Causes:

Obesity occurs when a person consumes more calories than they need for the energy they use. Several different factors may influence weight gain.

About 90% of people who lose weight through dieting gain every pound back regardless of their weight-loss method.

Genetic factors may play some part in 70 - 80% of obesity cases.

The Biological Pathway to Appetite

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 increases or decreases 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.

Specific Genetic Factors

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.

Medical or Physical Causes of Obesity

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.

Effects of Certain Medications

Some prescription medications contribute to weight gain, usually by increasing appetite. Such drugs include:

• Corticosteroids
• Female hormone treatments, including some oral birth control pills (effect is usually temporary), and certain progestins (such as Megestrol) used to treat cancer
• 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

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

Television and Sedentary Habits

Perhaps the primary reason for the dramatic rise in obesity is the sedentary (inactive) lives led by most Americans, including children and young people. Researchers found that labor saving devices had reduced a person's energy use by over 100 calories a day -- adding up to an extra 11 pounds a year. Half the difference in energy use was due to less walking. At the same time, according to the U.S. Centers for Disease Control and Prevention, between 1970 and 2000 the typical American man increased his caloric intake by 168 calories a day (good for 17 pounds a year) while the average woman added 335 calories a day.

Regular television watching has been singled as the most hazardous pastime. According to a major 2003 study, for every 2 hours a person spends in front of the TV each day, the risk for obesity increases by 23% and for type 2 diabetes by 14%. In the study, TV watching produced the lowest metabolic rates compared to sewing, playing board games, reading, writing, and driving a car. Just the act of watching TV encourages unhealthy snacking and eating patterns. In addition, the advertising on the television complicates the problem by promoting fast foods, cereal, and snack products that are high in salt, fats, and carbohydrates. Even worse, much of these advertisements are directed at children -- the most vulnerable group.

Modern Diet and Eating Habits

People are not only eating more food than they did 20 years ago but also replacing home cooking with packaged foods, fast food, and dining out. This behavior, according to studies, places people at higher risk for obesity. Fast foods may be more harmful than restaurant cooking. These foods tend to be served in larger portions. They generally contain more calories and unhealthy fats, and fewer nutritious ingredients, than homemade or restaurant meals. Snack foods and sweet beverages, including juice and soft drinks, are specific problems that add to the increasing rates of obesity.

Frequent small, healthy meals (instead of two or three large daily meals) have been associated with lower weights.

Enough food is produced in the US to supply 3,800 calories every day to each man, woman, and child in the country. This is far more than the average person needs to sustain life. In general, the people who gain weight eat more and their portions are larger than those who do not.

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.