Human Nutrition

Agriculturally-produced foods can meet human nutritional requirements. What are those requirements?

1. Caloric need: ~2,000 – 2,600 Kcal/day depending on sex, size, and activity level

2. Essential amino acids: isoleucine, leucine, lysine, threonine, valine, tryptophan, histidine, methionine, phenylalanine. All these must be obtained from the diet. A helpful mnemonic (thanks to Dr. Crosby): I Love Lysine, Though Val Thinks His Meth’s Preferable.

3. Essential Fatty Acids: linoleic acid, linolenic acid, arachadonic acid. These, too, must be consumed in the diet.

4. Vitamins: 2 groups – fat soluble and water soluble

Fat soluble: A (retinol), D (calciferol), E (tocopherol) and K (phylloquinone)

Water soluble: C (ascorbic acid), folic acid, B vitamins – B1 (thiamine), B2 (riboflavin), B3 (niacin), B5 (pantothenic acid), B6 (pyroxidine), B12 (cobalamin)

5. Minerals: Major – Ca, P, K, S, Na, Cl, Mg Minor – Fe, I, Zn, Se, Mn, Cr, Cu, Co, Mb, F

Think about how the typical North American diet compares to that of the !Kung San bushmen and to the requirements indicated in this list. You may want to check out Chapter 10 and the USDA Food Pyramid in fig.10.5.

To understand the importance of a proper diet, let’s examine the ‘diseases’ associated with various dietary lacks and excesses:

The first problem is what happens when the diet provides insufficient calories (and probably also a similar insufficiency in protein intake and other nutrients). The ‘disease’ is called marasmus.

This image is of a child during WWII.

When the diet includes sufficient caloric intake, but insufficient protein, the ‘disease’ is called kwashiorkor. This is common in underdeveloped countries where the diet is almost all starch after weaning.

Symptoms include edema (caused by hypoalbuminemia) evident in puffy skin and swollen belly, frequently dermatitis, behavioural listlessness, liver diease (fatty liver), loss of muscle mass, and changes in pigmentation

Both marasmus and kwashiorkor, if not reversed in growing infants, will result in mental retardation and stunted growth. Once these conditions occur, they are permanent. Earlier symptoms are readily reversible with improvement in the diet.

What about the opposite general condition – overnutrition? That’s what’s common in North America. About 40% of us are now either overweight or obese. That is a serious health problem. Obesity greatly increases the risk of many serious problems, including: hypertension, cardiovascular disease, non-insulin dependent (type II) diabetes mellitus, asthma, glaucoma, and a number of cancers.

Type II diabetes used to be a disease of middle age and beyond. Now it is appearing in many young adults (age ~20) and even in adolescents. Lifetime cost of medication is high.

Your textbook presents the latest measure to indicate overweight or obese conditions: body mass index or BMI.

BMI = body weight in kg/(height in m)2

Optimum is a BMI of 20-25

Overweight is a BMI of 25-30

Obese is a BMI of >30

Severe and dangerous obesity is a BMI >40

Underweight is a BMI <20

With no intention of embarrassment, let’s check the BMIs of a few members of the class…

Fiber in the diet: There are two basic types of fiber – soluble and insoluble.

Soluble fiber includes materials like:

• pectins (used to thicken jams),

• gums (Look at the ingredients on your next package of ice cream. You are likely to find guar gum being used as a thickener and ‘smoothing’ agent. It’s also what makes oatmeal porridge thick)

• hemicelluloses (alginates, made from hemicelluloses in cell walls of algae, are also frequently found as thickeners in yogurt and ice cream)

Some soluble fibers are thought to lower cholesterol levels, but may have their main effect as bulk, moving food through the gut faster and lowering time for cholesterol uptake.

Insoluble fiber is not soluble in water. As such, most is indigestible and adds no nutrient to the diet, but does make for ‘regularity’ and may (??) reduce the occurrence of polyps in the large intestine. Those polyps are considered percursors of colon cancer.

