Whether starvation occurs on a long-term or short-term basis, metabolic changes occur in your body. The human body has many compensatory mechanisms that help it adapt to the withdrawal of glucose, protein, carbohydrates and fats from foods. For this reason, someone can live many days in the face of starvation. Eventually, however, even the adaptation techniques fail, and the body does not recover.
Glucose Under Normal Conditions
After you eat a meal, glucose and amino acids transfer from your intestines to your blood. In response to glucose entering the blood, the liver releases insulin. The insulin matches with glucose molecules and escorts them into the cells of various organs, including the brain. Insulin signals the synthesis of protein and storage of fuels. Through a cascade of reactions, insulin stimulates the liver and the muscles to make glycogen -- glucose that the liver recovers from the blood and stores.
Short-Term Starvation
For one reason or another, you have probably experienced temporary starvation. Maybe you fasted or simply missed the opportunity to eat. Your body has a scenario in place that adapts to the lack of glucose and protein. It starts when the liver fails to release insulin in response to not having glucose. Suddenly, the gears shift. Insulin is withheld and in its place the liver secretes glycogen, which it was saving for just such a situation. With the help of enzymes, glucagon enables glycogen to become a suitable replacement for glucose. Your body manages to keep your blood-glucose level over 80, yet you have no knowledge of its heroic efforts.
Long-Term Starvation
While your body can take these compensatory actions that keep you out of danger for short periods, it cannot bail you out forever. An average healthy person has enough stored fuels to meet his caloric intake for weeks or even months. However, once they get depleted, other drastic changes occur. Your body's priority is to supply your brain with glucose, which it requires for its energy. When your body can no longer supply the brain with glucose, it breaks down protein and uses the amino acids in its place. Your body has no protein stores, so it takes the protein it needs from muscle. Once again, the body adapts, but eventually it uses all the protein and leaves the muscles with no nutrition of their own. After several weeks of starvation, the body shuts down completely.
Compensatory Mechanisms
The human body, designed with a vast array of control mechanisms, has backup systems in case of failures. Organs pick up the slack when one of them can't function. Substances like amino acids substitute for glucose when necessary. This marvelous, intricate entity -- the body -- seeks to keep its host alive by any means necessary. Yet sometimes despite all of its efforts, the body fails.
References
- National Institutes of Health: Berg, et al: Food Intake and Starvation Induce Metabolic Changes
- "Textbook of Medical Physiology;" Guyton and Hall: Chap. 68; 1996
