The empirical relationship between longevity and physical stature may be obscured by unobserved genetic diversity in the optimal metabolic rate.

It is hypothesized that an inheritable "optimal" metabolic rate is associated with the lowest attainable mortality risk, greatest attainable height (in youth) and a desirable body weight. Positive deviations from this rate are reflected in overweight and higher mortality, negative deviations in shorter stature (stunting), low body mass index (BMI), and higher mortality. In a heterogeneous population the optimal rate may differ among genotypes, and those with a higher optimal rate are the ones who can translate more energy intake into growth in height such that under optimal net-nutritional intake, they are able to reach a greater final adult height with a lower body mass index than genotypes with a lower optimal rate. Hence, the optimal height and weight - in terms of longevity - may differ substantially among genotypes, so that an empirical estimation of the relationship may be blurred in a heterogeneous society. Conversely, the "true" association (within genotypes) may be stronger than the measured one, which may have some implications for the contribution of the secular increase in human height to the rise in life expectancy or the optimal BMI in persons who experienced stunting in youth.