Appetite is determined by processes that occur both in the brain and gastrointestinal tract. Eating patterns are controlled by areas in the hypothalamus and pituitary glands (in the brain).

The body produces a number of molecules that increase or decrease appetite, including leptin. Leptin is a hormone that fat cells release. Some scientists think this hormone may also be released by cells in the stomach. Leptin appears to play an important role in insulin resistance and fat storage in the body, but its role in obesity is unclear. The most likely scenario is that leptin levels rise as the cells store more fat. This increase in leptin levels decreases appetite. Falling levels of leptin make you feel hungry. In people who have genetically lower levels of leptin, however, the brain may be tricked into thinking that it is always starving because there is no leptin to decrease appetite. This can lead to weight gain.

Genetics may directly contribute to severe obesity in people with family histories of the problem. Genetic factors such as slow metabolisms may also make people more likely to be overweight, and there have been some genetic mutations identified in rare causes of severe obesity. Large epidemiological studies have not been able to identify specific location on chromosomes related to the regulation of BMI or the occurrence of obesity. However, recent studies of thousands of preteen twin pairs found that genetic factors have a considerable influence on BMI and obesity. Nevertheless, a study of over 4,300 twins found that physical activity can override the genetic predisposition for high body mass index and wide waist circumference.

Environmental factors were less influential in older children, but interacted significantly with genetic factors in younger children. Genetics also determines the number of fat cells a person has. Some people are simply born with more. It should be noted that even when genetic factors are present, a person can still control their diet.

A number of medical conditions may contribute to being overweight, but rarely are they a primary cause of obesity.

Hypothyroidism is sometimes associated with weight gain. But, patients with an underactive thyroid generally show only a moderate weight increase of five to 10 pounds.
Very rare genetic disorders, including Froehlich syndrome in boys, Laurence-Moon- Biedl syndrome, and Prader-Willi syndrome, cause obesity.
Abnormalities or injury to the hypothalamus gland can cause obesity.
Cushing's disease is a rare condition caused by high levels of steroid hormones. It results in obesity, a moon-shaped face, and muscle wasting.
Obesity is also linked to polycystic ovarian syndrome, a hormonal disorder in women

Some prescription medications contribute to weight gain, usually by increasing appetite. Such drugs include: corticosteroids, antidepressants and anti-psychotic drugs, including lithium and valproate, insulin and insulin-stimulating drugs used to treat diabetes, a particularly unfortunate conflict of interest for obese individuals with type 2 diabetes, female hormone treatments, including some oral birth control pills (effect is usually temporary), and certain progestins (such as Megestrol) used to treat cancer

Do NOT stop taking any medications without talking to your health care provider first.

Obesity is dramatically increasing in not only American children and adults but also every country that has adopted similar cultural habits. The World Health Organization now considers obesity to be a global epidemic and a public health problem as more nations become "Westernized." In spite of the proven health risks of obesity, the government, insurance companies, and the medical profession do not spend nearly enough money to balance the commercial and cultural pressures that are producing millions of overweight people.