have been shown to result in decreases in the levels of anabolic hormones even inlacto–ovo vegetarians.91Male, endurance athletes on a lacto–ovo vegetarian diet exhibited lower totaltestosterone levels compared with those using a diet mixed with meat.92 Also, another study showedthat in older men the consumption of a meat-containing diet contributed to greater gains in fat-freemass and skeletal muscle mass with resistance training than did a lacto–ovo vegetarian diet.93

A plant-based diet facilitates high-carbohydrate intake, which can be essential to supportprolonged exercise. A well-planned vegetarian diet can provide athletes with adequate amounts ofall known nutrients, although the potential for suboptimal iron, zinc, trace element, and proteinintake exists if the diet is too restrictive.

This concern, however, exists for all athletes, vegetarian or nonvegetarian, who have poordietary habits. Athletes who consume diets rich in fruit, vegetables, and whole grains receive highamounts of antioxidant nutrients that help reduce the oxidative stress associated with heavyexertion. Whereas athletes are most often concerned with performance, vegetarian diets also seemto provide long-term health benefits and a reduction in risk of chronic disease.

The bottom line is that if the decision to become a vegetarian is not a choice made because ofmoral or ethical principles, it may be more realistic and productive for athletes to include some meatin their diet.

COMMON VITAMIN AND MINERAL DEFICIENCIES

It is important to remember that vegetarians have special nutritional needs and concerns. Thus,vegetarians have to be careful that they receive their daily quota of the macronutrients, essentialfatty acids, and vitamins and minerals.

Because vegans do not eat dairy products or eggs, they are at a higher risk of having deficienciesfor calcium, vitamin B12, and vitamin D. These nutrients, while abundant in dairy products, are lessavailable in plant sources. Calcium is found in fortified foods, green leafy vegetables, beans, andtofu products.

Vitamin D can be a problem for vegans with low sun exposure. However, exposure to the sun oflimited parts of the body for 10–15 min=day is enough to supply the daily requirement of vitamin D.

Because vitamin B12 is only found in animal products, vegans must obtain it from fortified foodsor from nutritional supplements. The American Dietetic Association strongly recommends supple-ment use to ensure the proper quantity of B12.




SPECIFIC NUTRITIONAL NEEDS OF VEGETARIANS

Lack of calories is common with vegans because of the bulk of food that must be consumed sinceplant foods are generally less dense calorically and a lot of fiber is consumed. Additionally, lacto–ovo vegetarians that allow eggs and dairy products in their diet and vegans, who do not allow anyanimal products in their diets, can face some nutrient deficiencies.

Getting enough dietary protein is generally not a problem with lacto–ovo vegetarians becauseeggs and dairy products are nutritionally dense and contain first-quality protein. However, vegans,especially those who exercise vigorously or take part in sports, have to be extremely careful inplanning their diets so as to supply enough energy since plant foods tend to be nutritionally lessdense. Also, vegans have to mix proteins so that an overall amino acid balance is achieved.

Considering the importance of dietary protein, especially for athletes, this topic deserves in-depth attention.

PROTEIN

The quantity of protein in the diets of athletes, while important, is rarely a concern regardless ofwhether they are meat eaters or nonmeat eaters. Increased dietary intake can be accomplished easilyby both choosing high-protein foods or by supplementing with protein powders. Nevertheless, thequality of protein can pose a problem. Well-processed soybean protein is equal in quality to animalprotein. However, other legumes do not contain a full complement of the essential amino acidsrequired for the efficient manufacture of protein by the human body.

Previous vegetarian dietary guidelines recommended that a variety of plant-protein sources(such as grains and beans) be combined simultaneously at one meal to complement each other andprovide a complete protein source. Current research supports the notion that by eating a variety oflegumes, as well as all other food groups throughout the day, one can obtain the full array ofessential amino acids required for efficient protein metabolism.

Thus, by combining various plant proteins and making use of soybean protein, most vegetarianscan easily get an adequate level of dietary protein. Vegetarian athletes, on the other hand, may findthat combining complementary proteins at one meal may be beneficial.

ENSURING ADEQUATE AND HIGH-QUALITY PROTEIN

The mixture of proteins from grains, legumes, seeds, nuts, and vegetables provide a complement ofamino acids so those deficits in one food are made up by another. At present it is felt that for mostvegetarians not all types of plant foods need to be eaten at the same meal, since the amino acids arecombined in the body’s protein pool.

In general, proteins of animal origin contain adequate amounts of the essential amino acids andhence they are known as first-class proteins. On the other hand, many proteins of vegetable originare relatively deficient in certain amino acids, notably lysine and the sulfur-containing amino acids.However, soy protein has been shown to be nutritionally equivalent in protein value to proteins ofanimal origin and, thus, can serve as the sole source of protein intake if desired.

The essential amino acid lysine is consistently at a much lower concentration in all major plant-food protein groups than in animal foods. Since lysine is the limiting amino acid, the addition oflimited amounts of lysine to cereal diets improves their protein quality. Studies in Peru andGuatemala have demonstrated that growing children benefited by this addition.94 In addition, thesulfur-containing amino acids are distinctly lower in legumes and fruits and threonine is lower incereals compared with amounts found in proteins of animal origin.

COMPLEMENTARY PROTEINS

Although humans have used some 300 plant species for food and at least 150 species have beencultivated for commercial purposes, most of the world’s population depends on approximately 20




different plant crops, which are generally divided into cereals, vegetables (including legumes),fruits, and nuts. The most important groups for human nutrition are cereal grains and food legumes,including oilseed legumes.

Because of completeness and digestibility, some proteins are nutritionally better than others;they contain a more balanced range of the essential and conditionally essential amino acids. Ingeneral, proteins of animal origin contain adequate amounts of the essential amino acids and hencethey are known as first-class proteins. On the other hand, many proteins of vegetable origin arerelatively deficient in certain amino acids, notably lysine and the sulfur-containing amino acids.

Mixtures of plant proteins can serve as a complete and well-balanced source of amino acids formeeting human physiological requirements.95 However, the combination of right foods is necessaryto obtain the necessary levels of both the essential or indispensable and conditionally indispensableamino acids.

There are important differences among and between food products of vegetable and animalorigin including the concentrations of proteins and indispensable amino acids that they contain.The concentration of protein and the quality of the protein in some foods of vegetable origin maybe too low to make them adequate, sole sources of proteins. In some of the poorer parts of theworld, diets are based predominantly on a single plant (e.g., corn) and they frequently lead tomalnutrition.

Foods of plant origin contain many amino acids, but single food items usually do not contain allthe essential amino acids in adequate amounts to support good health. Vegetarian diets are plannedso that the amino acid content is adequate to support good nutritional health. Amino acids are thebuilding blocks of protein, the nutrient that is supplied in greatest quantity by meat, fish, poultry,eggs, milk, and cheese.

The mainstays of strict vegetarian diets are fruits and vegetables, whole grains, legumes, nuts,and seeds. Because these plant foods do not contain an optimal balance of essential amino acidswhen eaten separately, the vegetarian must combine foods that complement each other, thusproviding the body with the right amino acid mix.95 The combining of right foods is necessary toobtain the necessary levels of both the essential or indispensable and conditionally indispensableamino acids. The addition of eggs or dairy products with meals or snacks greatly improves theoverall protein quality of the vegetarian diet.

Fortunately, the amino acid deficiencies in a protein can usually be improved by combiningit with another so that the mixture of the two proteins will often have a higher food value thaneither one alone. For example, many cereals are low in lysine, but high in methionine andcysteine. On the other hand, soybeans, lima beans, and kidney beans are high in lysine but lowin methionine and cysteine. When eaten together these types of proteins give a more favorableamino acid profile.

Rice, the staple food for more than half of the world population, is inadequate in its amino acidcontent when it is the sole protein source, but when combined with another type of plant protein,such as legumes, a complete protein mixture is formed. A mixture of red beans or black-eyed peasand rice is an example of complementary protein foods as is a peanut butter sandwich (peanuts arelegumes while the wheat of the bread is a grain). Other examples are corn tortillas with refried beansand tofu with fried rice.

Another example would be the combination of soybean, which is low in sulfur-containingamino acids, with cottonseed, peanut and sesame flour, and cereal grains, which are deficient mainlyin lysine. In general, oilseed proteins, in particular soy protein, can be used effectively incombination with most cereal grains to improve the overall quality of the total protein intake. Acombination of soy protein, which is high in lysine, with a cereal that contains a relatively goodconcentration of sulfur-containing amino acids results in a nutritional complementation; the proteinquality of the mixture is greater than that for either protein source alone.

Common examples of complementary food proteins include beans and corn (as in tortillas); riceand black-eyed peas; whole wheat or bulgar, soybeans, and sesame seeds; and soybeans, peanuts,




brown rice, and bulgar wheat. This kind of supplementation works when the deficient and comple-mentary proteins are ingested together or within a few hours of each other.

