Catabolysis is a biological process in which the body breaks down fat and muscle tissue in order to stay alive.

Catabolysis occurs only when there is no longer any source of protein, carbohydrate, or vitamin nourishment feeding all body systems; it is the most severe type of malnutrition.

Due to the normal metabolic rate of humans catabolysis becomes life-threatening only after 1–2 months from the cessation of nutrition going into the body.

The person may, during catabolysis, have large amounts of lipids, proteins, and amino acids in the bloodstream, due to the muscle fibers and adipose tissues being broken down and sent to the nervous system and brain.

One may also exhibit a fever, since the body is working hard to transfer the nutrients in the muscles and fat to the blood.