Body composition, geography and ethnicity: understanding the associations with health.

Human body composition varies substantially across populations, for a range of different reasons that remain poorly understood. Similar to body size and limb proportions, both fat-free and fat mass are geographically patterned, which may relate both to ancestry but also to persisting ecological stresses, whose effects may be transmitted through the life-course and across generations. The role of genetics in such variability remains uncertain, moreover relevant ecological stresses include not only physical (e.g. temperature) and biological (e.g. diet, subsistence, pathogen burden) components, but also societal factors including colonisation, conflict, migration and economic inequality. Body composition variability has major implications for health inequalities, and improving our understanding of the different underlying mechanisms may help us understand how to design public health interventions, and optimise the prevention and management of disease. However, the terminology used to address population variability in body composition needs to be reconsidered. Terms such as race/ethnicity, ancestry and geography are widely used to frame investigations and interpret findings, but the variable most often collected in studies is ethnicity, selected by research participants on the basis of their identity. Race has no biological validity, but may be a marker of exposure to racism and discrimination. Improved understanding of the multiple components of biology that relate to ethnicity may help understand the sources of population variability in body composition, and hence improve efforts to reduce health inequalities.