Techniques in energy metabolism studies and their limitations.

The determination of the calorific value of the material stored in or lost from the body during an interval of time can be made in two ways: by the comparative slaughter method and by the balance method. In the comparative slaughter method at the beginning and at the end of the time interval the heats of combustion of the entire bodies of two precisely similar animals are determined, and the retention of energy is determined by difference. Apart from identical twins, no two animals are precisely similar, so the comparative slaughter technique necessarily involves slaughter of fairly large groups of animals if the error attached to the estimate of retention is to be kept small. The method is suited to studies with small laboratory animals and poultry (Davidson, Mathieson, Williams & Boyne, 1964). I t is an expensive method full of technical difficulty due to sampling problems when applied to large animals (Mitchell, Kammlade & Hamilton, 1928; Milford & Minson, 1965; Morris & Moir, 1963) and cannot be applied to man. The comparative slaughter method, however, does provide a direct measure of retention, limited only by analytical accuracy and by the basic assumption that initially the two groups of animals have the same energy content. With proper numbers and randomization of animals, this latter assumption does not lead to systematic error. Balance methods to determine energy retention are of two types. In one the difference between the heat of combustion of the food and the sum of the heats of combustion of the excreta, including gases and the heat produced by the animal, is measured during an interval of time. This method is called the energy balance method. I n the other, from the intake of carbon and nitrogen in food, all losses of these elements in the excreta and as gas are deducted to give estimates of the retention of C and N. Certain assumptions about the chemical forms in which C and N are stored in the body are then made to provide an estimate of the fat and protein and hence of the energy retained. When hydrogen retention is also determined by these same balance methods, estimates of fat, protein and carbohydrate retention can be achieved (Atwater & Benedict, 1904). These methods are called the C and N or C, N and H balance methods. The advantages of balance methods are that they do not involve destruction of