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Carbohydrates are the principal dietary source of energy. They are comprised of simple sugars and complex carbohydrates. The most common simple sugars, or monosaccharides (i.e., one sugar unit) include glucose, fructose, galactose and mannose. Glucose is the main form in which carbohydrate is absorbed, exchanged between tissues, and channeled into metabolic pathways. One molecule of glucose contains 6 atoms of carbon, 6 atoms of oxygen and 12 atoms of hydrogen. The name carbohydrates (often abbreviated as 'CHO') derives from the fact that these compounds are primarily made up of carbon atoms and water. Disaccharides (i.e., two sugar units) include sucrose (also called saccharose), maltose and lactose

Oligosaccharides, such as raffinose and stachyose in legumes, are composed of 3-10 sugars. Polysaccharides are substances in which multiple, often hundreds of monosaccharides molecules, are linked together. The most abundant edible forms are the starches present in grains, potatoes, etc., which are polymers of glucose. The two major forms of these complex carbohydrates are amylose and amylopectin. In animals, the storage form of carbohydrates is glycogen. Dextrins are fragments of starch produced by dry heat, such as toasting.

Dietary fibers such as cellulose are polysaccharides in which glucose or other monosaccharides are linked by bonds which cannot be hydrolysed by the digestive enzymes produced in the intestine. Dietary fiber, dextrins and some oligosaccharides are thus not digestible, except through the action of enzymes produced by intestinal bacteria. Undigested dietary fiber passes to the large bowel where it is fermented by bacteria to yield lactic acid and the short-chain fatty acids acetic, propionic and butyric acid. Humans derive measurable amounts of energy from the metabolism of these products of bacterial fermentation. This process is very active in herbivores which can thereby derive most of their energy from cellulose. Other carbohydrate-related compounds include polyols (i.e., sugar alcohols such as sorbitol and xylitol) and organic acids, such as ascorbic acid (i.e., vitamin C), citric acid and malic acid. Although monosaccharides yield slightly less energy than starches (3.7 vs. 4.2 kilocalories per g), the energy value of dietary carbohydrates is generally considered to be an even 4 kilocalories per gram.

Deficiency: There is no absolute requirement for dietary carbohydrate, although the brain, the red blood cells and some cells in the kidney use glucose as a preferred source of energy. The liver has a considerable capacity to synthesize glucose from amino acids (from dietary and body protein) and glycerol, released from lipid hydrolysis. When intake of dietary sugars and starch is low or lacking, stored fat (triacyglyceride) is mobilized, providing fatty acids to sustain energy generation in the body. A carbohydrate-free diet is associated with increased protein breakdown and ketone body formation in the liver, which may produce dehydration and loss of cations, though not to the same dangerous extent as in diabetic ketoacidosis. Starvation ketosis is greatly attenuated by daily intakes of as little as 50-100 g of carbohydrate.
 

Diet recommendations: No formal diet recommendation for carbohydrates exists as intakes are adequate in US diets, though consumption of some 25 grams of dietary fiber (i.e. undigestible polysaccharides) is recommended. The Daily Value for carbohydrates used in nutritional labeling is 60% of total calories, or 300 g/day for a 2,000 Calorie diet. In 1985, the US average carbohydrate intake was 177 g in women and 287 g in men. Approximately 23% came from fruits and vegetables and 41% was from grain products. About half of the carbohydrate intake was in the form of mono- and disaccharides and the reminder as complex carbohydrates.
 

Food sources: Monosaccharides such as glucose and fructose are found in fruits and honey. Sucrose (glucose linked to fructose), or table sugar (produced from sugar cane or sugar beets) is the most commonly consumed disaccharide, found also in molasses, maple syrup and some fruits. Maltose (two glucose units) is a component of sprouting grains, malted cereals and syrups. By converting the glucose in corn syrups into fructose, isosyrups with increased sweetening power are produced which are used instead of cane sugar, notably in confectionary items and soft drinks. Lactose (a dissacharide of glucose and galactose) is the sugar in milk. Amylose is the dominant starch in wheat and rice. Amylopectin is found in corn, potatoes and tapioca.

Resistant starches are starches which are poorly digested in the human gastrointestinal tract, either because the starch is in granular form (e.g., uncooked) and therefore resistant to digestive enzymes, or in the form of retrograded amylose, which may form after cooling starch that has been gelatinized by moist heating. Up to 7-10% of starch from wheat, oats and potatoes and 20% from baked beans can pass through to the colon. Resistant starches which reach the colon are fermented there by the microflora and converted into short-chain fatty acids, a process which also yields small amounts of methane gas and hydrogen.
 

Recent research: Conversion of carbohydrate into fat allows animals to build up fat reserves even when their feed contains very little fat. It has therefore long been believed that excess carbohydrate can readily be converted into stored body fat. However, it is now well established that in man, fat synthesis is essentially negligible under conditions of unrestricted access to food. On another front, it has been shown recently that hyperactivity in children is not associated with sugar intake. Of continued major interest are studies on the beneficial effect of consuming more complex carbohydrates which have a relatively low glycemic index (i.e., they cause a less abrupt increase in blood glucose levels after meals) instead of the more rapidly absorbed sugars and starches. This is particularly important for individuals affected by insulin-dependent, as well as non-insulin dependent, diabetes mellitus and because complex carbohydrates tend to exert favorable effects on blood lipid levels in subjects at risk for cardiovascular disease. In addition there is continuing research into the role of non-starch polysaccharides (fiber) in the prevention of colonic cancer.
 

Toxicity: Ingestion of even large amounts of carbohydrates has no adverse effects, except that lactose-containing milk products can lead to diarrhea in lactase-deficient populations. In individuals affected by certain inherited carbohydrate-metabolizing enzyme deficiencies, carbohydrate intake may have to be carefully controlled to avoid possibly severe adverse reactions.
 

For further information:

Levin, R. J. (1998) In: Modern Nutrition in Health and Disease (Shils, M. E., Olson, J. A., Shike, M. & Ross, A. C., eds.), 9th ed., pp. 49-66. Williams & Wilkins, Baltimore, MD.

National Research Council. (1989) Diet and Health. Implications for Reducing Chronic Disease Risk. National Academy Press, Washington, DC