Catabolysis is a biological process in which the body will break down fat and muscle tissue in order to stay alive. Catabolysis only occurs when there is no longer any source of protein, carbohydrate, or vitamin nourishment feeding all body systems and is the most severe type of malnutrition.
Catabolysis is seen in starved and malnourished people. According to the Food and Agriculture Organization in the United Nations, over 20,000 people die from starvation each day. Also, the FAO estimates that over 800,000,000 people are "chronically undernourished" and that a child dies from starvation every five seconds.
Due to the normal metabolic rate of humans (which requires going approximately 12 hours without food), catabolysis only becomes life-threatening after 1–2 months from the cessation of nutrition going into the body. After this time, the damage to muscles and organs can be permanent and can also eventually cause death, if left untreated. Catabolysis is a last resort effort of the body to keep itself—particularly the nervous system—alive.
The situation can become dire when one begins to lose muscle mass; this is a sign that the fat has been expended and the body is now metabolizing the muscle tissue. This results in muscle atrophy, a loss of strength and, ultimately, a depletion of muscular tissue completely. Muscle weakness is not necessarily a symptom of catabolysis: the muscles will normally feel fatigued when they are not receiving enough energy or oxygen.
The body has a natural store of fat (also called adipose) that stores reserve energy, one can still stay alive while their body breaks down the fatty tissue (hence people wasting away from starvation).
The person may, during catabolysis, have large amounts of lipids, proteins and amino acids in their bloodstream, due to the muscle fibers and adipose tissue being broken down and sent to the nervous system and brain. They may also exhibit a fever, since the body is working hard to transfer the nutrients in the muscles and fat to the blood.
While catabolysis can be deadly over time, if the person is given medical treatment early enough, the effects of catabolysis can be reversed. However, the person may require intravenous nutrition, a blood transfusion, and/or oxygen replenishment. After that, it may be a few weeks to a few months before the person's muscle mass and fat deposits can build themselves up again; there is a possibility that they may never build back up, depending on the severity of the condition.