Genetic factors in human obesity.

The recent increase in the prevalence of obesity in many parts of the world is clearly driven by secular changes in exposure to environmental factors, including the increased availability and reduced costs of energy-den e food and a reduction in physical activity during work and lei ure time (1,2). The peed with which this 'obesity epidemic' has occurred has led to the widespread misconception that heritable factors play an insignificant role in determining which humans become obese. In fact, measures of fatness (also known as adiposity) are among the most heritable of human traits (3). Multiple studies of families, adoptees, twins and, mo t powerfully, adopted twin (4,5) have all confirmed that heritable factors are likely to be responsible for 4J::Z.5% of the inter-individual variation in body mass index (the commonly used measure of adiposity) (3,6-8). These heritable factors are likelz to be multiple and are likely to operate through the full range of potential mechanisms, including energy intake, energy expenditure and the partitioning of nutrients between fat and lean ti sue. Paradoxically, there ,is.a danger that the effects of such hereditary influences on ''It weight will be downplaved at a time when we are begin-ning, for the first time, to have a genuine understanding of the molecules involved in the control of body weight and how genetic variation in them can influence human obesity. Why search for human obesity genes?