handouts - carbohydrates

8/3/2019 Handouts - Carbohydrates

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THE CARBOHYDRATES

DEFINITION AND COMPOSITION

Carbohydrates are organic compounds that contain the elements carbon,hydrogen, and oxygen arranged as monosaccharide or multiples ofmonosaccharide. Most, but not all carbohydrates have a ratio of onecarbon molecule to one water molecule, thus the term carbohydrate.

Carbohydrates are polyhydric aldehydes and ketones In modem biochemistry, with the accumulation of much more current data,

the definition of carbohydrates has been modified and broadened toencompass many other compounds with little or no resemblance to theoriginal '"water of carbon".

SYNTHESIS

Photosynthesis is the process by which plants form carbohydrates fromcarbon dioxide and water in the presence of sunlight and plant enzymesacting upon chlorophyll in green leaves. Oxygen is produced and releasedin the process.

The plant uses the glucose produced for its immediate growth and repairneeds any extra is converted to starch and stored in the leaves, stems,roots, seeds, pods, and fruits.

CLASSIFICATION

Dietary carbohydrates are composed of the simple carbohydrates (thesugars, i.e., monosaccharides and disaccharides) and the complexcarbohydrates (the polysaccharides, i.e., glycogen, starches, and fibers).

The body converts these types of carbohydrates to its own energycurrency blood sugar also called glucose.

MONOSACCHARIDES:The Single Sugars

Monosaccharides are structurally the simplest form of carbohydrates, thusthey cannot be hydrolyzed to a simpler form.

Monosaccharides have only one sugar unit and may contain 3 to 7 carbonatoms: triose (3-C chain), tetrose (4-C chain), pentose (5--C chain),hexose (6-C chain), and heptose (7-C chain).

The sugars most important in nutrition are the 6-C monosaccharidesknown as hexoses: glucose, galactose, and fructose.


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Glucose

Also known as physiologic sugar, blood sugar, dextrose, corn sugar, andgrape sugar.

Principal product formed by the hydrolysis of more complex carbohydrates

during digestion. Primary energy source for all the cells especially the central nervous

system.

Galactose

Not found in a free state in nature. Formed during the hydrolysis of lactose. Converted to glucose in the liver. Constituent glycolipids and glycoproteins.

Fructose

Also known as levulose and fruit sugar. Formed during the hydrolysis of sucrose. Converted to glucose in the liver. Sweetest of the sugars. Used commercially in sweeteners such as high-fructose corn syrup.

DISACCHARIDES:The Double Sugars

The disaccharides are split to monosaccharide units by acid hydrolysis orby digestive enzymes. Each of the three common disaccharides consistsof glucose; the other monosaccharide is either fructose, galactose, oranother glucose:

sucrose = glucose + fructoselactose = glucose + galactosemaltose = 2 glucose

Sucrose

Also known as table sugar, sugar, cane sugar, beet sugar, andsaccharose.

Most widely distributed of the disaccharides and is the most commonnatural sweetener.

When hydrolyzed by digestive enzymes or acid, it is converted to invertsugar, a mixture of equal parts of glucose and fructose.


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Lactose

Also known as milk sugar. Only carbohydrate of animal origin that is of significance in the human

diet.

Comes from milk and milk products. Least sweet of the sugars; about 1/6 as sweet as sucrose.

Maltose

Also known as malt sugar and grain sugar. Does not ordinarily occur free in nature. Occurs in plants when seeds germinate. By-product of starch hydrolysis in humans. Used in fermentation to produce malted beverages such as beer and

whiskey.

With dextrins, it forms dextrimaltose and serves as the source ofcarbohydrate for some infant formulas.

POLYSACCHARIDES:The Complex Carbohydrates

Polysaccharides are complex carbohydrates made up of long chains ofmonosaccharide units that may number in the hundreds or eventhousands.

They are less soluble but more stable than the simple sugars and aredigested with varying degrees of completeness.

Three polysaccharides are important in nutrition: glycogen, starches, andfibers.

Glycogen

Also called "animal starch" Major form of stored carbohydrate in human and animal tissues which is

readily converted to glucose as needed. Rapidly synthesized from glucose in the liver and muscles where it is

stored. Found to a limited extent in meats and not at all in plants, therefore is not

a significant food source of carbohydrate and is not counted as one of thecomplex carbohydrates in foods.

