Energy requirements: the case for the factorial model

Members of Food and Agriculture Organization and the International Atomic Energy Agency (IAEA) have recently stated that it is time for an update of human energy requirements (ERs) and expressed concerns about previous methodologies used in their derivation. They suggest that a factorial approach to energy expenditure and requirements, which includes an estimate of physical activity energy expenditure (PAEE), is no longer needed. Instead, they propose that predictive equations for daily total energy expenditure (TEE) derived from an IAEA large database of doubly labeled water (DLW) measurements of TEE can be used to estimate population ERs from basic anthropometric data for most population groups. Even though predictive equations cannot account for variable PAEE, they argue that ecological DLW studies have demonstrated that the TEE of presumed physically active population groups, i.e., hunter gatherers and rural Nigerian females, did not reflect an active lifestyle compared with United States and European populations. Such reports have resulted in the suggestion that TEE is a relatively constrained product of our evolved physiology. It is argued that we adapt to maintain TEE within a narrow physiological range rather than increasing with physical activity in an additive, dose-dependent manner as is implicit in the factorial model. Here, the factorial model is re-examined. Weight-adjusted metrics of PAEE are derived, which are used to analyze published energy expenditure of traditional populations and compare it with a large DLW data set, assembled to derive the United Kingdom energy dietary reference values. This reveals very high rates of energy expenditure consistent with lifestyles with no obvious evidence of energy constraint. We conclude that any approach to updating human ERs will need to address the interindividual variation in PAEE.