Body Mass Index (BMI) vs body fat percentage

The BMI has been considered the most accurate way to determine the effect of weight on your health. It is widely used in medical research as an indicator of someone’s health status and disease risk. The CDC provides the following ranges for BMI values (calculated as your weight in kilograms over your height squared in centimeters) for adults:

Underweight: ≤ 18.5 kg/m2

Recommended: 18.6 to 24.9 kg/m2

Overweight 25.0 to 29.9 kg/m2

Obese ≥30 kg/m2

As your BMI increases, so does your risk of developing weight-related diseases, such as heart disease, stroke, type 2 diabetes and some cancers. However, BMI does not distinguish fat from muscle. Therefore, recent research has shown that body-fat percentage may be a better measure of your risk of weight-related diseases than BMI. The accuracy of BMI in diagnosing obesity is limited, particularly for individuals in the intermediate BMI ranges, in men and in the elderly. A BMI cutoff of ≥30 kg/m2 has good specificity but misses more than 50% of people with excess fat.

Estimates for body composition among the college-aged sample were strikingly different for gender and race. Asian men (23.7 kg/m2) and women (21.5 kg/m2) had the lowest mean BMI among the study sample, while Hispanic men (25.9 kg/m2) and women (23.5 kg/m2) had the highest mean BMI. However, percentage body fat did not show the same distribution pattern. While Asian women had the lowest BMI, they did not have the lowest percentage body fat. Asian women had 27.8% body fat, while Caucasian women, lower than Asian women, had 26.9%. Hispanic women had the highest percentage body fat (29.8%).

How much body fat is OK?

The idea is to shoot for is a range rather than a magic number.

How is body fat percentage measured?

The two most common methods used are skin-fold measurement and bioelectrical impedance analysis (BIA). The error rates for these can be as high as 8%. There are other methods which are highly accurate but much more complicated, such as X-ray analysis, water displacement, and others Bioelectric Impedance Analysis. BIA analysis is easy to administer and inexpensive, but it has questionable accuracy because the results may change with a change in hydration level.

References:

Maskarinec, G, et al. Diet Quality in Midadulthood Predicts Visceral Adiposity and Liver Fatness in Older Ages: The Multiethnic Cohort Study. Obesity, 2017;25:1442–1450

Carpenter CL, Yan E, Chen S, et al. Body Fat and Body-Mass Index among a Multiethnic Sample of College-Age Men and Women. Journal of Obesity. 2013;2013:790654

American Council on Exercise. What are the guidelines for percentage of body fat loss? https://www.acefitness.org/acefit/healthy-living-article/60/112/what-are-the-guidelines-for-percentage-of-body-fat; Accessed August 1, 2017.

Romero-Corral A, Somers VK, Sierra-Johnson J, et al. Accuracy of body mass index in diagnosing obesity in the adult general population. International Journal of Obesity. 2008;32(6):959–966