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Causes of obesity

Body weight

Heavier people require more calories to maintain their body weights than lighter ones. For example, a middle-aged male weighting 250 lbs. doing minimal amounts of physical activity may require 2700 calories to maintain his body weight. If this person goes on a 2000 calorie-per-day diet, he will lose weight. Eventually, however, even if he stays with a 2000-calorie daily diet, his weight will stabilize because his metabolic rate will gradually decrease. When this man reaches approximately 200 lbs., he will require perhaps only about 2000 calories per day to maintain his new weight. This is a normal process and takes place in all individuals.

Food preferences

High fat foods are obesity-promoting in animals and humans. In the last 25 years, the ready availability of high fat foods (such as, "fast foods"), combined with the decreased calorie requirements from decreased physical activity, is felt to be the major factor in the sharp rise in the prevalence of obesity. Thus, the current low carb, high fat diet craze, which encourages intake of fatty meats instead of vegetables ("carbs"), will, in the long run, result in an even a sharper rise in obesity and probably heart disease, as well.

Medications

Certain medications prescribed for inflammatory conditions, seizures, and mental illness tend to increase appetite and may also decrease metabolic rate.

Hereditary factors affecting appetite and metabolism

There has been speculation for several years that this genetic tendency for obesity results from a gene that produces an abnormal protein or hormone that interferes with the feeling of satiety ("fullness"). With this abnormal protein, an afflicted individual would have a lower metabolic rate and would not feel satiated ("full") after what normal-weight individuals would consider a large meal.

In the mid-1990's, the hormone leptin was discovered. However, as of May 2004, only a few unfortunate individuals have been shown to have abnormalities related to leptin (these individuals are profoundly obese).

Other possible abnormalities have been evaluated. There may be a differences in the mitochondria in obese individuals. The mitochondria are a part of  cells that are involved in energy (calorie) utilization. The mitochondria are only inherited from the mother. This may explain the high correlation of a child's weight to that of the biological mother's weight.

Several years ago (August 1995), an abnormality in a gene responsible for the Beta-3 adrenergic receptor (a receptor found primarily in the fat tissue in the abdominal area) was associated with obesity. This abnormality results in diminished heat production (which requires energy stored in fat cells) and diminished ability to utilize fat. The net result is that an afflicted individual will have a lower resting metabolic rate, thus making it easier for them to gain weight. This, however, affects only a few individuals, and usually to a mild degree.

A tendency to be obese is actually of survival value. If a person tends to be obese, he or she may have a more efficient metabolism, which means that they can survive on less food. In times of famine, people with this more efficient metabolism can survive on fewer calories. In most countries, adequate food, much of it high fat, is relatively plentiful for the majority of the population, making a more efficient metabolism a disadvantage contributing to the obese state.  Hereditary factors are a major factor contributing to obesity.

Incidentally, inheriting a tendency toward obesity does not mean that you cannot lose weight —it just means that you have to work harder at it.

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