The part played by variation of energy expenditure in the regulation of energy balance

Energy balance is the difference between the amounts of energy gained and lost by an organism over any period.Since energy is conserved, non-zero energy balance is necessarily associated with an equal change in the energy content of the body; since this is in chemical form, there must be a change in the weight of some body constituent, and so in general in body-weight.Energy balance must therefore approximate to zero over adult life.This would not come about by chance.The necessary control might be exerted on energy intake, expenditure, or both.Even though it may not be subjectively obvious to humans, there is much evidence that 'voluntary' energy intake is regulated in the long term to match energy expenditure with impressive precision (Hervey, 1975).Energy expenditure, however, must to a considerable extent at least be determined inflexibly by physiological requirements and the demands of the outside world for activity and thermoregulation.There is no doubt that imposed changes in energy intake are followed by changes in energy expenditure.Although precise information is surprisingly scarce, general experience of feeding humans and animals less energy than they would eat voluntarily seems to show that they still maintain steady, though lower, body-weights.Such measurements of energy expenditure in chronically underfed animals and humans as have been reported confirm that it is reduced (Keys et al. 1950).The mechanism of this is unknown, but it seems to take time and to have the characteristics of a regulatory response.Perhaps understandably in an 'affluent society', the effect of increased energy intake upon expenditure has attracted more interest.When more energy is absorbed, energy expenditure must be expected to increase.Feeding has long been known to be followed by increased heat output, which presumably reflects costs of digestion, absorption and metabolism; this is probably best called Heat Increment of Feeding (Webster, 1981)' a descriptive term that does not prejudge the mechanism.It must be expected to increase with energy intake.Further, since no process can be perfectly efficient, whatever is done with energy absorbed in excess of requirements must cost some energy.In particular, storing excess energy will entail expenditure in synthesizing fat or building tissue.In the context of regulation of energy balance, the important questions are: is there a component of energy expenditure accessible to physiological control and used as an effector; and if so, what contribution does it make?T o find answers, the regulatory component (or components) must be distinguished from other, inevitable changes in expenditure, such as those due to the heat increment of feeding and the cost of fat synthesis.