# Race and body fat percentage in men and women and Body mass index

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.

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:

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 percent cutoffs.

Age BMI Fat % 36.5 27.7 25.0% 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).

What about those body fat percentage formulas?

The Deurenberg formula is on Wikipedia, but there are reasons to be skeptical of them.

Suppose you were a young adult woman with a BMI of 20. The linear formula would tell you a body fat of about 25%. But look at the curvy chart a little higher up, and you’ll see the body fat value should be, about 18 percent or less.

I knew it! Those online calculators made by people selling diets, have insulted me more than once.

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.

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