The functions of carbohydrates can be described as follows:
(i) As a source of Energy
Each gramme of carbohydrate when oxidized yields approximately 4 cal. of energy. Some carbohydrates in the form of glucose are used for immediate tissue energy needs. Small amount are as glycogen in the liver and muscles and some are stored as adipose tissue for later energy needs. Glucose is the sole form of energy for the brain and nervous tissue.
(ii) Protein sparing Action
Carbohydrate is used as a source of energy thus sparing protein for tissue building. Any deficiency in calorie requirement in the diet is made up by using adipose tissue and protein tissue. The first physiological demand of the body is for energy which must be satisfied before the nutrients are used for other functions. Thus this function of carbohydrate to spare protein for its primary purpose of body building and repair of tissues is an important one.
(iii) Regulation of fat Metabolism
For the normal oxidation of facts some carbohydrate is necessary. When carbohydrate is severely restricted in diet, fats will be metabolized faster than the body can take care of the immediate products. The complete oxidation of fact to the metabolic end-product of carbon dioxide and water requires an organic acid called pyruvic acid which is formed during the oxidation of carbohydrates. In the absence of adequate carbohydrate large amount of ketone bodies are produced. The accumulation of ketone bodies increases the acidity (reduces the alkalinity) of blood. This condition, called ketosis may result in a coma if the alkalinity of blood is reduced considerably, ketosis may occur in diabetes where the cells cannot utilize carbohydrates. It may also occur in starvation where the cell must use the fat sources of the body for energy.
(iv) Role in gastro-intestinal function
The carbohydrate plays an important role in the gastro-intestinal function of mammals. They serve as a source of energy for the micro-oraganisms that synthesise some B-Complex vitamins in the intestinal tract and the cellulose provides fiber and bulk that promote healthy intestinal hygiene. Carbohydrates foods also add variety, flavor and bulk to the diet.