The propensity to obesity in animals and man identifies those individuals who are genetically favoured to survive when food supplies are scarce. Obese subjects are limited in their ability to produce heat, either in a cold environment or after food, because of a reduced activity in skeletal muscle of a "futile" cycle in glucose metabolism. The impaired thermogenesis reduces the maintenance requirement for energy in the pre-obese individual so that a "normal" energy intake can only be balanced by excessive exercise or the expansion of adipocytes. The basal metabolic rate rises as obesity develops and compensates for the impaired thermogenic mechanism.