Starches

Most significant source of carbohydrate in the diet; grains are the richestfood source of starch. Storage form of carbohydrate in plants.


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Partial hydrolysis yields dextrins and maltose; complete hydrolysis yieldsglucose units.

Occurs in both the amylose form (long straight chains of glucose units)and amylopectin form (long branched chains of glucose units),

Resistant Starches

Three main types of resistant starch occur naturally in the human diet:

1. Physically trapped starch, found in coarsely ground or chewed cereals2. Ungelatinized starch granules, which are resistant to digestion, such as

green banana and high-amylose starch3. starch polymers, mainly amylose, which are produced when starch is

cooled (as in cooled cooked potato) after gelatinization

Fibers

Fibers are the structural parts of plants and thus are found in all plant-derived foods such as vegetables, fruits, grains, and legumes.

It is often described as non-starch polysaccharides which includecellulose, hemicellulose, pectins, gums, and mucilages.

Fibers also include some nonpolysaccharides such as lignins, cutins, andtannins.

Fibers cannot be broken down by human digestive enzymes. Thus, fibersdo not contribute monosaccharides to the body.

The bacteria of the GI tract can break some fibers down, however, andthis is important to digestion and to health.

The various components of fiber:

Cellulose Indigestible polysaccharide composed of long, straight chains of

glucose molecules in beta-linkage Primary constituent of plant cell walls Important as a bulking agent and source of energy for intestinal

bacteria

Hemicelluloses Indigestible polysaccharide which can be hydrolyzed by dilute acids Fewer glucose molecules in beta-linkage Digested by bacteria in the human GI tract. Some are soluble, while others are not Main constituent of cereal fibers


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Pectin Indigestible polysaccharide which forms bulk by absorbing large

amounts of liquids and swells into a colloidal mass In the food industry, it is used to thicken jelly, keep salad dressings

from separating and control texture and consistency because they

readily form gels in water Commonly found in vegetables and fruits, especially citrus fruits

and apples

Gums and Mucilages Noncellulose polysaccharide made up of glucose units combined

with other polysaccharides Found in plant secretions and seeds Gums (e.g., gum Arabic) and mucilages (e.g., guar and

carrageenan) are used as additives and stabilizers by the foodindustry

FUNCTIONS of CARBOHYDRATE

1. Source of energy. Carbohydrates are the cheapest and most availablesource of energy. Each gram provides 4 calories.

2. Protein-sparing action. Carbohydrates prevent the use of protein forenergy, thus sparing it for building and repair of body tissues.

3. Prevent ketosis. Fats "burn in the fire of carbohydrates", i.e., adequatecarbohydrates prevent the rapid oxidation of fats which results in theaccumulation of ketone bodies (ketosis). This causes acidosis,sodium imbalance, and dehydration.

4. Role in Gastrointestinal tract (GI tract) function:- Lactose promotes the growth of desirable bacteria, some of which

are useful in the synthesis of B-complex vitamins- Lactose enhances the absorption and utilization of calcium.- Fiber maintains gut integrity and ensures normal elimination

of waste.5. Carbohydrates are constituents of body compounds that regulate

metabolism:- Glucuronic acid in the liver acts as a detoxifying agent- Hyaluronic acid forms the matrix of connective tissues- Heparin prevents the clotting of blood.

DIGESTION ofCarbohydrates

In the Mouth:

Chewing breaks down food into fine particles and mixes it with saliva. The enzyme salivary amylase (ptyalin) starts the digestive action on starch


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hydrolyzing it to dextrin and maltose. The food mass is conveyed to thestomach by peristalsis.

In the Stomach:

The successive wavelike contractions of the muscle fibers of the stomachwall (peristalsis) further mixes food particles with gastric secretions tofacilitate chemical digestion.

The action of ptyalin continues until the gastric acid (HCl) penetrates thefood mass, now a thick, creamy chyme and lowers the pH sufficiently toinactivate the enzyme.

The chyme is emptied into the duodenum, the first portion of the smallintestine.

In the Small Intestine:

Chemical digestion of carbohydrate is completed in the small intestine byenzymes from 2 sources:

The pancreatic juice, which contains pancreatic amylase,(amylopsin) an enzyme that continues the breakdown of starch todextrins and maltose.

