Atoms of carbon, hydrogen and oxygen combine to form carbohydrates. There are three kinds of carbohydrate:

Monosaccharides - e.g. glucose and fructose

Disaccharides - e.g. sucrose, lactose and maltose

Polysaccharides - e.g. starch, fibre, and the glucose polymer glycogen.

The main function of carbohydrate is to serve as an energy fuel, particularly during exercise. The energy derived from the breakdown of blood-borne glucose and liver and muscle glycogen (Glycogenolysis - Process of reconverting glycogen to glucose) is ultimately used to power the contractile elements of muscle as well as other forms of biological work. Carbohydrates are stored in a limited quantity in the liver and muscle.

Muscle glycogen and blood glucose are the primary fuels during intense exercise . The body's glycogen stores have an important role in sustaining high levels of aerobic exercise and sparing breakdown of protein. A carbohydrate-deficient diet depletes muscle and liver glycogen and can profoundly affect both high intensity anaerobic and long duration aerobic exercise.