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GeneticsAs with many medical conditions, the caloric imbalance that results in obesity often develops from a combination of genetic and environmental factors. Differences in various genes controlling appetite, metabolism, and adipokine release predispose to obesity, but the condition requires availability of sufficient calories, and possibly other factors, to develop fully; ie: you have too eat too much to become fat.
Various genetic abnormalities that predispose to obesity have been identified (such as Prader-Willi syndrome, Bardet-Biedl syndrome, MOMO syndrome and leptin receptor mutations), but only about 5% of obese individuals have genetic changes that have brought about the obesity.
While it is thought that a large proportion of the causative genes are still to be identified, much obesity is likely the result of interactions between multiple genes, and non-genetic factors are likely also important.
LifestyleThe combination of an excessive nutrient intake and a sedentary lifestyle has been identified as the main cause for the rapid acceleration of obesity in Western society in the second half of the 20th century.
Despite the widespread availability of nutritional information in schools, doctors' offices, on the internet and on groceries, it is evident that overeating remains a substantial problem. For instance, reliance on energy-dense fast-food meals tripled between 1977 and 1995, and calorie intake quadrupled over the same period.
Less well established life style issues include a stressful mentality and insufficient sleep.
Medical illnessCertain physical and mental illnesses and particular pharmaceutical substances may predispose to obesity. Apart from the fact that correcting these situations may improve the obesity, the presence of increased body weight may complicate the management of others.
Medical illnesses that increase obesity risk include several rare congenital syndromes (listed above), hypothyroidism, Cushing's syndrome, growth hormone deficiency. Smoking cessation is a known cause for moderate weight gain, as nicotine suppresses appetite. Certain medications (e.g. steroids, atypical antipsychotics, some fertility medication) may cause weight gain.
Mental illnesses may also increase obesity risk, specifically some eating disorders (bulimia nervosa and binge eating disorder).
Society
Some obesity factors are resistant to the theory that the "epidemic" is a new phenomenon. In particular, a class co-factor consistently appears across many studies. Comparing net worth with BMI scores, a 2004 study found obese American subjects approximately half as wealthy as thin ones. When income differentials were factored out, the inequity persisted—thin subjects were richer than fat ones.
A lower level of education and tendencies to rely on cheaper fast foods is seen as a reason for this difference. Another study finds women who married into higher status are predictably thinner than women who married into lower status.
A 2007 study of more than 32,500 people indicated that people risked being obese if their friends, siblings or spouse were. The cohort was followed for 32 years. Friends (especially same-sex peers and even those many miles away) were the most important factor; this would indicate that social factors are a major determinant of body mass - either through behavioral issues or acceptance of increased body mass.
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