NUTRITIONAL RESPONSES TO COMBINING TWO DIETARY PROTEINS

Various nutritional responses are observed when two dietary proteins are combined. These havebeen classified by Bressani et al.96 into one of four types.

Type I is an example where no protein complementary effect is achieved. For example, thisoccurs with combinations of peanut and corn, where both the protein sources have a common andquantitatively similar lysine deficiency and are both also deficient in other amino acids.

Type II is observed when combinations are made of two protein sources that have the samelimiting amino acid, but in quantitatively different amounts. Corn and cottonseed flour, for example,are both limiting in lysine but cottonseed is relatively less inadequate than is corn.

Type III demonstrates a true complementary effect because there is a synergistic effect on theoverall nutritive value of the protein mixture; the protein quality of the best mix exceeds that of eachcomponent alone. This type of response occurs when one of the protein sources has a considerablyhigher concentration of the most-limiting amino acid in the other protein. An example of thisresponse, based on studies in children, is observed when corn and soy flour are mixed so that 60% ofthe protein intake comes from corn and the remainder from soy protein.

Type IV occurs when both protein sources have a common amino acid deficiency. The proteincomponent giving the highest value is the one containing a higher concentration of the deficient aminoacid. Combinations of some textured-soy proteins and beef protein follow this type of response.

These nutritional relationships have been determined from rat bioassay studies. However, themore limited results available from human studies with soy and other legumes confirm theapplicability of this general concept in human nutrition. This knowledge helps us to understandand evaluate how nutritionally effective combinations of plant protein foods can be achieved.

Even when combinations of plant protein foods are used there is still the concern of timing ofingestion of complementary proteins. Is there a need to ingest different plant proteins at the sametime, or within the same meal, to achieve maximum benefit and nutritional value from proteins withdifferent, but complementary, amino acid patterns? This concern may also extend to the question ofthe need to ingest a significant amount of protein at each meal, or whether it is sufficient to consumeprotein in variable amounts at different meals and even different days as long as the average dailyintake meets or exceeds the recommended or safe protein intakes.

According to FAO=WHO=UNU,97 estimates of protein requirements refer to metabolic needsthat persist over moderate periods. However, as discussed in the following chapters, the body doesnot store much protein outside of a meager free amino acid pool, and begins certain catabolicprocesses in the postabsorptive phase making the ingestion of regular amounts of protein critical formaximizing the anabolic effects of exercise.

There is a limited database that we can consult to make a definitive conclusion on the timing ofconsumption of complementary proteins or of specific amino acid supplements for proteins that aredeficient in one or more amino acids. An earlier work in rapidly growing rats suggested thatdelaying the supplementation of a protein with its limiting amino acid reduces the value of thesupplement.98,99 Similarly, the frequency of feeding diets supplemented with lysine in growing pigsaffects the overall efficiency of utilization of dietary protein.100

Studies in human adults showed that overall dietary protein utilization was similar whether thedaily protein intake was distributed among two or three meals.101,102 The general consensus at presentis that the various complementary proteins do not have to be consumed in the same meal in order tosupply the necessary amino acids but can in fact be consumed over the course of the day.103,104

In general, especially under conditions where intakes of total protein are high, it may not benecessary to consume complementary proteins at the same time. Separation of the proteins amongmeals over the course of a day would still permit the nutritional benefits of complementation.




However, in athletes trying to maximize protein synthesis and muscular hypertrophy it isnecessary to have a full complement of amino acids present for every meal in order to maximize theanabolic effects of exercise. There is a meal-related decrease in proteolysis and increase in proteinsynthesis. In one study whole-body leucine and a-ketoisocaproate (KIC) metabolism were estimatedinmature dogs fed a completemeal, ameal devoid of BCAAs, and ameal devoid of all amino acids.105

Using a constant infusion of [4,5–3H]leucine and [1–14C]KIC, combined with dietary[5,5,5–2H3]leucine, the rate of whole-body proteolysis, protein synthesis, leucine oxidation, andinterconversion of leucine and KIC were estimated along with the rate of leucine absorption.Ingestion of the complete meal resulted in a decrease in the rate of endogenous proteolysis, asmall increase in the estimated rate of leucine entering protein, and a twofold increase in the rate ofleucine oxidation. Ingestion of either the meal devoid of BCAAs or devoid of all amino acidsresulted in a decrease in estimates of whole-body rates of proteolysis and protein synthesis,decreased leucine oxidation, and a decrease in the interconversion of leucine and KIC.

The decrease in whole-body proteolysis was closely associated with the rise in plasma insulinconcentrations following meal ingestion. Together these data suggest that the transition from tissuecatabolism to anabolism is the result, at least in part, of decreased whole-body proteolysis. This meal-related decrease in proteolysis is independent of the dietary amino acid composition or content. Incontrast, the rate of protein synthesis was sustained only when the meal complete in all amino acidswas provided, indicating an overriding control of protein synthesis by amino acid availability.

NUTRITIONAL SUPPLEMENTS AND THE VEGETARIAN ATHLETE

Because of the necessity to be more careful and consistent with their diets, nutritional supplementscan be more of a necessity for vegetarian athletes than for nonvegetarian ones.106 It seems thatvegetarians rely more on nutritional supplements to get the nutrients they need.107,108

The most useful nutritional supplements for vegetarians are those in which they may bemarginally deficient. Also, like most athletes, they would benefit from increasing dietary proteinwith the use of high-quality soy protein for vegans and with one or more of whey, casein, egg, andsoy protein in lacto–ovo vegetarians. There are also other nutritional supplements, such as creatine(see later) and some of the amino acids, which may be useful for enhancing performance.

There is one caveat to using nutritional supplements. Vegetarians need to carefully examine theingredients of the various nutritional supplements to make sure they do not contain any of theingredients that are not on their food list. For example, many meal replacement powders use acombination of soy and milk proteins. Also there are many vitamin supplements that do not containanimal products.

Considering that many vegetarian diets exclude dairy foods, it is important to ensure adequateintake of calcium, as well as iron, riboflavin, and vitamin D. To make up for possible calciumdeficiencies in vegetarians, calcium supplements should be used by all vegan athletes, especiallywomen and children. Extra calcium should also be taken by vegan women during pregnancy andwhen breast-feeding.

A high-quality vitamin supplement should be included into a vegetarian’s diet. This wouldinclude vitamin D which may be needed if sunlight exposure is limited since sunlight activates asubstance in the skin and converts it into vitamin D. Vegan diets should include a reliable source ofvitamin B12 because this nutrient occurs only in animal foods. Vitamin B12 deficiency can result inirreversible nerve deterioration.

If a diet is low in essential fatty acids, one teaspoon of flaxseed oil or two tablespoonscanola=soybean oil may be added to the diet.

Some supplements normally found in abundance in red meat may be correct for vegetarians touse since these supplements are synthesized and as such do not contain any animal products. Anexample of this would be creatine monohydrate although you will have to make sure that thecreatine mixes do not contain any added nutrients that may be of animal origin.




CREATINE

The estimated daily need for creatine in humans is about 2 g, whereas the daily intake from meat orfish is about l g in the average American diet. The body makes up the deficit by producing creatinein the liver, kidney, and pancreas, using as precursors glycine and arginine. When dietary supply islow, the body steps up its production of creatine, but may not completely compensate, especiallyamong vegetarians, who have reduced body creatine pools.

Creatine stores vary greatly among individuals, and apart from diet, the reasons are unclear.Athletes with low stores might be most apt to benefit from supplementation. Creatine supplemen-tation of 20 g=day for at least 3 days has resulted in significant increases in total creatine for someindividuals but not others, suggesting that there are ‘‘responders’’ and ‘‘nonresponders.’’ Theseincreases in total concentration among responders are greatest in individuals who have the lowestinitial total creatine, such as vegetarians.

Research has shown that muscle and other creatine pools are significantly lower in vegetariansand that by using creatine supplements they as a group are more responsive to creatine supplemen-tation and reach the ceiling levels that nonvegetarians attain with creatine supplementation.109–114

On average, muscle creatine levels increase an average of 20% after 6 days of supplementingat 20 g=day (rapid creatine loading). These higher levels can be maintained by ingesting as lowas 2 g=day thereafter. A similar, but slower, 20% rise in muscle creatine levels occurs by ingesting3 g=day for 1 month, the ‘‘no-load’’ method.

FOOD PROCESSING

The processing of food with the use of heat and chemicals can adversely affect amino acidavailability.115 For example, lysine can be lost from mild heat treatment in the presence of reducingsugars such as glucose or lactose since these sugars react with free amino groups. This may alsooccur when the protein and sugar are stored together at low temperatures.

Under severe heating conditions, either with or without the presence of either sugars or oxidizedlipids, food proteins form additional chemical bonds and can become resistant to digestion so thatavailability of all amino acids is reduced. When protein is exposed to severe treatment with alkali,lysine and cysteine can be eliminated, with formation of lysinoalanine which may be toxic. The useof oxidizing agents such as sulfur dioxide (SO2) can result in a loss of methionine in the protein,through the formation of methionine sulfoxide.

