Is Brown Adipose Tissue-Mediated Adaptive Thermogenesis the Missing Component of the Constrained Total Energy Expenditure Model?

sis, the magnitude of the reduction of activity energy expenditure component points out to an additional reduction in non-muscular energy expenditure (e.g. reproductive activity or somatic maintenance) [1] . Clarifying the mechanism for the decrease in non-muscular energy expenditure is crucial to fully understand the PA-TEE relationship. Here, we hypothesize that the adaptive thermogenesis mediated by brown adipose tissue (BAT) might account for a large proportion of the exercise-induced decrease in non-muscular energy expenditure. There are 2 types of thermogenic BAT: classical brown and beige adipose tissues. The main function of both tissues is to burn glucose and lipids to produce heat (by uncoupled respiration). Although sharing its main metabolic role, classical brown and beige adipocytes have distinct developmental origins as well as gene expression patterns [3] . A huge interest in human BAT physiology emerged in 2009 due to its potential role in energy balance, as well as in glucose and lipid metabolism [3, 4] . Activated BAT influences energy expenditure in animal models, increasing both resting energy expenditure and adaptive thermogenesis (i.e. dietand cold-induced thermogenesis). In humans, the influence of activated BAT on resting energy expenditure in thermoneutral conditions seems negligible, and its role on diet-induced In simple terms, obesity is the result of chronic excess of energy intake over energy expenditure. However, the obese phenotype is likely to be the result of complex interactions between the individual’s genetic background, environmental, behavioral and socioeconomic factors. Although much has been done, it is evident that our current knowledge about energy balance is insufficient to combat the obesity pandemic and new approaches have to be harnessed and exploited. Recently, in an outstanding study, Pontzer et al. [1] showed evidence supporting the constrained total energy expenditure (TEE) model [2] , by objectively measuring TEE and physical activity (PA) in free living humans. The constrained TEE model proposes that TEE increases linearly with PA until it reaches a plateau above which the effect of PA on TEE is negligible. Free-living PA influences energy balance in a different manner than what structured exercise training does. It remains to be investigated in a longitudinal design whether the constrained model hypothesis also applies to long-term exercise training. Pontzer et al. [1] also showed that the plateau in the PA-TEE relationship is not explained by a decrease in resting energy expenditure [2] , but by a decrease in the activity energy expenditure component itself. Besides an obvious reduction in non-exercise activity thermogenePublished online: July 30, 2016