Now on to proteins. Some amino acids are essential. Those, listed earlier, cannot be synthesized by metabolic pathways we (and most other animals) possess.

Proteins from animal sources are almost all complete, meaning they include those essential amino acids (think logically: if animals need them in the diet to grow and prosper, when we harvest them, they must have obtained those essential amino acids). Plant proteins are mostly incomplete, lacking one or more of the essential amino acids.

How much protein do we need?

Much less than most of us eat! About 10% of caloric intake should come from protein.

For a 2000 kcal diet, about 200 kcal should come from protein. At 4 kcal/g, that means about 50g of protein per day.

Assume your average “1/4 pounder” is just less than 20% protein. ¼ pound is ~115 g. 20% of that is ~24 g, or about ½ your needed daily intake of protein. If you have it as a cheeseburger, with a 50 g slice of cheese, you’ve added about another 15 g protein.

You only need another 10 – 15 g of protein over the rest of your day to meet daily requirements.

Fats and Cholesterol: it is suggested that 30% of caloric intake should come from fats and oils.

Most fats and oils in foods are in the form of triglycerides – a glycerin to which 3 fatty acids are bound. Here’s the basic structure:

This is a saturated fat. There are no double bonds in the fatty acid chains

Saturated fats (and fatty acids) can pack tightly together, and thus are generally ‘solid’. Double bonds along the carbon chains (one = monounsaturated, >1 = polyunsaturated) cause bends in the chains. They cannot pack tightly together, and are therefore typically liquid (oils).

Note that both of these fatty acids are in cis configuration.

Lately there has been a lot of press about trans fats. The trans configuration means that where there is a double bond, the hydrogens attached to the carbons are ‘pointed’ in opposite directions:

H…-C=C- …

HTrans fatty acids mostly act like saturated fats, and are formed when lipids are ‘hydrogenated’ partially or fully to manufacture products like margarine. This kind of trans fat is now being eliminated (by KFC, MacDonald’s, and others).

Some trans fats in animal food products (red meats, milk, cream, and butter) cannot be eliminated. Conjugated linoleic acid is a major component, and may be an anticarcinogen.

We need fats in our diet.

They are raw materials for the biosynthesis of cell membrane components, a number of hormones (various sex hormones are all cholesterol derivatives), and cholesterol itself. Some cholesterol is synthesized in our livers from saturated fatty acids; some is absorbed from animal products in our diets: eggs, butter, cheeses and meats.

Fats, however, are insoluble in water. They must be bound to water-soluble protein to move in the circulation. So bound they are called lipoproteins. They fall into two classes: low density lipoproteins (LDL) typically called “bad cholesterol” and high density lipoproteins (HDL) called “good cholesterol”.

Reason for those common names: LDL is deposited on arteries, slowly clogging them. HDL tends to remove LDL from arterial walls and prevent accumulation.

When LDL deposits f

When LDL deposits develop in an artery, they partially block it. They also collect other junk, and a plaque develops. Platelets clump, trying to repair the arterial wall. The whole mess hardens. Slowly, blood flow beyond the blockage is reduced. It may finally become a complete blockage. If it occurs in an artery supplying the heart, a ‘heart attack’ (myocardial infarction) occurs, and downstream parts of the heart muscle die. If it occurs in the brain, a stroke occurs, and downstream brain cells die.

The U.S. FDA has set a standard that total cholesterol:HDL as a ratio should be less than 5:1.

Vitamins and Minerals: These are collectively called micronutrients, since we only need a little of each one in our daily diet.

We’ll consider vitamins first. Remember, there are two types: water soluble and fat soluble vitamins.

Too little of any vitamin causes some form of deficiency disease, but too much can also cause problems.

Water soluble vitamins: the vitamin B complex are mostly coenzymes, necessary to the proper function of various enzymes in the body. Since they are water soluble, they can be lost in food preparation that uses excessive amounts of water, then discards the water. Some can also be destroyed by high temperature cooking.

Vitamin B1 is otherwise called thiamine. It is a critical coenzyme in basic metabolism of carbohydrates. The disease of thiamine deficiency is beriberi. The symptoms are: muscle weakness, fatigue, confusion, cramping, numbness in the legs.

