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Measures of adiposity: the inappropriate use of the fat mass index

International Journal of Obesity volume 34, page 213 (2010)Cite this article

The fat content of the body is sometimes expressed as the fat mass index (FMI), calculated as fat mass/height2.1 This is advocated because, unlike percentage body fat (%BF), the FMI is independent of fat-free mass.2, 3 However, for most purposes as explained below, the height exponent should be three, not two.

The FMI was conceived, appropriately, as a component of the body mass index (BMI), that is, (body mass)/height2, and is indeed useful in analyzing BMIs in terms of fat and non-fat.3, 4 People understand the rationale of the BMI in various ways, but its height exponent—and therefore that of the FMI—is effectively a round-number approximation of a non-linear regression parameter that at least roughly optimizes the BMI as a predictor of adiposity.5 (This is especially true for men, but the optimum exponent can be close to one and three in women and children, respectively.5) The BMI thus has a predictive role based on the statistics of adult population samples. The FMI cannot have this role, because it is only calculable when %BF is already known. Because the FMI's height exponent of two derives from that of the BMI, it is also based in population statistics. It has no theoretical basis beyond that. Recently the FMI has been adapted for 9-year-old children using a regression-based height exponent of 5.8.6 The same objection applies to this index as to the standard FMI, but there is an additional difficulty in that values from the two versions cannot be directly compared.

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  1. Faculty of Biomedical and Life Sciences, Thomson Building, University of Glasgow, Glasgow, UK

    R F Burton

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Burton, R. Measures of adiposity: the inappropriate use of the fat mass index. Int J Obes 34, 213 (2010). https://doi.org/10.1038/ijo.2009.202

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