Carbohydrates

Carbohydrates - A group of compounds composed of carbon, hydrogen, and oxygen, which supply the body with its main source of energy. Carbohydrates are found in fruits, cereals, and root crops and fall into 2 groups.

These are available carbohydrates, which are metabolized into glucose for the body’s use, and unavailable carbohydrates, such as cellulose, which cannot be broken down by digestive enzymes and make up the bulk of dietary fibre (see fibre, dietary).

Available carbohydrates are predominantly starches (complex carbohydrates) and sugars (simple carbohydrates). In carbohydrate metabolism, the monosaccharides (simple sugars) glucose (grape sugar), galactose (a milk sugar), and fructose (fruit sugar) are absorbed into the bloodstream unchanged. The disaccharides (double sugars) sucrose, maltose and lactose (a milk sugar) are broken down into simple sugars before they are absorbed. Starches also have to be broken down into simple sugars.

Some glucose is burned up immediately (see metabolism) in order to generate energy for cells, such as brain cells, that need a constant supply. Galactose and fructose have to be converted to glucose in the liver before they can be used by body cells. Surplus glucose is conveyed to the liver, muscles, and fat cells where it is converted into glycogen and fat for storage. When blood glucose levels are high, glucose storage is stimulated by insulin, a hormone that is secreted by the pancreas.

When the blood glucose level becomes low, insulin secretion diminishes and glucagon, which is another hormone produced by the pancreas, stimulates the conversion of stored glycogen to glucose for release into the bloodstream. Although fat cannot be converted to glucose, it can be burned as a fuel in order to conserve glucose. In the disorder diabetes mellitus, carbohydrate metabolism is disturbed by a deficiency of insulin.