This woman is so weakened by beriberi that she cannot stand.

Major sources of thiamine: cereal bran, whole grains, seeds, nuts, legumes, and, to a lesser extent, eggs, milk and meat.

Vitamin B2 or riboflavin is another important coenzyme. You may recognize the electron transport component FAD – the F is for flavin. Deficiency symptoms include a number of ocular problems (eye strain and fatigue), sensitivity to light, and lesions of the mouth and lips.

Riboflavin deficiency – angular stomatitis of the mouth and lips, evident as irritation and fissuring in the corners of the mouth

Rich food sources include liver, meat, eggs, green leafy vegetables, enriched and whole grain breads and cereals.

Vitamin B3 or niacin (or nicotinamide) forms the coenzyme NAD, phosphorylated NADP, and reduced as NADPH. This coenzyme is critical in both photosynthesis and metabolism. Niacin deficiency results in pellagra. The sequence of effects are called the 4Ds: dermatitis, diarrhea, depression (or dementia) and death.

Rich sources include meat, peanuts and legumes, whole grain and enriched breads and cereals. Part of the niacin requirement is met by synthesis of niacin from tryptophan.

This ‘necklace’ is typical in the early stages of pellagra dermatitis. Facial lesions are also evident. His tongue would be ‘strawberry’ in colour.

Corn is poor in niacin and tryptophan. When corn was brought back from the New World, nearly epidemic pellagra resulted. In the New World, corn was prepared with lime, which releases niacin from the grain and prevented pellagra.

Vitamin B12 or cobalamin does not occur in plant-derived foods. It must come from meat, dairy products and eggs in the diet. Vegans must take dietary supplements.

The most common symptoms of deficiency are weakness, fatigue, palpitations, dizziness, and pallor. These are all related to the disease which can result: pernicious anemia. In pernicious anemia, red blood cells are malformed, poor oxygen transportation results, and there can be eventual peripheral nerve damage. Cobalamin is also involved in DNA synthesis, most evident in red blood cell formation.

Note that very few of these red blood cells look like the healthy ‘donuts’ we’d expect.

The last of the water soluble vitamins is folic acid. Basically, it functions with B12, and has the same deficiency effects (except peripheral neuropathy).

pernicious anemia

The fat soluble vitamins:

Vitamin A – there are plant and animal sources. From plants the source is β-carotene (e.g. from carrots). From animals the source is retinol. It is an important anti-oxidant.

Vitamin A deficiency leads to ‘night blindness’ due to insufficient synthesis of opsin proteins (rhodopsin – black and white vision and iodopsins for colour vision). Long term and severe deficiency leads to corneal degeneration and epithelial keratinization

Vitamin D is critical in the regulation of calcium and phosphorus: 1) it is important in absorption of these minerals from the gastrointestinal tract (from food), 2) it acts as a cofactor with parathyroid hormone (see below) to release calcium from bone when the blood level drops, and 3) it acts on the kidney to cause calcium retention.

Vitamin D is the only vitamin we can make. It is made from a cholesterol derivative in the skin under exposure to UV light (the product is a ‘preliminary’ form – provitamin D). The provitamin is converted to the active form in the liver.

Vitamin D deficiency leads to rickets (pictures with calcium regulation). Vitamin D excess can cause calcium deposition in the circulation and kidneys, even leading to death.

Vitamin C or ascorbic acid comes (obviously) from citrus fruits, but also from organ meats. It is involved in the synthesis of collagen, a protein of the connective tissue.

Deficiency causes scurvy, which was rampant in sailors and explorers before the vitamin and its sources were identified. Scurvy caused bleeding from the gums and loss of teeth. It also causes blood vessel rupture in the skin, so that small wounds do not heal and bruises formed and remained. It can be fatal.