Heating can also have favorable effects. Heating soybean flour improves the utilization ofprotein by making the amino acid methionine more available, and heating raw soybeans destroys theinhibitor of the protein digestive enzyme trypsin. Cooking eggs destroys the trypsin inhibitorovomucoid found in the white part of the egg.

MEASURING PROTEIN QUALITY

There are many factors that influence dietary protein quality including the amino acid compositionand bioavailability of the protein.

There are several ways to measure the protein quality of food. The four most quoted are theprotein efficiency ratio (PER), biological value (BV), protein digestibility (PD), and the proteindigestibility corrected amino acid score (PDCAAS).

PROTEIN EFFICIENCY RATIO

Up to this decade, the PER the best-known and most widely used procedure for evaluating proteinquality, and was used in the United States as the basis for food labeling regulations and for theestablishment of the protein RDA. In this procedure, immature rats are fed a measured amount of




protein and weighed periodically as they grow. The PER is then calculated by dividing the weightgain (in grams) by the protein intake (also in grams).

While simple and economical, the PER procedure is time consuming. Also, evidence fromstudies with rats indicates that the pattern of amino acids required for maintenance and tissue proteinaccretion is quite different in mature rats and in humans. Thus, the amino acid requirements ofgrowing rats are not the same as for those of mature rats, much less mature human beings. Indeed,the intracellular muscle free amino acid pool of rats is probably less suitable for the investigation ofamino acid metabolism, due to the great differences in its distribution in human and rat muscle.116

Nevertheless the PER is still used today although other methods are becoming more common. Astudy published over a decade ago compared the protein quality of different animal foods and oftheir mixtures with vegetable foods, mainly cereals, at the 30:70 for animal to vegetable proteinproportion with experiments performed under the same conditions.117 The animal foods were eggs,beef, pork, barbecued lamb, chicken, ham, sausage, and milk powder. The vegetable foods used inthe mixtures were rice, lime-treated corn flour, wheat flour, and cooked black beans. The proteinconcentrations in the raw and cooked materials were analyzed. The PER and digestibilities weredetermined in Fisher 344 weanling rats.

Based on the corrected PER, the foods with the best protein quality were egg (3.24), sirloin beef(3.16), lamb (3.11), and chicken breast (3.07), which were significantly different from milk powder(2.88) and beef liver and beef round (2.81 and 2.70, respectively). Ham (2.63) and pork loin (2.57)had a similar protein quality to that of casein (2.50). The lowest protein quality was found insausages (2.14). In most of the mixtures of animal and vegetable protein (30:70), the PER wassimilar to or higher than that of the animal food alone. Beans were the vegetable food that showedthe lowest response to the addition of animal food. The conclusion of the study is that some 30:70mixtures of animal to vegetable protein such as chicken, beef round, and pork with cereals could beutilized for regular meals because of their high PER and low cost.

BIOLOGICAL VALUE

To determine the relative utilization of a protein by the body, it is necessary to measure not onlyurinary, but also fecal losses of nitrogen when that protein is actually fed to human beings under testconditions. Even under these circumstances, small additional losses from sweat, sloughed skin, hair,and fingernails will be missed. This kind of experiment determines the BV of proteins, a measureused internationally.

The BV test involves two nitrogen balance studies. In the first, no protein is ingested and fecal andurinary nitrogen excretions are measured. It is assumed that under these conditions nitrogen lost in theurine is the amount the body always necessarily loses by filtration into the urine each day, regardlessof what protein is fed (endogenous nitrogen). The nitrogen lost in the feces (called metabolic nitrogen)is the amount the body invariably loses into the intestine each day, whether or not food protein is fed.In the second study, an amount of protein slightly below the requirement is ingested, and intake andlosses are measured. The BV is then derived using the following formula:

BV ¼ 100� food N� ðfecal N�metabolic NÞ � ðurinary N� endogenous NÞ½ �food N� ðfecal N�metabolic NÞ

The denominator of the BV equation expresses the amount of nitrogen absorbed: food Nminus fecal N (excluding the N the body would lose in the feces anyway, even without food).The numerator expresses the amount of N retained from the N absorbed: absorbed N (as in thedenominator) minus the N excreted in the urine (excluding the N the body would lose in the urineanyway, even without food). Thus, it can be more simply expressed as:

BV ¼ (N retained=N absorbed) � 100




The BV method has the advantages of being based on experiments with human beings and ofmeasuring actual nitrogen retention. Its disadvantages are that it is cumbersome, expensive,sometimes impractical, and is based on several assumptions that may not be valid.

For example, the subjects used for testing may not be similar physiologically or in terms of theirnormal environment or typical food intake (e.g., dietary protein intake history) to those for whomthe test protein may ultimately be used. Also, that protein is retained in the body does notnecessarily mean that it is being well utilized.

There is considerable exchange of protein among tissues (protein turnover) that is hidden fromview when only nitrogen intake and output are measured. One tissue could be shorted, and the testof BV would not detect this. At present, the relationship between protein intakes and optimum organfunction and health is poorly understood. Nevertheless, it would appear that the results obtained bythe BV method would be more applicable to humans.118

As examples we can compare the BV of two proteins that are on the opposite end of the scale,vegetable proteins and hydrolyzed whey protein. Plant proteins have a low BV and thereforesupplementation is a necessity for most plant-based vegetarians (as against lacto–ovo and lactovegetarians) athletes who wish to increase lean body mass.

A study measured the effects of a diet made up of plant protein in rats.119 The resultsdemonstrate the insufficiency of vegetable sources of food with respect to proteosynthesis and thecontent of limiting amino acids (decisive for the synthesis of peptide chains) in the period of theorganism development.

On the other hand, whey protein has a higher BV and is much more suitable for athletes wishingto increase muscle mass. The BV of whey protein also increases once it has been predigested (i.e.,its high molecular weight protein fractions have been broken down into more efficiently absorbedshort chain peptides: di-, tri-, and oligopeptides).

PROTEIN DIGESTIBILITY CORRECTED AMINO ACID SCORE

The latest way to assess dietary protein quality is PDCAAS, recommended by the FAO=WHOJoint Expert Commission on Protein Evaluation at Rome in 1990. Several factors such asinadequacies of PER (the poorest test) and other animal assays, advancements made in standard-izing methods for amino acid analysis and protein digestibility, availability of data on digestibilityof protein and individual amino acids in a variety of foods, and reliability of human amino acidrequirements and scoring patterns make the amino acid score—corrected for true digestibilityof protein—the most suitable routine method for predicting protein quality of foods forhumans.120

The PDCAAS method is a simple and scientifically sound approach for routine evaluation ofprotein quality of foods. The PDCAAS is directly applicable to humans, and incorporates factors formore real-life variables than either PER or BV. The amino acid pattern for humans 2–5 years is usedas the basis for determination of PDCAAS, since this age group matches or exceeds amino acidrequirement patterns of older children and adults. Corrections for digestibility of protein are alsotaken from human data. PDCAAS scores range from 1.0 to 0.0, with 1.0 being the upper limit ofprotein quality (able to support growth and health).

Interestingly, using PDCAAS results in a score of 1.0 for isolated soy protein, identical to otherproteins with scores of 1.0 (such as casein, lactalbumin, or ovalbumen).121 This finding, although itlikely reflects the successful, long-term use of soy protein as the exclusive protein source for infants,who have even higher amino acid requirements than children or adults, does not give us the fullstory as to the quality of soy protein.

Discrepancies between the PER and PDCAAS are related to the differences between younggrowing rats and humans. Growing rats have a larger requirement for sulfur-containing amino acidsto generate the larger amounts of keratin for whole-body coats of hair, which humans do notpossess.




Thus soy protein can be added to the list of complete proteins for humans, along with milk andegg proteins. Other vegetable proteins (pea, beans, peanuts, wheat, oats, etc.) still have less thanadequate protein scores.

Unfortunately the PDCAAS is far from perfect. First of all, our diets rarely contain just one typeof protein and the PDCAAS is not able to assess the impact that a mixture of proteins will haveon the quality of the combination of proteins that together, but not individually, make up ahigh-quality protein. This is especially evident when dealing with the combining of proteins doneby vegetarians.

For example, grain protein has a low PDCAAS because of the low lysine, but it has a highmethionine content. On the other hand, most pulses have a low PDCAAS because of low methio-nine, but they have high lysine content. Because they compliment each other, the two proteinstogether make up a high-quality protein source.

On the other hand, it contains more than enough methionine. White bean protein (and that ofmany other pulses) has a PDCAAS of 0.6–0.7, limited by methionine, and contains more thanenough lysine. When both are eaten in roughly equal quantities in a diet, the PDCAAS of thecombined constituent is 1.0, because each constituent’s protein is complemented by the other.

There is also the fact that several proteins that are different in some ways, since they meet thecriteria decided upon at the 1990 FAO=WHO meeting, all have identical scores of 1.0 since theiractual scores are rounded off. As such, soy protein, with a score of 0.9 is considered much closer inquality than it should be to egg protein, with an actual score of 1.2.

