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others have a predisposition to be too thin. Most people who lose weight on diets tend to put it back on. Most people who try to gain weight have difficulty keeping their weight up. These observations led to the set-point theory, that we each have a set point, or a preset natural body weight, determined by the number of fat cells in our body. When we eat less, our weight goes down and our fat cells contract, which seems to trigger processes that result in decreased metabolism and increased hunger. When we eat more, our weight goes up and our fat cells increase in size, which seems to result in increased metabolism and decreased hunger. If we continue to eat more, we can continue to gain weight, and our set point can go up. Some scientists theorize that many chronic dieters are restrained eaters who stringently control their eating impulses and feel guilty when they fail. They become disinhibited and eat excessively if their control is disrupted, which contributes to weight gain.

Eating Disorders

Slim models and actresses in the media are pictured as ideals in America and in some European countries. Some people are highly motivated to achieve this ideal weight, and develop eating disorders. Underweight people who weigh less than 85% of their normal body weight, but are still terrified of being fat, suffer from anorexia nervosa. People with this disorder are usually young women who follow starvation diets and have unrealistic body images. Anorexia is associated with perfectionism, excessive exercising, and an excessive desire for self-control. Bulimia nervosa is a more common eating disorder characterized by eating binges involving the intake of thousands of calories, followed by purging either by vomiting or using laxatives. People with this disorder are also usually young women who think obsessively about food, but who are also terrified of being fat. Results of research suggest that some people suffering this disorder secrete less cholecystokinin than normal, have a low level of serotonin, have been teased for being overweight, participate in activities that require slim bodies, have been sexually abused, or are restrained eaters.

Thirst

Regulation of thirst is similar to regulation of hunger. The lateral hypothalamus seems to be the “on” button for both hunger and thirst. When stimulated, this area of the hypothalamus will start drinking behavior, but if it is lesioned or removed, the individual refuses liquids, even to the point of dehydration. Different neurotransmitters are involved in hunger and thirst. Mouth dryness plays a minor role in stimulating us to drink. More important is the fluid content of cells and the volume of blood. Osmoreceptors are sensitive to dehydration of our cells. When osmoreceptors detect shrinking of our cells, we become thirsty. The hypothalamus stimulates the pituitary to release antidiuretic hormone (ADH), which promotes reabsorption of water in the kidneys, resulting in decreased urination. When we vomit, donate blood, or have diarrhea, the volume of our blood decreases, resulting in decreased blood pressure. This stimulates kidney cells to release an enzyme that causes synthesis of angiotensin, which stimulates thirst receptors in our hypothalamus and septum. Drinking behavior and reabsorption of water in the kidneys result. Not only is thirst affected by internal cues, it is affected by external cues too. We often get thirsty when we see other people drinking in real life or advertisements. These external stimuli can act as an incentive that stimulates drinking behavior, even when we have had enough to drink. What we drink is affected by customs as well as the weather.

Pain Reduction

Whereas hunger and thirst drives promote eating and drinking behavior, pain promotes avoidance or escape behavior to eliminate causes of discomfort. (Additional information about pain is in Chapter 8.)

Sex

Like hunger and thirst, the sex drive involves the hypothalamus, but unlike hunger and thirst, the sex drive can be aroused by almost anything at any time and is not necessary for survival of an