If food intake is greater than the energy demands, the body promotes the storage of excess energy, usually fat. This is a useful adaptation in expectation of times of shortage, during which the stored fuels will be mobilized, resulting in the reduction of fat stores and body weight. A fat content equal to 12 to 18% of body weight in men and 18 to 24% in women is considered normal and possibly necessary for health. For example, a marked reduction in fat content, particularly in women, may be detrimental to a normal menstrual cycle and reproductive functions.
Excessive accumulation of fat (obesity), however, may also be disadvantageous, at least for individuals who are genetically predisposed to diabetes and hypertension. Clearly, obesity is a matter of degree, but when body weight exceeds 30% of the norm (due to excess fat), obesity is present. In adults, fat storage is accompanied by increase in the size of fat cells (i.e, hypertrophic obesity). This condition can be reversed by weight reduction measures. In children, excess food intake, in addition to causing increase in the size of fat cells, may also lead to increase in their number, a condition called hyperplastic-hypertrophic obesity, which is probably not reversible with weight reduction. Moreover, as adults, these children may have a higher propensity toward excessive obesity and its possible pathological consequences.
Fuel Mobilization During Starvation
During prolonged fasting or starvation, all fuel reserves are mobilized to ensure survival of the body, particularly the brain and heart. The few hundred grams of carbohydrates (glucose and glycogen) are quickly used up during the first day, along with some fatty acids and labile amino acids of the liver. During the next days and weeks, all fat deposits and the remaining reserves of the labile proteins are mobilized. Finally, after six weeks, structual proteins of muscle and bone are catabolized to mobilize their amino acids for gluconeogenesis. During starvation, BMR is reduced and gluconeogenesis and ketone body metabolism are promoted. The massive utilization of the body fuel stores affects all body tissues, resulting in weight loss and thinness, but the brain and heart are spared. The maximum time the body can tolerate complete fuel-food starvation (but with water, minerals and vitamins provided) is probably two months.
Regulation of Food Intake
Two centers in the brain hypothalamus regulate food intake, thus indirectly controlling weight gain and loss. Increased activity in the feeding center promotes appetite, feeding behavior, and food intake. The feeding center is inhibited by another hypothalamic center the satiety center. The satiety center neurons are sensitive to blood glucose levels: high levels increase and low levels decrese their activity. Consequently, low blood sugar reduces the inhibition of the feeding center by the satiety center, resulting in the sensation of hunger and activation of feeding. Stimulation of sensors in the mouth and distention of the stomach during ingestion reduce the activity in the feeding centers. After food is absorbed, increased blood sugar activates the satiety center, which in turn inhibits the feeding center, resulting in cessation of appetite and eating behavior. Certain types of obesity as well as the loss of appetite and extreme thinness seen in anorexia nervosa may be of nervous and emotional origin, involving disturbance of these brain regulators of food intake.
Daryl Conant, M.Ed.