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Ann Hum Biol. 2007 Nov-Dec;34(6):656-63.

Why is the body mass index calculated as mass/height2, not as mass/height3?

Author information

1
Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow, UK. R.F.Burton@bio.gla.ac.uk

Abstract

BACKGROUND:

The body mass index or BMI, mass/height(2), is used to predict fatness and health. It is an approximation to the Benn index, mass/height(p), where p (typically 1.1-2.5 for adult populations) makes the index uncorrelated with height. Mass/height(3) is an index of body build that is independent of scale and statistics.

AIM:

To explain why p varies and is less than three, show how statistical methods can distort perceptions of mass-height relationships, and clarify the nature of the BMI.

METHODS:

A hypothetical adult population is modelled statistically, with mass being approximately proportional to height(3) and with neither variable determining the other. Values of p are calculated both for the model and for real adults.

RESULTS:

In both cases p increases with the correlation between mass and height. Both p and that correlation are usually lower for women than for men.

CONCLUSIONS:

In adult populations mass must vary more nearly with height(3) than with height(2), although, for reasons explained, conventional statistical techniques suggest otherwise. Nevertheless the BMI is a valid predictor of fatness from mass and height in adults and is properly divisible into fat mass and fat-free mass indices. The validity of the latter three indices for children is questionable.

PMID:
18092209
DOI:
10.1080/03014460701732962
[Indexed for MEDLINE]

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