Ethnic groups vary in cardiometabolic risk, but the underlying mechanisms remain unclear. Several components of body composition variability (fat/lean ratio, fat distribution, lean mass composition and metabolism, and adipose tissue biology) are increasingly linked with cardiometabolic risk and vary substantially across ethnic groups. Constituents of lean mass are proposed to contribute to ‘metabolic capacity’, a generic trait favouring the maintenance of homeostasis. Adiposity is proposed to contribute to ‘metabolic load’, which at higher levels challenges metabolic homeostasis, elevating cardiometabolic risk. Ethnic differences in body composition, representing different load-capacity ratios, may therefore contribute to ethnic variability in cardiometabolic risk. Ecological and evolutionary factors potentially contributing to ethnic variability in body composition are explored. In contemporary populations, clinicians encounter an increasing range of ethnicity, along with many individuals of mixed-ethnic ancestry. Increasing understanding of the contribution of body composition to cardiometabolic risk may reduce the need to treat ethnic groups as qualitatively different. A conceptual model is proposed, treating insulin sensitivity and stroke risk as composite functions of body composition variables. Operationalizing this model may potentially improve the ability to assess cardiovascular risk across the full ethnicity spectrum, and to predict cardiometabolic consequences of excess weight gain.