Animal models for the study of energy balance

The most obvious outcome of the failure to regulate energy balance effectively in animals and man is obesity.To learn how obesity arises and may be prevented, we must understand how animals regulate energy balance or metabolism, but a concentration on obesity as such may focus too much research attention on the adipose tissue, a proliferation of which is the outward sign of a tendency towards positive energy balance though not necessarily the cause of it.Rather than attempting a complete review, I shall select several current areas of research to illustrate specific points in connexion with the question: why do we need 'models'? The concept of energy balanceAn animal is said to be in energy balance when his body energy stores remain constant.His energy intake (metabolizable energy of food) then equals his energy expenditure (total heat output).Energy balance is positive if intake exceeds output; negative if output exceeds input. E,, -Eow = A body energyAt least two terms in the equation should be measured.This is accomplished more simply and with greater accuracy over longer periods with animals than with man: a compelling reason for studying animal models to understand basic mechanisms.Changes in body energy are quite difficult to assess properly in man.Deposition of adipose tissue is associated with increases in protein and water as well as fat.Changes in carcass energy can be measured accurately in animals by direct chemical analysis, but it is surprising how frequently this is omitted.It is common for authors to refer only to weight gain as though this can fully describe a change in energy balance.Energy balance can change quite dramatically without any change in weight as illustrated by an example from our work with piglets (Gurr et al. 1980).A group of piglets was fed on a diet of low-protein content (approximately 2% of metabolizable energy) ad lib.The protein level was adjusted so that animals maintained weight.A second group was fed on a high-protein diet (27% of ME) so as to maintain the same weight.The fundamentals of this experiment were lirst described by Miller & Payne (1962).At the beginning and end of the experiment, the animals were slaughtered and carcass composition measured.In low-protein pigs, there was a significant increase in carcass energy without a significant change in weight (Table I ) and this was accompanied by a higher proportion of carcass fat (&30?6) and a lower proportion of body water oo294651/80/