M.D. Since anorexia nervosa is a condition of self-starvation, its effects can only be understood by knowing the physical and psychological consequences of starvation. The human body has an awe-inspiring ability to conserve itself, an evolutionary gift of survival from prehistoric days when food supplies were erratic at best, when large supplies of food were available only periodically and long periods of fasting occurred in between. In those days the ability to store calories was basic to survival, since the body called upon its stored calorie supply during these periods of fasting. The human body has retained this mechanism to the present day.
Food calories come from carbohydrate, protein, and fat.
Glycogen is used to help maintain a steady level of blood sugar. Protein calories are used to replace the protein broken down in the tissues since the last meal. Finally, surplus calories from any source are stored as fat. When starvation begins, the body undergoes hormonal and metabolic changes that lead to selective mobilization of these stored calories for energy. Muscle and liver stores of glycogen contain less than 2,000 calories and supply energy for a very short time.