Non-restrictive methods for measuring energy expenditure.

Knowledge of whole-body energy expenditure is important if we are to make accurate estimates of energy requirements for optimum health and performance in man and optimum health, performance and production in animals. Energy requirements vary with nutritional, genetic and environmental influences and these can interact in complex ways, therefore it is necessary to isolate and identify specific factors which are important in controlling energy expenditure. In order to do that it is useful to reduce whole-body energy expenditure to smaller component parts; e.g. basal metabolic rate (BMR), dietary-induced thermogenesis, the energy cost of activity, growth and lactation, etc. These components are normally measured in a calorimeter. The chambers in which the subjects are housed for the period of measurement are often relatively small; examples at the Rowett Research Institute include four animal chambers each with a floor area of 1.5 m2 and two human chambers each with a floor area of 6.5 m2. In the highly controlled and defined conditions of such calorimetry chambers it is possible to investigate the effects of nutritional, genetic and environmental influences on each of the components of energy expenditure separately. However, because of the physical restriction neeessary in calorimetry chambers and precisely because of the controlled conditions of temperature, activity, food intake, etc. such measurements may not be representative of the true energy expenditure (and hence energy requirements) of free-living animals and man. In order to measure these we need techniques which will allow the measurement of energy expenditure without restriction. Outside of calorimeter chambers free-living energy expenditure can be estimated in human subjects from recorded food intake and changes in body energy stores. In animal studies changes in body composition can be determined more exactly in serial-slaughter experiments, but it is more difficult to measure energy intake in the field. These techniques are based on the simple relation: