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Obesity is a condition in which the natural energy reserve, stored in the fatty tissue of humans and other mammals, is increased to a point where it is associated with certain health conditions or increased mortality.

Causes and Mechanisms

Lifestyle

Most researchers have concluded that the combination of an excessive nutrient intake and a sedentary lifestyle are the main cause for the rapid acceleration of obesity in Western society in the last quarter 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.

However, dietary intake in itself is insufficient to explain the phenomenal rise in levels of obesity in much of the industrialized world during recent years. An increasingly sedentary lifestyle also has a significant role to play. More and more research into child obesity, for example, links such things as the school run, with the current high levels of this disease.

Genetics

As with many medical conditions, the calorific 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 conditions that feature obesity have been identified 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.

Medical illness

Certain 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 hypothyroidism, Cushing's syndrome, growth hormone deficiency.