Title of article:

The effect of sex, age and race on estimating percentage body fat from body mass index: the Heritage Family Study

Authors: Jackson AS, Stanforth PR, Gagnon J, et al
Journal: Int J Obes Relat Metab Disord, Jun 2002;26(6):789-96

Abstract

OBJECTIVE: To study the effects of sex, age and race on the relation between body mass index (BMI) and measured percent body fat (%fat). DESIGN: Cross-sectional validation study of sedentary individuals. SUBJECTS: The Heritage Family Study cohort of 665 black and white men and women who ranged in age from 17 to 65 y. MEASUREMENTS: Body density determined from hydrostatic weighing. Percentage body fat determined with gender and race-specific, two-compartment models. BMI determined from height and weight, and sex and race in dummy coded form. RESULTS: Polynomial regression showed that the relationship between %fat and BMI was quadratic for both men and women. A natural log transformation of BMI adjusted for the non-linearity. Test for homogeneity of log transformed BMI and gender showed that the male-female slopes were within random variance, but the intercepts differed. For the same BMI, the %fat of females was 10.4% higher than that of males. General linear models analysis of the women's data showed that age, race and race-by-BMI interaction were independently related to %fat. The same analysis applied to the men's data showed that %fat was not just a function of BMI, but also age and age-by-BMI interaction. Multiple regression analyses provided models that defined the bias. CONCLUSIONS: These data and results published in the literature show that BMI and %fat relationship are not independent of age and gender. These data showed a race effect for women, but not men. The failure to adjust for these sources of bias resulted in substantial differences in the proportion of subjects defined as obese by measured %fat.

Comments and Key points

This study has some data comparing white and black American's BMI. I've summarized it below, for the benefit of people looking for this type of data.

  Male Female
  BMI % fat BMI % fat
Black 26.9 22.9% 28.1 24.8%
White 26.5 23.0% 36.0% 30.0%

This study found no difference in men, Black or White. Although Black women are heavier, their ratio of fat to BMI is similar. Actually, this study found a slight difference between black women and white women, but only when their BMI is less than 25. No big deal.

This graph of Body Fat percentage versus Body Mass Index, shows that women's fat percentage is higher than mens. The curving data trendlines shown below look similar to other studies of body fat versus BMI.

fat versus BMI

The article referred to 4 different equations that can predict body fat percentage, from BMI.

Deurenberg formula1 #1: Adult Body Fat % = (1.20 x BMI) + (0.23 x Age) - (10.8 x gender) - 5.4
Deurenberg formula2 #2: Adult Body Fat % = (1.29 x BMI) + (0.20 x Age) - (11.4 x gender) - 8.0
Gallagher formula3: Adult Body Fat % = (1.46 x BMI) + (0.14 x Age) - (11.6 x gender) - 10
Jackson-Pollock formula4,5: Adult Body Fat % = (1.61 x BMI) + (0.13 x Age) - (12.1 x gender) - 13.9

These formulas are graphed and compared in the chart below:

Various Body Fat verus BMI formulas

These formulas graphed are all fairly similar. Trouble is, they are linear formulas (giving straight lines), but the real relationship between BMI and body fat is a curved one, as you saw higher up on this page.

This article also suggested it's own formula for predicting body fat percentage from BMI, age and gender:

Heritage study: Adult Body Fat % = (1.39 x BMI) + (0.16 x Age) - (10.34 x gender) - 9

One important conclusion of this study, is to notice that AGE is a significant part of all these body fat formulas. This indicates that it is not suitable to use a fixed BMI across all ages as a definition of obesity. Also, these formulas tend to overestimate the amount of fat in young men with a BMI over 30.

"results are consistent with published data showing the need to consider age and gender when defining the prevalence of obesity with BMI".

This article suggests that for body fat, 25% for men, and 33% for women, are suitable cutoff thresholds for defining obesity6. Using these fat % cutoffs, and using their formula to relate BMI to body fat, and using the mean Ages in the study, this table shows the BMI values that could achieve the body fat% cutoffs.

  Age BMI Fat %
Men 36.5 27.7 25.0%
Women 34.4 26.3 33.0%

 

So, to summarize that last table, a BMI of 27.7 for men, and BMI of 26.3 for women, could be suitable BMI thresholds to define obesity ( although sensitivity and specificity might need adjusting).

References

  1. Deurenberg P, Westrate JA, Seidell JC. Body mass index as a measure of body fatness: age- and sex-specific prediction formulas. Br J Nutr 1991; 65:105-114.
  2. Deurenberg P, Yap M, van Staveren WA. Body mass index and percent body fat. A meta analysis among different ethnic groups. Int J Obes Relat Metab Disord 1998; 22:1164-1171.
  3. Gallagher D, Visser M, Sepulveda D, Pierson RN, Harris T, Heymsfield SB. How useful is body mass index for comparison of body fatness across age, sex and ethnic groups. Am J Epidemiol 1996; 143:228-239.
  4. Jackson AS, Pollock ML, Ward A, Generalized equations for predicting body density of women. Med Sci Sports Exercise 1980; 12:175-182.
  5. Jackson AS. Research design and analysis of data procedures for predicting body density. Med Sci Sports Exercise 1984; 16:616-620.
  6. Wellens, RJ, Roche AF, Khamis HJ et al. Relationships between body mass index and body composition. Obes Res 1996; 4:35-44.

Review & comments by Steven B. Halls, MD, Last edited 23-June, 2008, Copyright.
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