The extent of genetic influences on human obesity has been assessed by twin, adoption, and family studies. This influence is dramatically illustrated by the marked increase in the prevalence of obesity in the past decade. Socioeconomic status is an important influence on obesity, particularly among women. The negative correlation between socioeconomic status and obesity reflects an underlying cause. Longitudinal studies have shown that growing up with lower socioeconomic status is a powerful risk factor for obesity.
Socioeconomic factors are major influences on both energy intake and energy expenditure.
A large food intake is associated with obesity. For many years, it was believed that obscure metabolic disturbances caused obesity and that food intake was normal. However, the doubly labeled water method, using stable isotopes of hydrogen and oxygen, shows that obese persons have a large energy expenditure, which requires in turn a large food intake. Furthermore, this large food intake usually includes a large fat intake, which independently predisposes to obesity. A sedentary lifestyle, so prevalent in Western societies, is another major environmental influence promoting obesity.