Title of article:

How useful is body mass index for comparison of body fatness across age, sex, and ethnic groups?

Authors: Gallagher D, Visser M, Sepulveda D, Pierson RN, Harris T, Heymsfield SB.
Journal: Am J Epidemiol, Feb 1996;143(3):228-39

Abstract

This study tested the hypothesis that body mass index (BMI) is representative of body fatness independent of age, sex, and ethnicity. Between 1986 and 1992, the authors studied a total of 202 black and 504 white men and women who resided in or near New York City, were ages 20-94 years, and had BMIs of 18-35 kg/m2. Total body fat, expressed as a percentage of body weight (BF%), was assessed using a four-compartment body composition model that does not rely on assumptions known to be age, sex, or ethnicity dependent. Statistically significant age dependencies were observed in the BF%-BMI relations in all four sex and ethnic groups (p values < 0.05-0.001) with older persons showing a higher BF% compared with younger persons with comparable BMIs. Statistically significant sex effects were also observed in BF%-BMI relations within each ethnic group (p values < 0.001) after controlling first for age. For an equivalent BMI, women have significantly greater amounts of total body fat than do men throughout the entire adult life span. Ethnicity did not significantly influence the BF%-BMI relation after controlling first for age and sex even though both black women and men had longer appendicular bone lengths relative to stature (p values < 0.001 and 0.02, respectively) compared with white women and men. Body mass index alone accounted for 25% of between-individual differences in body fat percentage for the 706 total subjects; adding age and sex as independent variables to the regression model increased the variance (r2) to 67%. These results suggest that BMI is age and sex dependent when used as an indicator of body fatness, but that it is ethnicity independent in black and white adults.

Comments and Key points

A study comparing New York's white and black persons.

	Men		Women
	BMI	fat %	BMI	fat %
Black	25.8	21.7%	27.0	35.6%
White	25.2	21.2%	23.3	30.3%

Men had similar BMI and body fat percentage, and although black women had higher values than white women, the ratio of body fat to Body Mass Index is extremely close. This is illustrated in the chart below, where black and white women's data of body fat percentage and BMI is completely overlapping. (Also true for men).

This article has some data about leg length, saying that Black's have longer legs than Whites. This theoretically affects BMI, but it didn't seem to matter in this study.

Regarding Elderly people, this article says that, for people with BMI of 23, the body fat percentage increases 1.0 to 1.1 percent (men) or 0.7 to 1.0 percent (women) per decade.

Regarding Gender differences, this article says that, for 20-year olds with a BMI of 23, the men would have 13.3% body fat, and women would have 26.0%. For 80-year olds, men's body fat would be 23.9% and women's body fat percentage would be 32.6%. In other words, it seems natural for young women to have 12.7% greater body fat than men. (That seems a little high).

This article suggests some formulas to predict body fat %, based on BMI, Age, Gender and Race.

Review & comments by Steven B. Halls, MD, Last modified 123-June, 2008, Copyright
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