The main constituents of food are carbohydrates, lipids, and proteins but they are found in complex forms that cannot be directly utilized by the body. Hence, they are broken down into simpler units by the process of digestion. The simpler forms canreadily be absorbed by the intestines. Subsequently, they are utilized by the body

(a) to provide energy needed for its constant activities and also (b) for the production of various components needed by the body.

The excess carbohydrate is stored in the form of glycogen in liver and muscle. Excess lipid is stored in the form of triglycerides in adipose tissue. There is no specific storage form of excess protein. These reserved stored forms are utilized for the production of energy whenever there is short supply of food from outside. These functions are carried out by a process called metabolism. Digestion, absorption and metabolism are carried out by the biocatalyst called enzymes.