DIETARY PROTEIN REQUIREMENTS

The term protein requirement means that amount of protein which must be consumed to provide theamino acids for the synthesis of those body proteins irreversibly catabolized in the course of thebody’s metabolism.

The dietary requirement for protein comprises two components: (1) a requirement for totalnitrogen and (2) a requirement for the nutritionally indispensable amino acids. The nutritionallyindispensable amino acids alone do not maintain body nitrogen balance; a source of ‘‘nonspecific’’nitrogen from dispensable amino acids, such as from glycine and alanine or other nitrogencompounds, is also required.122

The recommended allowances for protein (nitrogen) are based upon experiments in whichnormal requirement is defined as the intake necessary to achieve zero balance between intake versusoutput. Most estimates assume normal digestion and absorption and normal metabolism. In somecases, estimates of daily turnover are used to determine the amount of nutrient required to maintainbody stores.

Thus the intake of nitrogen from protein must be sufficient to balance that excreted; this basicconcept is called nitrogen balance.

The minimum daily requirement, that is the minimum amount of dietary protein which willprovide the needed amounts of amino acids to optimally maintain the body, is impossibleto determine for each individual without expending a good deal of time and effort for everyperson.

To eliminate the necessity of determining individual nutrient requirements, a system called theRDA has been devised. As research accumulates for each of the many essential nutrients, itsassociated RDA is revised.

The RDAs (1941–1989) were established to try to cover the nutritional needs of all normal,healthy persons living in the United States without the necessity of determining individual nutrientrequirements. Canada had a similar system called the recommended nutrient intake (RNI). Severalcountries have their own RDAs.

In the past, RDAs were established for protein (there was no RDA for fat or carbohydrate),vitamins A, D, E, K, B1, B2, B3, B6, B12, folacin, C, calcium, magnesium, iron, iodine, phosphorus,




selenium, and zinc. The RDAs were set to allow for the vast majority of all normal healthy personsin the United States, but do not cover people with illness or chronic disease. In order to cover someof the outliers, there is a margin of safety built into the RDAs so that the average, healthy person canconsume one-third less than the RDA and still not run into any deficiencies. The Food and NutritionBoard of National Academy of Sciences set the values for the RDAs based on human and animalresearch. They meet every 5 years or so to review contemporary research on nutrients. As researchaccumulated for each of the many essential nutrients, its associated RDA was revised. The lastrevision for the RDAs was in 1989.123

In the last decade, some significant changes have taken place in the detail and scope of officialdietary recommendations. In 1997, the Food and Nutrition Board of the National Academy ofSciences introduced dietary reference intakes (DRIs), changing the way nutritionists and nutritionscientists evaluate the diets of healthy people. There are four types of DRI reference values: theestimated average requirement (EAR), the RDA, the adequate intake (AI), and the tolerable upperintake level (UL). The primary goal of having new dietary reference values was to not only preventnutrient deficiencies, but also reduce the risk of chronic diseases such as osteoporosis, cancer, andcardiovascular disease. Ten DRI reports have been completed since 1997 and several articles andhundreds of papers discussed them.124–126

In 2000, an expert nutrition review panel, consisting of both American and Canadian scientistsselected by the National Academy of Sciences, was formed to establish the DRIs for macronutrientsand energy. The panel’s mission was, based on the current scientific literature, to update, replace,and expand upon the old RDAs in the United States (NRC 1989) and RNIs in Canada (Health andWelfare Canada 1990). In 2002, the DRI report was released with new recommendations for healthyindividuals for energy, dietary carbohydrates and fiber, dietary fats which include fatty acids andcholesterol, dietary protein, and the indispensable amino acids.127

Although the panel was instructed to consider increased physical activity as a factor affectingrequirements, the DRIs for protein that applied to most healthy individuals were felt to be adequatefor those that were physically active. After reviewing the scientific literature investigatingthe protein needs of athletes, the panel stated: ‘‘In view of the lack of compelling evidence to the

Dietary Reference Intakes DefinitionsRecommended dietary allowance (RDA): The average daily dietary intake level that is suffi-cient to meet the nutrient requirement of nearly all (97%–98%) healthy individuals in aparticular life stage and gender group.

Adequate intake (AI): A recommended intake value based on observed or experimentallydetermined approximations or estimates of nutrient intake by a group (or groups) of healthypeople, that are assumed to be adequate—used when an RDA cannot be determined.

Tolerable upper intake level (UL): The highest level of daily nutrient intake that is likely topose no risk of adverse health effects for almost all individuals in the general population. Asintake increase above the UL, the potential risk of adverse effects increases.

Estimated average requirement (EAR): A daily nutrient intake value that is estimated to meetthe requirement of half of the healthy individuals in a life stage and gender group—used toassess dietary adequacy and as the basis for the RDA.

Acceptable macronutrient distribution range (AMDR): A range of intakes for a particularenergy source that is associated with reduced risk of chronic diseases while providing adequateintakes of essential nutrients.

Estimated energy requirement (EER): The average dietary energy intake that is predicted tomaintain energy balance in a healthy adult of a defined age, gender, weight, height, and level ofphysical activity consistent with good health.




contrary, no additional dietary protein is suggested for healthy adults undertaking resistance orendurance exercise.’’128

The general criterion used to establish the EAR for protein in the adult, and subsequently theRDA, was to determine the point at which the body is able to maintain body protein content atits current level (i.e., maintenance of nitrogen balance). This daily value was 0.66 g=kg of bodymass per day for adults (men and women 19 years of age and older), which resulted in an RDAof 0.80 g=kg. This is the same daily value as the 1989 RDA and slightly lower than the 1990 RNI of0.86 g=kg (Health and Welfare Canada 1990).129

In the case of protein and amino acid requirements, the RDA is age and gender dependent and isset at twice the minimum value of the subject who required the most protein and=or amino acid in allthe studies conducted. By greatly increasing the recommended intake figure over that experimen-tally determined it was hoped that the protein and amino acid needs of the majority or 95% of theAmerican population would be met.

The RDA for protein was originally quite high. For many years, it was set at 1 g=kg body weightfor the average adult male. The average adult male was assumed to weigh 70 kg (about 155 pounds)so the RDA was 70 g=day. In the last three decades, however, the RDA for protein has been adjustedsteadily downward until the recommendations of 1989.

HOW WAS THE RDA ESTABLISHED?

Ideally, sufficient research is conducted to show that (1) a given nutrient is needed by the human, (2)certain deficiency signs can be produced, (3) these signs can be avoided or reversed if the missingnutrient is administered, and (4) no further improvement is observed if the nutrient is administered atlevels above that which reversed the deficiency symptoms.

Next, studies are conducted on a variety of subjects to determine their minimal need. Sincehumans vary so much, it is not possible to measure the requirements over a broad range of humanvariability. To allow for this variability, a safety factor is added on to the determined minimumneeds of the group of subjects studied. As more subjects are studied and more data accumulated, theadded safety factor becomes smaller.

For the purposes of determining protein needs, some shortcuts are taken. For example,since proper nitrogen balance studies are laborious and are performed over a period of severaldays, a shortcut is often taken and an estimate of nitrogen balance is made by collecting andmeasuring nitrogen in the urine, since the end products of protein metabolism leave the bodymainly via this route, and other losses are also estimated. Estimations are made of the nitrogenlost in the feces and the small losses of protein from skin, hair, fingernails, perspiration, and othersecretions.

About 90% of the nitrogen in urine is urea and ammonia salts—the end products of proteinmetabolism. The remaining nitrogen is accounted for by creatinine (from creatine), uric acid(products of the metabolism of purines and pyrimidines), porphyrins, and other nitrogen-containingcompounds.

Urinary nitrogen excretion is related to the basal metabolic rate (BMR). The larger the musclemass in the body, the more calories are needed to maintain the BMR. Also, the rate of transamina-tion is greater as amino acids and carbohydrates are interconverted to fulfill energy needs in themuscle. Approximately, 1–1.3 mg of urinary nitrogen is excreted for each kilocalorie required forbasal metabolism. Nitrogen excretion also increases during exercise and heavy work.

Fecal and skin losses account for a significant amount of nitrogen loss from the body in normalconditions, and these may vary widely in disease states. Thus, measurement of urinary nitrogen lossalone may not provide a predictable assessment of daily nitrogen requirement when it is mostneeded. Fecal losses are due to the inefficiency of digestion and absorption of protein (93%efficiency). In addition, the intestinal tract secretes proteins in the lumen from saliva, gastricjuice, bile, pancreatic enzymes, and enterocyte sloughing.




