carbohydrate

(kăr″bō-hī′drāt)

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[L. carbo, carbon, + Gr. hydor, water]
One of a group of organic chemicals, including sugars, glycogen, starches, dextrins, and celluloses, that contain only carbon, oxygen, and hydrogen. Usually the ratio of hydrogen to oxygen is 2 to 1. Glucose and its polymers (including starch and cellulose) are estimated to be the most abundant organic chemical compounds on earth, surpassing in quantity even the great stores of fuel hydrocarbons beneath the earth's crust. Carbohydrates are one of the six classes of nutrients needed by the body (the others are proteins, fats, minerals, vitamins, and water).
Green plants use the sun's energy to combine carbon dioxide and water to form carbohydrates. Most plant carbohydrates (celluloses) are unavailable for direct metabolism by vertebrates. However, the bacteria present in the intestinal tracts of some vertebrates break down cellulose to molecules that can be absorbed. The human intestinal tract lacks the enzyme that splits cellulose into sugar molecules, but humans do split starch into maltose by means of their salivary and pancreatic amylases.

CLASSIFICATION
Carbohydrates are grouped according to the number of carbon atoms they contain and how many of the basic types are combined into larger molecules. The most common simple sugars, monosaccharides, contain five or six carbon atoms and are called pentoses and hexoses, respectively. Two monosaccharides linked together are called a disaccharide. A series (chain) of monosaccharides or disaccharides is called a polysaccharide. Ribose and deoxyribose are the most important pentoses; glucose, fructose, and galactose are the most important hexoses in human metabolism. The disaccharide sugars in the diet are maltose (2 d-glucose molecules), sucrose or cane sugar (glucose and fructose), and lactose or milk sugar (d-glucose and d-galactose). These sugars are split and eventually converted to glucose by enzyme action. The two important polysaccharides are starch and glycogen; the latter is called animal starch. The basic monosaccharide building block for both of these large polymers is glucose. Dietary starch and glycogen are metabolized first to glucose and then to carbon dioxide and water in humans. SEE TABLE: Classification of Important Carbohydrates

FUNCTION
Carbohydrates are a basic source of energy. They are stored in the body as glycogen in virtually all tissues, but principally in the liver and muscles. Glucose, an important source of reserve energy, can be mobilized from these sites.

DIGESTION AND ABSORPTION
Cooked but not raw starch is broken down to disaccharide by salivary amylase. Both cooked and raw starches are split in the small intestine by pancreatic amylase. Disaccharides cannot be absorbed until they have been split into monosaccharides by the enzymes present in the brush border of cells lining the intestinal tract. Glucose and galactose are the actively absorbed sugars. Fructose is absorbed by diffusion. SEE TABLE: Digestion of Carbohydrates

METABOLISM
Although very complex at the molecular level, carbohydrate metabolism can be explained as follows. Carbohydrates are absorbed as glucose, galactose, or fructose. Fructose and galactose are converted to glucose by the liver and are then available for energy production, or they may be stored after conversion to glycogen. The glycogen is available for metabolism to glucose whenever reserve energy is needed.
SEE: muscle metabolism

SOURCES
Carbohydrates are present in food in digestible and indigestible forms. The digestible type are an important source of energy. Those that cannot be used, usually some form of cellulose, are beneficial in adding bulk to the diet. Whole grains, vegetables, legumes (peas and beans), tubers (potatoes), fruits, honey, and refined sugar are excellent sources of carbohydrate. Calories derived from sugar and candy have been termed “empty” calories because these foods lack essential amino acids, vitamins, and minerals.
SEE: fiber, dietary

NUTRITION
Carbohydrates contain 4.1 kcal/g and are esp. useful as a quick source of energy as they are readily digested.
Dietary sources of complex carbohydrates

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