Latitudinal Gradient in the Body Mass Index (BMI), and the BMI's Geometric and Statistical Relationships to the Surface Area: Volume Ratio and Body Shape

The body mass index (BMI), weight/height² (W/H²), is currently the index of choice for assessment of nutritional status. Statements in the literature about the BMI as a potential expression of “cold adaptation” or “Bergmann’s Rule” beg the question: What does that BMI measure in terms of size, shape, and the surface area:volume (SA:V) ratio? Geometric modeling shows that the BMI captures both size and shape and is inversely related to the SA:V ratio. This admixture of size/shape information, combined with the unmeaningful absolute value of the BMI, preclude precise understanding of what it measures. A new weight-height-based variable was derived –the mean effective breadth (MEB)- which more clearly relates to the SA:V ratio and heuristically represents what weight-for-height does: it alters body breadth.

Previous findings of a geographical cline in the BMI in Native Americans were expanded to a worldwide sample of 328 adult populations. The BMI and MEB increased with increasing latitude, while the SA:V ratio decreased. All three ratios were also correlated with variables that alter the biological SA:V ratio: sitting height, relative sitting height, and bi-acromial and bi-iliac breadths. The MEB showed higher correlations with latitude, weight, height, sitting height, relative sitting height, and bi-acromial breadth than did the BMI, though coefficients were similar to those of the SA:V ratio.

The BMI’s geometric and statistical associations with the SA:V ratio and measures of proportion or shape corroborate and amplify others’ findings that the BMI is not a shape-independent index of body size or nutritional status. The W/H² ratio was originally conceived by Quetelet as a “proof” of body proportionality. Nutritional epidemiologists should beware these associations when using BMI cutoff categories to diagnose chronic energy deficiency or obesity.