Taking all these losses into consideration and using nitrogen balance as a tool, the minimumdaily dietary allowance for protein may be derived on the following basis:

1. Obligatory urinary nitrogen losses of young adults amount to about 37 mg=kg of bodyweight.

2. Fecal nitrogen losses average 12 mg=kg of body weight.3. Amounts of nitrogen lost in perspiration, hair, fingernails, and sloughed skin are estimated

at 3 mg=kg of body weight.4. Minor routes of nitrogen loss such as saliva, menstruation, and seminal ejaculation are

estimated at 2 mg=kg of body weight.5. Total obligatory nitrogen lost—which must be replaced daily—amounts to 54 mg=kg, or in

terms of protein lost this is 0.34 g=kg (0.0543 6.25 � 1 g N¼ 6.25 g of protein).6. To account for individual variation the daily loss is increased by 30%, or 70 mg=kg. In

terms of protein, this is 0.45 g=kg of body weight.7. This protein loss is further increased by 30%, to 0.6 g=kg of body weight, to account for the

loss of efficiency when consuming even a high-quality protein such as egg.8. Final adjustment is to correct for the 75% efficiency of utilization of protein in the mixed

diet of North Americans. Thus, the RDA for protein becomes 0.8 g=kg of body weight fornormal healthy adult males and females, or 63 g of protein per day for a 174 lb (79 kg) manand 50 g=day for a 138 lb (63 kg) woman.

The need for dietary protein is influenced by age, environmental temperature, energy intake, gender,micronutrient intake, infection, activity, previous diet, trauma, pregnancy, and lactation.

The minimum daily requirement, that is the minimum amount of dietary protein which willprovide the needed amounts of amino acids to optimally maintain the body, is impossibleto determine for each individual without expending a good deal of time and effort for everyperson.

At present, the normal amount of protein recommended for sedentary people is 0.8 g ofprotein per kilogram (0.36 g=lb) of body weight per day. This RDA for protein presumes that thedietary protein is coming from a mixed diet containing a reasonable amount of good-qualityproteins. For the average person subsisting on mixtures of poor-quality proteins, this RDA maynot be adequate.

RECOMMENDED DAILY INTAKES FOR ATHLETES

The RDAs make little provision for changes in nutrient requirements for those who exercise. Energyrequirements increase with exercise as the lean (muscle) mass increases and as resting metabolicenergy expenditure increases. Increased physical activity at all ages promotes the retention of leanmuscle mass and requires increased protein and energy intake.

In athletes, several factors can increase the amount of protein needed, including duration andintensity of exercise, degree of training, and current energy and protein intake of the diet. Athleteswho train hard need more protein than the average individual. This holds true for both enduranceand power sports.

HISTORICAL OVERVIEW

The history of protein requirements for athletes is both interesting and circular. In the mid-1800s thepopular opinion was that protein was the primary fuel for working muscles.130 This was an incentivefor athletes to consume large amounts of dietary protein.

In 1866, a paper based on urinary nitrogen excretion measures (in order for protein to provideenergy its nitrogen must be removed and subsequently excreted primarily in the urine), suggested




that protein was not an important fuel, and contributed about 6% of the fuel used during a 1956 mclimb in the Swiss Alps.131 This paper and others led to the perception that exercise does notincrease one’s need for dietary protein. This view has persisted to the present.

Recently, however, there is some evidence to show that protein contributes more than isgenerally believed at present. The data in the 1866 study likely underestimated the actual proteinuse for several methodological reasons. For example, the subjects consumed a protein-free dietbefore the climb, post-climb excretion measures were not made, and other routes of nitrogenexcretion may have been substantial.

However, based largely on these data, this belief has persisted throughout most of the20th century. This is somewhat surprising because Cathcart,132 in an extensive review of theliterature prior to 1925, concluded ‘‘the accumulated evidence seems to me to point in no unmis-takable fashion to the opposite conclusion that muscle activity does increase, if only in small degree,the metabolism of protein.’’ Based on results from a number of separate experimental approaches,the conclusions of several more investigators support Cathcart’s conclusion.133

Several studies have shown that the current RDA for protein is not enough for many people,including athletes and the elderly.

The current dietary protein recommendations for protein (0.8 g=kg) may be insufficient forathletes and those wishing to maximize lean body mass and strength. These athletes may wellbenefit from protein supplementation. With exercise and under certain conditions, the use of proteinand amino acid supplements may have significant anabolic and anticatabolic effects.

Athletes who train hard, because of the increased use of amino acids for energy metabolism andprotein synthesis, need more protein than the average individual.

Over a decade ago, Peter Lemon of the Applied Physiology Research Laboratory, Kent StateUniversity, addressed the issue of the protein requirements of athletes.134 Lemon remarked thatcurrent recommendations concerning dietary protein are based primarily on data obtained fromsedentary subjects. However, both endurance and strength athletes, he says, will likely benefit fromdiets containing more protein than the current RDA of 0.8 g=kg=day, though the roles played byprotein in excess of the RDA will likely be quite different between the two sets of athletes.

For strength athletes, Lemon states that protein requirements will probably be in the range of1.4–1.8 g=kg=day, whereas endurance athletes need about 1.2–1.4 g=kg=day. There is no indicationthat these intakes will cause any adverse side effects in healthy humans. On the other hand, there isessentially no valid scientific evidence that protein intakes exceeding about 1.8–2.0 g=kg=day willprovide an additional advantage.

EFFECTS OF EXERCISE ON DIETARY PROTEIN REQUIREMENTS

High protein intake has been the mainstay of most athletes’ diets. Athletes in general and strengthathletes and bodybuilders in particular consume large amounts of protein.135 One reason for theirincreased protein consumption is their increased caloric intake. Another is that most athletesdeliberately increase their intake of protein-rich foods and often use protein supplements.

As we have seen, many scientific and medical sources feel that protein supplementation andhigh-protein diets are unnecessary and that the RDA supplies more than adequate amounts ofprotein for the athlete.136 In fact, overloading on protein is felt to be detrimental because of theincreased load to the kidneys of the metabolic breakdown products formed when the excess proteinis used as an energy source.

The results of a number of investigations involving both strength and endurance athletesindicate that, in fact, exercise does increase protein=amino acid needs.137–141 In a review, the overallconsensus has been that all athletes need more protein than sedentary people, and that strengthathletes need the most.142

Over a decade ago, a group of researchers at McMaster University in Hamilton, Ontarioconcluded that the current Canadian RNI for protein of 0.86 g=kg=day is inadequate for those




engaged in endurance exercise.143 Moreover, their results indicated that male athletes may have aneven higher protein requirement than females. A recent study found that endurance athletes need aprotein intake of at least 1.2 g=kg to achieve a positive nitrogen balance.144

Butterfield145 performed a review of the literature and recommended high protein intakes (up to2–3 g=kg) for physically active individuals. She found evidence for the existence of an intricaterelationship between protein and energy utilization with exercise:

When energy intake is in excess of need, the utilization of even a marginal intake of protein will beimproved, giving the appearance that protein intake is adequate. When energy intake and outputare balanced, the improvement in nitrogen retention accomplished by exercise seems to be fairlyconstant at protein intakes greater than 0.8 g=kg=d, but falls off rapidly at protein intakes below this.When energy balance is negative, the magnitude of the effect of exercise on protein retention may bedecreased as the activity increases, and protein requirements may be higher than when energy balance ismaintained.

Somewhat in agreement with Butterfield’s conclusions are the results of another study by Piattiet al.146 They investigated the effects of two hypocaloric diets (800 kcal) on body weight reductionand composition, insulin sensitivity, and proteolysis in 25 normal obese women. The two diets hadthe following composition: 45% protein, 35% carbohydrate, and 20% fat (high-protein diet); and60% carbohydrate, 20% protein, and 20% fat (high-carbohydrate diet). The results, according to theauthors, suggest that (1) a hypocaloric diet providing a high percentage of natural protein canimprove insulin sensitivity and (2) conversely, a hypocaloric high-carbohydrate diet decreasesinsulin sensitivity and is unable to spare muscle tissue.

In another study it was shown that a protein intake as high as four times the recommended RDA(3.3 g=kg of bodyweight per day versus RDA of 0.8 g=kg=day) resulted in significantly increasedprotein synthesis even when compared to a protein intake that was almost twice the RDA.147 Thisobservation that a protein intake of approximately four times the RDA, in combination with weighttraining, can promote greater muscle size gains than the same training with a diet containing what isconsidered by many to be more than adequate protein is in tune with what many bodybuilders andother weight-training athletes believe.

The effects of two levels of protein intake (1.5 or 2.5 g=kg=day) on muscle performance andenergy metabolism were studied in humans submitted to repeated daily sessions of prolongedexercise at moderate altitude.148 The study showed that the higher level of protein intake greatlyminimized the exercise-induced decrease in serum BCAAs.

Several studies and papers in the last decade have championed the need for protein above theRDA for athletes and those involved in physical exercise. The reasons are many and involve thoseathletes in strength and endurance sports, and those wanting to increase muscle mass.

Many support the idea that the protein needs of athletes are substantially higher than thoseof sedentary subjects because of the oxidation of amino acids during exercise and gluconeogenesis,as well as the retention of nitrogen during periods of muscle building.149–151 Intense muscular activityincreases both protein catabolism and protein utilization as an energy source.152,153 Thus, a high-protein diet may decrease the catabolic effects of exercise by several means—including the use ofdietary protein as an energy substrate thus decreasing the catabolism of endogenous protein duringexercise.

