Appetite—the desire to eat food—serves to balance energy intake with metabolic needs. It is regulated by a complex process that involves the digestive tract, hormones, and the brain. While the hypothalamus region of the brain is most closely linked with regulating appetite, certain brain chemicals are also being investigated for their effects.

Normal brain activity relies on electrochemical communication between networks of neurons and other cells in the body. The primary messengers are known as neurotransmitters. One type of neurotransmitter, serotonin, is what brain specialists call a modulator. It acts somewhat like volume control for the brain and nervous system. Serotonin is involved with a wide range of functions, such as regulating mood and sleep cycles. Recent research suggests that serotonin levels also influence appetite.

People with anorexia and those recovering from it have elevated levels of serotonin, which acts to suppress appetite. A negative feedback loop involving what the brain perceives is happening to serotonin levels and what is actually happening has the effect of prolonging the disorder. And while the higher levels of serotonin might actually result from the malnutrition associated with anorexia, some researchers suggest that disruption of serotonin levels may lead to increased anxiety, which is what initially triggers the anorexia.

People who suffer from eating disorders, which range from compulsive overeating to starvation diets, can experience serious physical health complications, including heart conditions and kidney failure. Those who overeat compulsively tend to consume too much dietary fat. Too much dietary fat can, of course, be detrimental to health, leading to obesity and cholesterol buildup in the arteries. In contrast, people who suffer from anorexia typically eliminate fat from their diet. A nutritional plan that deprives one's body of fat is also unhealthy.

Fat's main purpose is to store energy. The breaking of bonds within fat molecules releases energy that the body uses. But fats also play a critical role in many other important body functions, including digestion, insulation, and central nervous system function. If the body is deprived of fats, all of these functions can be impaired.

In addition, fat is also needed to produce hormones, including estrogen. Estrogen plays a critical role in menstruation; it also regulates skeletal growth and bone density. So for women, a starvation diet with no fats can lead to abnormal menstrual cycles and weakened bones. This can be significant, as new bone does not typically grow after one's early 20s. Rather, the body can only replace what is lost to resorption, or the natural process of bone removal. This replacement, called remodeling, is limited to repairs of small cracks and other damage to the skeleton that can result from repeated stresses.

Explore in this NOVA classroom activity how healthy men and women are portrayed in the media.

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