Our main energy store is fat, an energy intake greater than energy expenditure unavoidably causes an increase of adipose tissue, which is always accompanied by an increase of lean body mass, and thus an increase of body weight.
Since body energy expenditure is proportional to body weight, an increase of the latter also causes an increase of energy expenditure. The body weight will then stabilize when the energy expenditure of the weight gain will equal the increase of energy intake, the balance condition being re-established. Exactly the opposite happens if the energy intake is permanently reduced.

Level of physical activity : There are patient who reports a history of progressive lifelong weight gain with no progressive lifelong increase in food intake. Moreover, it is quite possible that the energy intake has actually remained the same, the progressive weight gain being caused by decreased energy expenditure due either to the progressive reduction of physical activity that generally accompanies weight gain 

Aging :The progressive reduction of lean body mass which inevitably occurs with aging causes increase in weight due to fall in the body's capacity to burn fat. The physiological loss of lean body mass that occurs with aging, if the energy intake remains the same, compulsorily causes the gain of a body weight which, as we know, is composed of lean and fat, and thus consumes less and must be gained in a greater amount to produce the same consumption of what was lost.

Body Composition: Individuals with equal body weight may have different energy intakes, and individuals with the same energy intake may have different body weights, and a very heavy individual may have an energy intake much smaller than that of another much lighter individual, and vice versa. In summary, certainly the energy intake determines body weight and its variations, but the size of body weight and of its variations is determined by body composition and by its variations. 

Genetic factors: Obesity tends to run in families, suggesting a genetic cause. The mechanism through which genetics might effect weight gain include reduced dietary-induced thermogenesis, lower Resting Metabolic Rate, decreased thyroid functioning, decreased lipoprotein lipase activity, lower Basal body temperatures and decreased amount of more metabolically active brown fat.  Families also share diet and lifestyle habits that may contribute to obesity. Separating these from genetic factors is often difficult. Even so, science shows that heredity is linked to obesity.

Socio-economic : In affluent countries obesity is more common in the lower socio-economic groups. In developing countries it occurs in the prosperous elite. Society too carries its pressure on individuals to either be too thin (model-like) or fat as a sign of prosperity. 

Lifestyle behaviors - such as what a woman eats. Americans tend to eat high-fat foods, and put taste and convenience ahead of nutrition. Although you cannot change your genetic makeup, you can change your eating habits.

Psychological factors may also influence eating habits. Many people eat in response to negative emotions such as boredom, sadness, or anger. Most overweight people have no more psychological problems than people of average weight. Still, up to 10 percent of people who are mildly obese and try to lose weight on their own or through commercial weight loss programs have binge eating disorder. This disorder is even more common in people who are severely obese.

During a binge eating episode, people eat large amounts of food and feel that they cannot control how much they are eating. Those with the most severe binge eating problems are also likely to have symptoms of depression and low self-esteem. These people may have more difficulty losing weight and keeping it off than people without binge eating problems. If you are upset by binge eating behavior and think you might have binge eating disorder, seek help from a health professional such as a psychiatrist, psychologist, or clinical social worker.

Endocrine factors : An endocrine influence on body fat is seen both in normal physiological situations and in pathological state. The normal fat content of young adult women is about twice that of young men and pregnancy is characterized by an increase in body fat. Obesity in women begins at puberty, during pregnancy or at menopause. Obesity frequently but not invariably, accompanies hypothyroidism, hypogonadism, hypopituitarian and Cushing's Syndrome. 

Other causes of obesity

Some illnesses can lead to obesity or a tendency to gain weight. These include depression, and certain neurological problems that can lead to overeating. Also, drugs such as steroids, oral contraceptives, phenothiazines, insulin and some antidepressants may cause appetite stimulation & consequently weight gain. A doctor can tell whether there are underlying medical conditions that are causing weight gain or making weight loss difficult

Understanding Causes of Obesity

Waist-to-Hip ratio

Clinical Features

Management Strategy

Low Energy

Diet

FDA-Approved Medications

for Treating Obesity.

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