Athletes have for years maintained that a high-protein diet is essential for maximizing lean bodymass. And even though there have been attempts to discourage it, the popularity of high-proteindiets has not waned. Athletes seem to feel intuitively that they need higher levels of protein than theaverage sedentary person. This intuitive feeling is backed up by their claims of the ergogenic effectsof high-protein diets.

Are these effects simply psychological? Not according to studies that have shown the anaboliceffects of increased dietary protein intake. For example, in one study done in rats,154 dietary energyhad no identifiable influence on muscle growth. In contrast, increased dietary protein appeared to




stimulate muscle growth directly by increasing muscle RNA content and inhibiting proteolysis, aswell as increasing insulin and free T3 levels.

Supplements that may work through a placebo effect but have no intrinsic effects, althoughperhaps popular for a while, eventually fall by the wayside, and are abandoned by the majority.High-protein diets are used because they work. The use of protein supplements is also popularbecause of their effectiveness above and beyond a whole-food, high-protein diet.

Although there has been some concern about the effects of a high dietary protein intake on thekidney, there seems to be no basis for these concerns in healthy individuals.141,155 In fact, someanimal studies have pointed to a beneficial effect of high-protein diets on kidney function.156

A recent review of some of the issues of high dietary protein intake in humans suggested that themaximum protein intake based on bodily needs, weight-control evidence, and protein toxicityavoidance would be approximately 25% of energy requirements at approximately 2–2.5 g=kg ofweight corresponding to 176 g protein per day for an 80 kg individual on a 12,000 kJ=day or 3,000calories=day diet.157 The authors also stated that this is well below the theoretical maximum safeintake range for an 80 kg person (285–365 g=day).

There has also been some concern about the adverse effects of high-protein diets on the serumlipid profile and on blood pressure. However, it would seem that these concerns also have little basisin fact.

In one study, a diet higher in lean animal protein, including beef, was found to result in morefavorable HDL (‘‘good’’ cholesterol) and LDL (‘‘bad’’ cholesterol) levels.158 The study involved 10moderately hypercholesterolemic subjects (6 women, 4 men). They were randomly allocated toisocaloric high- or low-protein diets for 4–5 weeks, after which they switched over to the other.Protein provided either 23% or 11% of energy intake; carbohydrate provided 65% or 53%; and fatsaccounted for 24%. During the high-protein diet, mean fasting plasma total cholesterol, LDL, andtriglycerides were significantly lower, HDL was raised by 12%, and the ratio of LDL to HDLconsistently decreased. Other studies by the same group of researchers found that increased levels ofdietary proteins resulted in beneficial changes in blood lipids in both normolipemic and dyslipemichumans.159,160 In addition, recent studies have found that increasing dietary plant and animal proteinlowers blood pressure in hypertensive persons.161,162

Intense muscular activity increases protein catabolism (breakdown) and protein use as an energysource. The less protein available, the less muscle you are going to be able to build. A high-protein dietprotects the protein to be turned into muscle by, among other things, providing another energy sourcefor use during exercise. The body will burn this protein instead of the protein inside the muscle cells.

In fact, studies have shown that the anabolic effects of intense training are increased by a high-protein diet. When intensity of effort is at its maximum and stimulates an adaptive, muscle-producing response, protein needs accelerate to provide for that increased muscle mass. It is alsowell known that a high-protein diet is necessary for anabolic steroids to have full effect.

Once a certain threshold of work intensity is crossed, dietary protein becomes essential inmaximizing the anabolic effects of exercise. Exercise performed under that threshold, however, mayhave little anabolic effect and may not require increased protein. As a result, while serious athletescan benefit from increased protein, other athletes who do not undergo similar, rigorous trainingmay not.

Whether or not you need to supplement your diet with extra protein depends on your goals. Forthose of us who do not have to worry about gaining some fat along with the muscle (traditionallyathletes in sports without weight classes or those in the heavyweight classes in sports that do, wheremass is an advantage, e.g., in the shot put and in weight lifting), high-caloric diets will usuallysupply all the protein you need, provided you include plenty of meat, fish, eggs, and dairy products.With the increased caloric intake and including high-quality protein foods, you will get your extraprotein at the dinner table without thinking about it.

Most athletes, however, need the economy of maximizing lean body mass and minimizing bodyfat. These athletes, both competitive and recreational, are on a moderate or at times a low caloric




intake. In order to increase their protein intake, they need to plan their diets carefully and in manycases use protein supplements since they can not calorically afford to eat food in the volumenecessary to get enough protein.

On the average, I recommend a minimum of 1 g of high-quality protein per pound of body-weight (2.2 g=kg) every day for any person involved in competitive or intense recreational sportswho wants to maximize lean body mass but does not wish to gain weight or have excessive musclehypertrophy. This would apply to athletes who wish to stay in a certain competitive weight class orthose involved in endurance events.

However, athletes involved in strength events such as the Olympic field and sprint events; thosein football or hockey; or weight lifters, power-lifters, and bodybuilders, may need even morequantity than my recommendation to maximize body composition and athletic performance. Inthose attempting to minimize body fat and thus maximize body composition, for example in sportswith weight classes and in bodybuilding, it is possible that as they diet, protein may well make upover 50% of their daily caloric intake.

This is because if you are trying to lose weight or body fat it is important to keep your dietaryprotein levels high. The body transforms and oxidizes more protein on a calorie deficient diet than itwould in a diet that has adequate calories. The larger the body muscle mass, the more transaminationof amino acids occurs to fulfill energy needs. Thus, for those wishing to lose weight but maintain oreven increase lean body mass in specific skeletal muscles, I recommend at least 1.5 g of high-qualityprotein per pound of bodyweight. The reduction in calories needed to lose weight should be at theexpense of the fats and carbohydrates, not protein.

PROTEIN (NITROGEN) BALANCE

Protein balance is a function of intake relative to output (utilization and loss; see Figure 4.1). Intakecomes from dietary sources and the recycling of proteins, especially of cellular protein sloughed offin the intestinal lumen, while losses occur due to the use of the carbon skeletons of amino acids withthe excretion of nitrogenous compounds, as well as protein losses in feces, urine, sweat, and theintegumentary system.163

Nitrogen balance in adult

ProteinsSweat, sebum,nails, hair, skin

Nitrogenouscompounds

Amino acids

Secretion

Intestinal lumenDiet

N intake = N losses

NH3 Urea

Colon

Carbon skeleton Urine

Feces

FIGURE 4.1 Metabolic pathways determining nitrogen balance. Nitrogen balance is the result of N intake(from the diet) and N losses, which consists of N recovered in urine and feces and miscellaneous losses. (FromTomé, D. and Bos, C., Dietary protein and nitrogen utilization, J. Nutr., 130(7): 1868S–1873S, 2000.)




Body proteins are in a constant state of flux, with both protein degradation and protein synthesisconstantly going on. Normally, these two processes are equal with no net loss or net gain of proteintaking place. Protein intake usually equals protein lost.

However, if protein synthesis (anabolism) is greater than protein degradation (catabolism), theoverall result is anabolic with a net increase in body protein. If protein degradation is greater thanprotein synthesis, the overall result is catabolic with a net decrease in body protein.

Although the end result may be the same, protein balance and protein turnover are notsynonymous. Protein turnover is a measure of the rate of protein metabolism, and involves bothprotein synthesis and catabolism.

Protein catabolism is necessary to eliminate proteins that are no longer required by the cell, orthat have become dysfunctional, and to provide substrates for energy production. Thus, degradationof proteins involves enzymes, hormones, and structural as well as contractile proteins.

Protein and amino acids are lost from the body through the urine; feces; sweat; seminal, vaginal,buccal, and menstrual fluids; the desquamation of skin and mucosal areas; and hair and nail growthand loss. Nitrogen derived from amino acids is mainly lost in urine and feces and from sweat.

Unlike the fats and carbohydrates that can be stored in the form of triglycerides and glycogen,there is no storage form of protein or amino acids. All the protein and amino acids (except for a verysmall amount of free amino acids that make up the plasma and intracellular amino acid pool) serveeither a structural or metabolic function. Excess amino acids from protein are transaminated, andnonnitrogenous portion of the molecule is transformed into glucose and used directly or convertedinto fat or glycogen. The unneeded nitrogen is converted to urea and excreted in the urine.

The larger the body muscle mass, the more transamination of amino acids occurs to fulfillenergy needs. Each kilocalorie needed for basal metabolism leads to the excretion of 1–1.3 mg ofurinary nitrogen. For the same reason, nitrogen excretion increases during exercise and heavy work.

Urea accounts for over 80% of urinary nitrogen. The remaining nitrogen is excreted as creati-nine, porphyrins, and other nitrogen-containing compounds. Thus total urine loss of nitrogen¼urinary urea nitrogen (mg=dL)3 daily volume (dL)=0.8. Urinary nitrogen is related to the restingmetabolic rate.

