The doubly-labelled water technique for measurement of energy expenditure in free-living animals.

Energy is important at many different levels of organization in the living animal. Investigations of energetics may enhance our understanding of the evolution of animal traits, and in humans help towards tackling problems of energy balance, such as obesity. The doubly-labelled water technique is a non-in vasive method for direct measurement of the energy expenditure of free-living animals. This paper considers the theoretical foundation of the technique and details the simplifying assumptions. Validation studies, which compare the measurement made by the technique with simultaneous evaluations using conventional laboratory techniques, suggest the technique gives an estimate of energy expenditure with an accuracy of about 5%, when it is applied to vertebrates. Offsetting this, however, is the fact the technique provides invaluable information which can be collected in no other way.