The measurement of food intake has long been used to describe ‘adaptation’ to low energy intakes in certain tropical peoples. However, the methods available to quantify food intake are unlikely to reflect accurately real energy intakes in free living peoples. Alternatively, estimating energy expenditure shows some promise—particularly the measurement of basal metabolic rate (BMR). The BMR may be measured effectively in males, but females show wide intra-individual variation in BMR during their menstrual cycle, which makes BMR measurements more difficult to interpret in the context of adaptation. The use of double-labelled water may be the only method suitable to quantify and define ‘adaptation’ to low intakes in women.