Fecal and skin losses account for a large proportion of nitrogen loss from the body (about 40%)in normal circumstances. The magnitude of these losses, however, varies in disease states. Thus,measurement of urinary nitrogen often provides a useful index of daily nitrogen requirement.

Minimal nitrogen loss (in grams per day) from a 70 kg person on a diet that is nitrogen free butenergy adequate approximates 1.9–3.1 in urine, 0.7–2.5 in stool, and 0.3 from skin for a mean totalloss of 4.4 g=day and a maximum total loss of 5.9 g=day.

Equivalent protein loss can be calculated by multiplying nitrogen loss by 6.25 so that mean totalloss by metabolism of protein is 4.43 6.25 or 27.5 g=day or about 0.4 g=kg body weight for a 70 kgperson and maximum total loss by metabolism or protein is 5.93 6.25 or 36.9 g=day or about0.5 g=kg body weight for a 70 kg person. The recommended protein allowance for adults variesfrom 0.6 to 0.9 g=kg to allow for a margin of safety.

This protein allowance is raised considerably if low-quality protein is the main source of protein.Low-quality proteins including certain vegetable proteins do not support growth as well as proteinfrom milk, eggs, or meat. The differences in the nutritional value of protein are largely due to thehigher content of essential amino acids in animal proteins and to differences in digestibility.

Protein requirements are highest during growth spurts and it is during these times that proteindeficiency is most harmful.

Protein requirements are raised if other energy sources are not ingested in adequate amounts—as is sometimes seen in athletes and other people who are dieting to decrease their body-fat levels.Amino acids ingested without other energy sources are not efficiently incorporated into proteinpartly because of the energy lost during amino acid metabolism. Moreover, incorporation of eachamino acid molecule into peptides requires three high-energy phosphate bonds. Consequently,excess of dietary energy over basal needs improves the efficiency of nitrogen utilization.




EFFECT OF DIETARY PROTEIN ON PROTEIN METABOLISM

The dietary history of the individual is an important factor affecting protein turnover. This wouldinclude not only the long-term effects of the previous diet, but also the short-term effects of recentprior meals. Anabolic and catabolic activities can be influenced by the amount as well as thefrequency and timing of protein intake.

The overall and specific metabolic effects of diets containing inadequate amounts of proteinhave been extensively studied.164–174 Following short-term restriction of dietary protein, there is adecrease in the rates of whole-body and organ protein synthesis and turnover.175–179 Amino acidsare catabolized less and used more as precursors for endogenous compounds such as nucleic acidsand creatine and for necessary protein synthesis.

High levels of protein in the diet usually result in increased hepatic amino acid catabolism,especially in the beginning when there is a dramatic increase in the level of dietary proteinor individual amino acids. The liver, because of its ability to adapt intracellular metabolizingenzyme180�182 and transport processes,183 has a major role in maintaining serum amino acid levelswithin a certain physiological range.175

However, even with this increased catabolic effect, higher-protein diets or diets modified fortheir amino acid content, result in higher serum and tissue levels of amino acids under a variety ofconditions.184–189 Supplementation with amino or keto acids also usually results in increased serumand tissue levels.72,190–192

These higher tissue levels translate into increased protein synthesis as shown by a number ofstudies using both oral amino acids and amino acid infusions.58,60,193–196 For example, one study byusing femoral arteriovenous catheterization and quadriceps muscle biopsies, measured muscle-protein synthesis and breakdown, and amino acid transport during intravenous infusion of anAAM in young and elderly subjects.197 Peripheral amino acid infusion significantly increasedamino acid delivery to the leg, amino acid transport, and muscle protein synthesis, independentlyof the age of the volunteers. Despite no change in protein breakdown during amino acid infusion, apositive net balance of amino acids across the muscle was achieved.

The same team of researchers published another study several years later and determined that abolus oral ingestion of amino acids also produce a similar response in young and elderly individuals.198

For more information on the effects of oral supplementation of amino acids please see laterunder the individual amino acids.

Altered intakes of protein and amino acids modulate the rates of the major systems (proteinsynthesis, protein degradation, and amino acid oxidation) responsible for the maintenance oforgan and whole-body protein and amino acid homeostasis. Switching from a low-protein dietto a high-protein diet or vice versa, results in adaptive responses that reflect the metabolic state ofthe organism.

Protein deficiency results in decreased proteolysis and decreased protein synthesis, changes thatspare amino acids. Switching from a low-protein to a high-protein diet results in an adaptation phasein which there is a high rate of protein synthesis (a catch-up phase) and increased amino acidoxidation.177,199,200 Similarly, supplementing the diet with certain amino acids can also increaseprotein synthesis and amino acid oxidation.72,201

A review looked at the mechanisms and nutritional significance of metabolic responses to alteredintakes of protein and amino acids, with reference to nutritional adaptation in humans.176 Theyconcluded that for oxidation, amino acid availability is a primary determinant and protein synthesisis affected particularly at the initiation phase. Thus, the immediate response to a change in dietaryprotein levels is often the opposite that occurs once the body has adapted to the change. For example,the rate of degradation of myofibrillar protein in skeletal muscle rises just after the beginning offasting,202 but decreases once the body is accustomed to the dietary change (see below).

Studies have shown that the body feels the amino acid pinch after only two protein-restrictedmeals. In one study,203 the mechanism governing short-term adaptation to dietary protein restriction




was investigated in nine normal adults by measuring their metabolic response to a standard mixedmeal, first while they were adapted to a conventional, high-protein diet (day 1) and then again afterthey had eaten two low-protein meals (day 2). Urea appearance (measured as the sum of its urinaryexcretion and the change in body urea pool size), body retention of 15N-alanine included in each testmeal, and whole-body protein turnover were calculated over the 9 h following meal consumption oneach day. Postprandial urea nitrogen appearance decreased on day 2. Whole-body nitrogen flux,protein synthesis, and protein breakdown all decreased significantly on day two.

The authors concluded that short-term metabolic adaptation occurs within two meals of reducedprotein intake. The mechanism appears not to involve selectively an increased first-pass retention ofdietary amino acids, but rather a general reduction in fed-state whole-body protein breakdown. Itwas also shown in the study that inadequate protein intake for as few as two meals induces a promptcompensatory increase in body protein retention when protein is returned to the diet.

The authors of another study 204 relate: ‘‘The argument here is that body protein is lost in thepost-absorptive state and that the extent of these losses is likely to be variable according to the extentto which changes in protein synthesis and degradation in response to fasting is conditioned bydietary protein intake.’’

Thus, if there is an induction of protein-oxidizing enzymes and a high rate of protein turnover asdietary protein intake rises and if this effect persists for an appreciable period of time when the dietis changed, after a period on a high-protein diet, a switch to a low-protein diet ‘‘should result in amarked negative balance with insufficient fed-state gain for balance whilst the high post-absorptivelosses persist.’’

AMINO ACID METABOLISM

Amino acids are not stored in the body other than as integral parts of protein. However, there is asmall amount of extracellular and intracellular free amino acids that is referred to as the free aminoacid pool. This free amino acid pool in muscle is labile, that is, constantly changing and is affectedby the quantity and quality of dietary protein which in turn affects protein metabolism.187

During exercise, most protein synthesis is suppressed in muscle, although the synthesis ofcertain proteins remains unchanged or even increases. The general suppression of protein synthesisin muscle leaves much of the free amino acid pool unused. As a result of the suppression and anincrease in muscle protein catabolism, an increased pool of available free amino acids is created.The main use of free amino acids is connected with the energy requirement of muscular activity,through the oxidation of BCAA and the use of alanine in gluconeogenesis (see later).205

The increases in the free amino acid pool, in the rate of the glucose–alanine cycle, and in the useof amino acids in the liver are stimulated by an increased level of glucocorticoids and a decreasedlevel of insulin during exercise. During recovery after exercise, the use of amino acids for adaptiveprotein synthesis is intensified.

Should any amino acids required for the synthesis of new contractile proteins be limiting inavailability, the training response may suffer as a result. Moreover, as the dietary provision of aminoacids is always inefficient,206 the need for a continual and ready supply of amino acids may be ofparticular importance to those individuals hoping to maximize training-induced lean tissue gains.

Fasting causes energy levels to drop. Catabolism of muscle proteins is enhanced at first, and freeamino acids such as alanine and glutamine are liberated which can be metabolized for energy via theTCA cycle or be used to form glucose via gluconeogenesis.

In the postabsorptive state, amino acids are released from the periphery to provide precursors forprotein synthesis in the splanchnic organs. In skin, for instance, integrity is maintained in theabsence of amino acid intake by using amino acids resulting from the net breakdown of muscleprotein as precursors for synthesis.

The importance of the prevailing amino acid concentration has been suggested by studiesshowing that increased amino acid availability alone may augment skeletal muscle protein synthesis




in healthy men.207,208 It appears that increasing amino acid availability is a major component in theresulting increase seen in protein synthesis.

