Will “fat mass index” measure a fatter or thinner America?

Athletes – particularly athletes who do a lot of weight training – have long exposed a key qualitative flaw in the way medical researchers calculate whether a person is overweight or obese. Body Mass Index (BMI) is determined by dividing your weight by the square of your height.  But because BMI cannot distinguish between fat and muscle,  many athletes – in fact, many people who simply lift weights regularly – are likely to be classified as overweight or possibly even obese (the National Institutes for Health defines a person as having a normal weight if his or her BMI is between 18.5 and 24.9, and as being obese if their BMI is 30.0). To further complicate matters, BMI doesn’t account for gender or ethnic differences either.

Now, researchers using new data from the National Health and Nutrition Survey (NHANES), an ongoing, nationally representative health survey, have estimated reference values for a range of body composition parameters. The most notable is “fat mass index,” (FMI) which they believe could replace BMI as a more accurate way of assessing obesity. As they write:

“At age 25, the FMI data in Table 3 indicates that there are substantial differences in adiposity between genders, with mean values for women ranging from 8.9 to 10.9 kg/m2 and mean values for males between 5.6 to 6.8 kg/m2 for the three ethnic groups. From these data it appears likely that lacking gender or ethnicity adjustments, BMI may be overestimating obesity in some groups and underestimating it in others.”

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