The intestinal juice, which contains 3 disaccharidases (sucrose,lactase, and maltase). These enzymes act on their respectivedisaccharides (sucrose, lactose, and maltose) to produce themonosaccharides glucose, galactose, and fructose. Thesemonosaccharides are ready for absorption.

Summary of carbohydrate digestion.

Organ Enzyme Digestive Action

mouth salivary amylase Starch dextrins maltose

stomach ---- Above action continues to aminor degree

smallintestine

pancreaticamylasesucraselactasemaltose

starch dextrins maltosesucrose glucose + fructoselactose glucose +galactosemaltose 2 glucose


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ABSORPTION and TRANSPORTATION

Glucose and galactose are absorbed across the intestinal wall by activetransport presumably by a carrier that is sodium-dependent; can also beabsorbed by passive diffusion when their intraluminal concentrations arehigh.

Fructose is absorbed by facilitated diffusion which uses a carrier but is notenergy dependent.

Following transport across the intestinal wall, the monosaccharides passthrough the walls of the blood capillaries for transport to the portal

circulation. From there they are delivered to different tissues, primarily theliver, muscle and adipose tissue. Glucose enters these cells by facilitated diffusion. In skeletal muscles and

adipose tissues, the process is insulin dependent, while in the liver, it isinsulin independent.

Galactose and fructose are readily taken up by liver cells via specifichepatocyte receptors and are subsequently metabolized. Both can beconverted to glucose and stored as liver glycogen or catabolized forenergy according to the body's energy demand.

METABOLISM and STORAGE

Galactose and fructose from the diet are rapidly converted to glucose inthe liver. Thus, carbohydrate metabolism is essentially glucosemetabolism.

There are 2 distinct phases in the metabolism of glucose:

1. anaerobic phase (glycolysis) which produces 5% of the total energy in thebody

2. aerobic phase (Krebs Cycle) which produces 95% of the total energy inthe body

Excess glucose not immediately used for energy is converted to glycogenwhich is stored in the liver and muscles.

The glucose that remains after energy needs and glycogen stores hasbeen attended to is converted to fat and stored in adipose tissues.

FOOD SOURCES


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Carbohydrates are widely distributed in nature, particularly in thevegetable kingdom* Fruits and vegetables contain varying amounts ofsugars and polysaccharides, while cereals, roots, and tubers containmainly starch.

Milk, which contains lactose, is the only animal food that contributes to the

daily carbohydrate intake.

RECOMMENDED ENERGY and NUTRIENT INTAKE (RENI)

There is no RENI for carbohydrate; a wide variation in carbohydrate intakeis compatible with health because of the interrelationships with fatty acidsand amino acids in meeting the energy needs of the body.

A daily intake of 50-100 grams of available carbohydrates is adequate to

prevent ketosis, excessive breakdown of tissue protein, loss of sodiumand other cations, and involuntary dehydration. A daily intake higher thanthis minimum range is desirable for a diet to be acceptable.

It is best to eat complex carbohydrates in an unrefined or minimally refinedstate to avail of their natural fiber and nutrients.

GLYCEMIC EFFECTS OF FOODS

The glycemic effect of a food is its effect on person's blood glucose leveland insulin response. It refers to how fast and how high the blood glucoserises, and how quickly the body responds by bringing it back to normal.

A low glycemic effect is desirable. It is characterized by slow absorption ofcarbohydrates, a modest rise in blood glucose, and a smooth return tonormal blood glucose level.

A high glycemic effect is not desirable. It is characterized by fastabsorption of carbohydrates, a surge in blood glucose, and anoverreaction that plunges blood glucose below normal. A high glycemicdiet seems to promote overeating in overweight people.

Absorption and glycemic effect of carbohydrate foods can be affected bymany factors:

1. Gastric emptying and small intestinal absorption.2. Food Factors

ROLE OF FIBER IN VARIOUS DISEASE STATES

Constipation and diarrhea Diverticulosis Colon cancer Hemorrhoids


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Appendicitis Diabetes Weight Control Heart Disease

ISSUES RELATED TO CARBOHYDRATE

Obesity Weight Control Dental Caries Diabetes Sugar and nutrient intake Lipid profile Heart Disease Cancer Mineral Deficiencies

Honey. Misbehavior and Hyperactivity Glycemic effect / glycemic index

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