The hormonal response to protein and amino acids and the direct effects of these substratesconstitute the two mechanisms by which feeding influences amino acid turnover and oxidation. Inrats, it has been shown that the anabolic drive from dietary proteins and amino acids is mediatedin part by their effects on insulin, thyroid hormone, and IGF-1.209 In humans, an increase seen ininsulin secretion, along with increased levels of amino acids, has been shown to result in an overallincrease in protein deposition. The hyperinsulinemia decreased proteolysis but did not stimulateprotein synthesis, and the hyperaminoacidemia stimulated protein synthesis but did not suppressendogenous proteolysis.210,211

A study found that amino acids fed alone, which exert a minor stimulus to insulin secretion,increase plasma amino acid levels, markedly stimulate protein synthesis and oxidation, and inhibitprotein degradation in humans.212 The authors of this study concluded that the feeding response ofprotein synthesis, degradation, and amino acid oxidation reflects the combined impact of insulin,inhibiting degradation, and tissue amino acids both inhibiting degradation and stimulating synthesisand oxidation.

In another study, the authors concluded that in the human adult, amino acid catabolism isnecessarily the major fate of dietary protein at low concentrations, with the shift to depositionrequiring increasing dietary protein concentrations.213 Thus, with lower protein intake the stimulusfor protein synthesis is depressed while with higher protein intakes there is an increase in proteinsynthesis. In this study, the regulatory influence of dietary amino acids on anabolic processes onlyoccurred with higher protein intakes. It would appear that the anabolic drive induced by low-protein(0.36 g=kg=day) and medium-protein (0.77 g=kg=day) diets was inadequate.

Changes in dietary protein or amino acid intake may alter transport of certain neutral amino acidsinto skeletal muscle via changes in plasma amino acid pools. In one study, amino acid transportsystems A and L, which transfer preferentially small neutral amino acids and large neutral aminoacids, respectively, were studied in the isolated soleus muscle.214 Selective differences were observedin transport by skeletal muscle of model amino acids for the A and L systems: increased transportresulting from various stimuli was limited to the model for the A system, and transport of both modelswas depressed with mixtures containing physiological levels of amino acids.

THE DIETARY PROTEIN PARADOX—THE PROBABLE NEED FOR PROTEIN AND AMINO

ACID SUPPLEMENTS EVEN IN DIETS HIGH IN DIETARY PROTEIN

However, simply increasing dietary protein through the use of increased amounts of whole-protein-containing foods may not be enough. Some studies have suggested that increased dietaryintake of protein may inhibit growth. In one study, 28-day-old male Sprague–Dawley rats were fed,either ad libitum or in restricted amounts, isoenergetic diets containing 2%, 5%, 10%, 15%, 25%, or50% lactalbumin protein and 5%, 11.9%, or 21.1% fat for 8 weeks, and were then killed.215 Weeklyfood consumption, body weight, terminal weight, body water and lipid, and liver weight, DNA,RNA, protein, and lipid were measured.

The growth rate increased progressively with each increase in the level of dietary protein upto 25% protein and then declined. Growth was also accelerated by a high-fat diet but was retardedby restriction of energy intake. Total body lipid correlated directly with the level of fat in thediet. Multiple regression analysis showed that the maximum rate of weight gain of 58.8 g=weekoccurred when the diet contained 23% protein. Growth rate declined when the diet contained ahigher protein level.

More recently, Moundras et al.216 found that increasing the dietary protein level in rats led to areduced availability of some amino acids for peripheral tissues. This was accompanied by adepressed weight gain in animals fed the highest protein diet (60% casein). The authors suggestedthat the depression in growth rate might be due to energy wastage caused by catabolism of excess




amino acids, the reduction in the availability of certain amino acids, or decreased insulinemia in ratsfed the high-protein diets.

In this study, set up to evaluate the effect of changes in dietary protein level on overallavailability of amino acids for tissues, rats were adapted to diets containing various concentrationsof casein (7.5%, 15%, 30%, and 60%) and were sampled either during the postprandial orpostabsorptive period. In rats fed the protein-deficient diet, glucogenic amino acids (except threo-nine) tended to accumulate in plasma, liver, and muscles.

In rats fed high-protein diets, the hepatic balance of glucogenic amino acids was markedlyenhanced and their liver concentrations were consistently depressed. This response was the result ofa marked induction of amino acid catabolism (a 45-fold increase of liver threonine–serine dehy-dratase activity was observed with the 60% casein diet). The muscle concentrations of threonine,serine, and glycine underwent changes parallel to plasma and liver concentrations, and a significantreduction of glutamine was observed.

During the postabsorptive period, adaptation to high-protein diets resulted in a sustained catab-olism of most glucogenic amino acids, which accentuated the drop in their concentrations (especiallythreonine) in all the compartments studied. The time course of metabolic adaptation from a 60% to a15% casein diet was also investigated. Adaptation of alanine and glutamine metabolism was rapid,whereas that of threonine, serine, and glycine was delayed and required 7–11 days. This wasparalleled by a relatively slow decay of liver threonine–serine dehydratase activity in contrast to therapid adaptation of pyruvate kinase activity after refeeding a high-carbohydrate diet.

Thus in a high-protein diet in which the protein is obtained from high-protein whole foods, thedecreased availability of amino acids, specifically threonine, glycine, and serine as well as glutamineand alanine, may result in a decrease in protein synthesis. The use of these amino acids in a proteinsupplement may correct the relative deficiency and increase the amount and efficiency of proteinsynthesis.217

It also seems that some amino acid deficiencies may result from the use of different proteinfoods in both isocaloric and hypocaloric diets. For example, in one study the substitution of soybeanprotein for casein in a high-protein diet resulted in a taurine deficiency in cats.218 When a casein-based diet containing either 25% or 50% protein was given to cats for 6 weeks, no differencein plasma taurine concentration was observed; however, substituting soybean protein forcasein resulted in a significant decrease in plasma taurine concentration of cats in the 50% soybeanprotein group, but not in the 25% soybean protein group. In a second part of this study, when thefood intake of cats was limited, cats fed 60% soybean protein or casein diets had significantly lowerplasma taurine concentrations than cats fed a 30% casein diet, with the 60% soybean protein dietcausing the greater decrease.

In high-protein diets there is an increased intake of BCAAs which have been shown to stimulateprotein synthesis (see earlier or later). However, with a high-protein diet the BCAAs are quicklycatabolized to alanine and glutamine in muscles, which in turn are used for gluconeogenesis and bythe gut and immune system respectively (see below).

Even with the formation of glutamine from BCAAs, a feature of high-protein, whole-food dietsis the decrease in hepatic, plasma, and muscle glutamine concentrations. This decrease may haveimportant implications for effective protein synthesis. Increasing the endogenous levels of glutamineby the use of exogenous glutamine might have important effects on increasing protein synthesis inhigh-protein, whole-food diets.

Low levels of alanine in between high-protein meals results in low levels of plasma alanine.216

This decrease in plasma alanine may result in an increase in protein catabolism and decrease inprotein synthesis. Supplemental alanine may be important for the regulation of protein metabolismand increasing the anabolic drive.

High-protein diets may also result in decreased levels of threonine, glycine, and serine in liver,plasma, and muscle (see later). Since threonine is an essential amino acid, its decrease might causesevere metabolic alterations and significant decreases in protein synthesis.




PROTEIN NEEDS IN CALORIE-RESTRICTED DIETS

Because of the increased oxidation of endogenous protein during weight loss, an increased level ofexogenous protein is needed to minimize the autophagy (breakdown of muscle protein) that occurs.In a randomized trial in 17 healthy obese women, a diet containing 1.5 g protein per kilogram idealbody weight was found to result in significantly better protein sparing than an isocaloric dietproviding only 0.8 g protein per kilogram ideal body weight.219 Patients lost weight at the samerate on the two diets, but since there was less nitrogen loss on the diet without carbohydrate, it canbe assumed that more fat loss occurred than on the diet where carbohydrate replaced some of theprotein.

Bodybuilders who are dieting for a contest and undergoing glycogen depletion would oxidizemore protein for energy and thus likely need even higher dietary protein to prevent musclecatabolism. Also, since it has been shown that individuals with lower glycogen stores oxidizedmore than twice the protein than those with high initial stores, this would further increase the needfor dietary protein.220

Bodybuilders preparing for competition cut calories to lose body fat. In order to maintain andperhaps even build muscle tissue, dietary protein must be increased even further than with theirnormally high-protein diets. In addition, as has been stated earlier, those on calorie-restricted dietsand those with increased muscle mass need more protein in their diets than normal.

In order to increase dietary protein but minimize the caloric increase, foods must be chosen thatare low in energy and high in protein. This leads to a diet that is traditionally high in chicken breasts,egg whites, and white fish. This type of restricted diet is a common dietary practice amongcompeting bodybuilders and some other athletes involved in sports with weight classes.221

Unfortunately a diet that consists mainly of these foods may be counterproductive. One of thereasons lies in the nonessential amino acid alanine (see discussion under the amino acid alanine).

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