carbohydrate

carbohydrate Carbohydrates are the major source of metabolic energy, the sugars and starches. Chemically they are composed of carbon, hydrogen, and oxygen in the ratio C_n : H_2n : O_n. The basic carbohydrates are the monosaccharide sugars, of which glucose, fructose, and galactose are the most important nutritionally.

Disaccharides are composed of two monosaccharides: nutritionally the important disaccharides are sucrose (a dimer of glucose + fructose), lactose (a dimer of glucose + galactose) and maltose (a dimer of two glucose units).

A number of oligosaccharides occur in foods, consisting of 3–5 monosaccharide units; in general these are not digested, and should be considered among the unavailable carbohydrates.

Larger polymers of carbohydrates are known as polysaccharides or complex carbohydrates. Nutritionally two classes of polysaccharides can be distinguished: (1)starches, which are polymers of glucose units, either as a straight chain (amylose) or with a branched structure (amylopectin) and are digested; and(2)a variety of other polysaccharides which are collectively known as non‐starch polysaccharides (NSP), and are not digested by human digestive enzymes.

The reserve of carbohydrate in the liver and muscles is glycogen, a glucose polymer with a similar structure to the amylopectin form of starch, but more branched.

Carbohydrates form the major part of the diet, providing between 50 and 70% of the energy intake, largely from starch and sucrose. The metabolic energy yield of carbohydrates is 4 kcal (17 kJ)/g. More precisely, monosaccharides yield 3.74 kcal (15.7 kJ), disaccharides 3.95 kcal (16.6 kJ), and starch 4.18 kcal (17.6 kJ)/g. Glycerol is a 3‐carbon sugar alcohol, and classified as a carbohydrate; it yields 4.32 kcal (18.1 kJ)/g.

See also sugar; sugar alcohols.