Explorers who depended on pemmican and other dried meat suffered from scurvy until they learned that fresh meat and blubber acted as antiscorbutics. In the exploration of the Antarctic, Scott’s polar party suffered from scurvy, but Amundsen, who started with extra dogs, and when they couldn’t pull, they were slaughtered and eaten (by both humans and remaining dogs) did not.

How much do we need? 10 mg/day prevents scurvy; 60 mg/day is the RDI. However, Linus Pauling claimed vitamin C could prevent the common cold, and suggested taking 2,000 mg/day. Others think saturation occurs with 100 mg/day, and anything more is excreted in urine. Expensive waste. The evidence that excess vitamin C prevents colds is weak.

Lastly, the minerals.

Calcium – bones and teeth are made up of calcium phosphate, but calcium is also critical in muscle contraction, blood clotting, and nerve impulse transmission. Calcium in the body fluid is kept within a narrow range under the control of two hormones: calcitonin and parathyroid hormone.

Calcitonin, made in the thyroid, reduces circulating Ca.

Parathormone, made by the parathyroid glands, increases circulating Ca. It causes cells in bone (osteoclasts) to release Ca from bone mineral into circulation. Calcitonin has the reverse effect.

Calcium deficiency at advanced age leads to osteoporosis, or degeneration of bone due to loss of calcium. In osteoporosis, bones become fragile and easily broken.

When calcium deficiency occurs during bone growth, the disease rickets occurs; bone weakness leads to deformation, which commonly is seen as ‘bending’ of the long bones of the legs.

Milk, milk products, and dark green leafy vegetables are the best sources of calcium. The RDI is 1,200 – 1,500 mg/day.

Excess calcium (generally more than 12,000 mg/day) can also lead to problems. The most painful one is kidney stones.

Iron – Fe is vital to the formation and function of red blood cells. We can get iron from plants (green leafy vegetables, fruits, legumes, and whole grains and the products made from them. We also get iron from animal tissues, both in the form of heme-iron (hemoglobin, myoglobin) and non-heme-iron. Vitamin C influences absorption of non-heme iron.

Deficiency leads to iron deficiency anemia. Fatigue, paleness, and weakness are its symptoms (also low hematocrit).

deficient normal

Finally, iodine – iodine is critical to the formation of thyroid hormone (thyroxine) and its precursors (tri- and tetraiodothyronine). Today, in areas that were once centers of thyroid insufficiency (e.g. the Great Lakes area), goiters are rare. Our table salt is iodized, and that is the source of enough iodine to prevent goiter. Many of us use sea salt, which is rich in iodine, and/or eat seafood, that is similarly rich in it. In other areas of the world, goiter persists as a problem.

Here, there are a number of reasons, including autoimmune diseases like Hashimoto’s syndrome (thyroiditis), for thyroxine insufficiency. Treatment uses adjusted doses of L-thyroxine to treat the diseases.

Hashimoto’s thyoiditis is an example of a disease that is being explored using molecular tools. It seems that there are particular genes in the MHC (major histocompatibility complex) that are much more commonly found in individuals who have the thyroiditis. There are also associated problems: rheumatoid arthritis (another autoimmune condition), problems in the kidneys, and other problems.

There was a time, in the Renaissance, when a moderate swelling due to goiter was considered a mark of beauty.

Now that you know what we need to have in our diets, what do we eat?

Too many calories. We need ~2000 kcal per day, but consider a typical McDonalds lunch:

A Big Mac - 585 caloriesLarge fries - 554 calories Large vanilla shake - 1108 calories.Total: 2,247 calories.

That's enough for the whole day and it's only lunch.

Obesity is an increasing problem. Data from your text indicates an increase in obesity from 14.5% of the U.S. population in 1971 to 31% in 2000, and now (from more recent news reports) ~40%.

Repeating (and expanding) the risks of obesity, you may be at risk for a wide range of serious diseases and conditions including:

• hypertension or high blood pressure; • coronary heart disease; • Type 2 diabetes; • stroke; • gallbladder disease; • osteoarthritis; • sleep apnea and other breathing problems; • some cancers such as breast, colon and endometrial cancer; and • mental health problems, such as low self-esteem and depression.