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Appetite and hunger are basic drives that help ensure survival. Whenever your body's nutritional stores are running low, the brain sends out hunger signals, letting you know that it is time to eat — the body's equivalent of refueling. The signs of hunger are easily recognized: most people begin to feel tense or somewhat restless and irritable. As the body's fuel reserves drop, these vague signals give way to so-called hunger pangs — a gnawing feeling in the stomach. Interestingly, these symptoms are experienced even by people who have had their stomachs removed.

Appetite and hunger are controlled by the hypothalamus, the part of the brain that regulates the endocrine system and numerous other body functions. Habit also influences hunger. For example, if you usually eat at a certain time each day, you are likely to begin to feel hungry then.

Hunger also is affected by daily activities, since the body demands more fuel when more calories are being consumed by physical effort. Growth may influence appetite, as in the case of teenage boys who seemingly eat ceaselessly during growth spurts and yet do not become fat.

Many external factors can play a role in whether you feel hungry. For example, when it is cold, you are likely to feel hungrier than usual because your body needs more energy to maintain a normal temperature. Conversely, you may experience a loss of appetite when it's hot. And there are instances in which hunger may be a sign of illness. Hunger that is not satisfied by eating or is accompanied by other symptoms, such as unusual weight loss or gain, warrants medical investigation.

Causes of hunger

Bulimia
Bulimia is an eating disorder characterized by binge eating of huge quantities of food and then inducing vomiting or using laxatives (purging) to avoid weight gain. It most often affects young women in their teens and 20's, though it may go on for many years. The disorder is poorly understood but is thought to be caused by a combination of psychological and perhaps hormonal factors. The

frequency of binge eating may range from once or twice a week to several times a day. Many bulimics say they are "hooked" on certain foods, and this addictive tendency seems to carry over into other areas, including abuse of drugs and alcohol and kleptomania (compulsive stealing). Many bulimic people also suffer from serious depression, and they have a high suicide rate.

Cystic fibrosis
This hereditary disease usually appears in early childhood and is caused by a dysfunction of the exocrine glands, which produce saliva, mucus, and sweat. An abnormally thick mucus clogs the lungs, resulting in a chronic cough, wheezing, and increased susceptibility to respiratory infections. The disease also is characterized by obstructed flow of certain enzymes to the small intestine. As a result, the child is unable to fully digest and metabolize food. The baby may appear normal at birth and even for a few years. But growth and weight gain eventually lag, despite a ravenous appetite, large food intake, and persistent hunger.

Depression
Emotional upsets often result in changes in eating patterns. For example, when a person feels upset or sad, he or she may not feel like eating or may turn to food for comfort. These are normal responses, and they usually disappear when the emotional upset is resolved. Persistent changes in eating patterns, however, are among the many symptoms of clinical depression — one of the most common forms of mental illness.

Diabetes
Diabetes occurs when insufficient amounts of a hormone called insulin are produced by specialized cells in the pancreas or when the body is unable to fully utilize its insulin. There are two types of diabetes: Type I or juvenile-onset, in which the pancreas

stops making insulin, and Type II, or adult-onset, in which the pancreas may produce insulin, but the body is unable to use it. Without insulin, the body cannot properly metabolize blood sugar (glucose), which is its major fuel. In effect, the body is starved for fuel, which produces the excessive hunger characteristic of diabetes.

Medications
Certain medications can produce changes in appetite. Most commonly, medicines cause a loss of appetite, but there are some that have an opposite effect. For example, people taking corticosteroids often find they feel ravenously hungry — one of the reasons why people taking steroids tend to gain weight.

Overactive thyroid
An overactive thyroid gland, resulting in overproduction of thyroid hormone, is referred to as hyperthyroidism. The problem is much more common in females than in males and occurs predominantly in young women. Common symptoms include a loss of weight despite a healthy appetite, increased nervousness and emotional outbursts, excessive sweating and feeling too warm in normal weather, and hyperactive behavior.