In this analysis, we bring together two research fields that have never been associated before: the clinical issue 'Quételet's body-mass index and longevity' and the comparative biological issue 'body-brain allometry'. Comparison of medical and biological data supports the view that body mass index is just a one-to-one mapping of the body-brain-energy balance-a biological variable indicating that an individual maintains its systemic energy homeostasis and therefore is likely to perform well in the coming years.