Spock the Baptist's Journal: Body Mass Index

The BMI: Body Mass Index = m/h^2.

Density: p = M/V

The above formulae clearly illustrate the problem with the BMI. The BMI in SI units would be kg/m^2, whereas the density is kg/m^3. Further the m^2 is from only one dimension, rather than from three mutually normal dimensions. The purpose of the BMI is to determine adiposity. For this purpose it may serve as a semi-useful metric in statistical studies of a epidemiological, or actuarial nature.

However, as a diagnostic metric in a clinical setting it is worse than useless. This is due to the fact that it (the BMI) does not, in any way, measure body density. It is the body density that allows for a determination of adiposity. As density is the ratio of mass to volume then a single dimension is totally inadequate to provide even a guess as to the volume of the object in question.

Consider three men all 1.7 meters tall. A is both thin in terms of depth, and has a narrow width. B is thin in terms of depth, but broad in width. C is broad in both depth, and width. If all three are of the same average body denisty, then clearly A will have less mass than B, and B in turn will be of lower mass than C, due to the greater body volume in turn of A, B, and C.

Futher, even if A, B, and C all have the same *standing* height, depth, and breadth. The ratio of their respective sitting height to standing height may differ. The one with the larger sitting to standing hight will be the most massive, with the one with the smallest sitting to standing height ratio being the least massive.

To conclude the BMI fails to take in to account variations in in body dementions which effect the volume of a persons body where the persons body is of optimal average density.