Food intake and growth in the newly-born.

The regulation of voluntary food intake 127 (4) Obese subjects make greater use of the hexose monophosphate pathway so providing NADPH, which is needed for synthesis of fatty acids. T h e evidence for this is that if obese and lean subjects are given glucose labelled in the C, and C, positions, a larger proportion of the labelled C, appears in the expired air of the obese than in the normal subject. T h e C, carbon atom is primarily involved in the oxidation of glucose via the hexose monophosphate pathway. ( 5 ) Obese patients have a diminished urinary excretion of the adrenal androgen, dehydroepiandrosterone. This hormone inhibits the activity of glucose-6-phosphate dehydrogenase which is involved in the first stage of the hexose monophosphate pathway. However, it is not clear whether any of these changes are the cause or the consequence of the obese state. None of them allows a sharp demarcation of the obese from the non-obese. There is no evidence that the specific dynamic action is less well-marked in the obese than in normal subjects. Homo sapiens evolved in an environment in which the food supply was often scarce. He learnt to adapt to this and is able to survive when his food intake is reduced to about half the normal amount. It is only recently that a significant proportion of the species has lived in an environment with an abundant supply of food, which can be obtained without physical effort. There is no evidence of any regulatory mechanism which can protect him against overeating and the consequent storing of excess energy as fat. Although, as indicated above, there are possible mechanisms whereby a higher proportion of dietary energy could be dissipated as heat without being associated with the corresponding amount of work, it seems very unlikely that obesity arises as a result of any such changes. In the vast majority of cases of human obesity, the condition appears to arise from changes in behavioural activity which are not compensated by any physiological control system.