What causes Obesity?

When food energy intake exceeds energy expenditure, fat cells (and to a lesser extent muscle and liver cells) throughout the body take in the energy and store it as fat. In its simplest conception, therefore, obesity is only made possible when the lifetime energy intake exceeds lifetime energy expenditure by more than it does for individuals of "normal weight."

In all individuals, the excess energy utilized to generate fat reserves is minute relative to the total number of calories consumed. This means that very fine perturbations in the energy balance can lead to large fluctuations in weight over time. To illustrate, an obese 40 year old who carries 100 lb of adipose tissue has only consumed about 25 more calories per day than he has burned on average - or the equivalent of an apple every three days. In comparison a very lean 40-year-old who carries only 15 lb of body fat will have exceeded his daily energy expenditure by about four calories a day - the equivalent of an apple every 18 days.

Factors that have been suggested to contribute to the development of obesity include:

* Sedentary lifestyle
* Genetic factors
* A high glycemic diet (i.e. a diet that consists of meals that give high postprandial blood sugar)
* Weight cycling, caused by repeated attempts to lose weight by dieting
* Underlying illness (e.g. hypothyroidism)
* Genetic disorders (e.g. Prader-Willi syndrome)
* Eating disorders (such as binge eating disorder)
* Stressful mentality
* Insufficient sleep
* Psychotropic medications
* Smoking cessation

As with many medical conditions, the caloric imbalance that results in obesity often develops from a combination of genetic and environmental factors. Polymorphisms 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. Various genetic abnormalities that predispose to obesity have been identified (such as Prader-Willi syndrome and leptin receptor mutations), but known single-locus mutations have been found in only about 5% of obese individuals. 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.

Some eating disorders are associated with obesity, especially binge eating disorder (BED). As the name indicates, patients with this disorder are prone to overeat, often in binges. A proposed mechanism is that the eating serves to reduce anxiety, and some parallels with substance abuse can be drawn. An important additional factor is that BED patients often lack the ability to recognize hunger and satiety, something that is normally learned in childhood. Learning theory suggests that early childhood conceptions may lead to an association between food and a calm mental state.