Luxuskonsumption, diet-induced thermogenesis and brown fat: a critical review.

That a steady body weight can be maintained over long periods of life is a remarkable fact. Human food intakes vary widely I l l , and do not seem subjectively to be closely controlled; it is an old observation that they sometimes vary oppositely to what might be expected from body weight-it will be recalled that in Pharaoh’s dream [21 the lean kine were the over-eaters. How, then, is constancy of weight achieved? Apart from fluctuations in water content, body weight must ultimately be governed by conservation of energy. Any difference between the amounts of energy gained and lost by the body changes the body’s energy content: since the latter is in chemical form, this implies weight change. In man the store of energy is such that energy imbalance over single clays causes little disturbance. Over periods of years, however, simple calculation shows that even a tiny systematic error in the balance between energy intake and output would cause gross change in weight: for example, Evans’ hypothetical subject who in 10 years gained 100 Ib more than her twin through indulging daily in “a little deeper cut in the pat of butter” [31. Measurements of intake and output in man show no relationship over single days, but an .approach toward equality over days to weeks [4, 51. Evidently some increasing acduracy as time progresses 161. The nature of the feedback signal and the central control involved in weight maintenance raise questions of great interest [6, 71. The currently topical question, however, concerns the effector through which control is exerted. In principle this must be variation of energy intake, or energy expenditure, or both. Human subjective impressions notwithstanding, there is much evidence that energy intake from food is controlled to maintain balance. The classical demonstration, readily repeated, is that of E. F. Adolph 18, 91. Rats in a constant environment ‘eat for energy’: when the concentration of energy in their diet is changed they accurately adjust the amount they eat. Adjustment takes 1-2 weeks in rats and probably more in man [lo]; this may explain why it is not perceived subjectively. If food intake is the effector of a negative feedback control system -that regulates energy stored as fat, it is also possible to explain why food intake and body weight tend to be related paradoxically. If, for example, energy expenditure is low and food attractive and readily available (as is the case for man in ‘Western’ society), excess of intake over expenditure will cause fat to accumulate; as this happens, however, it is-by some means-sensed as a proportionately increasing inhibitory signal by the control system. When the signal eventually becomes strong enough to hold food intake down to match expenditure, a steady state is attained, in which high body fat accompanies low food intake. This Correspondence: Professor G. R. Hervey, Department of Physiology, The University of Leeds, Leeds LS2 9JT, U.K.