The critiques over the use of BMI as a criterion for fatness/obesity are not new. Eric Oliver's excellent book, Fat Politics, which demonstrates the arbitrariness of BMI over the decades, was published in 2005. ("Arbitrary" is one possible description; "socially constructed" would be perhaps a more descriptive phrase). Kate Harding has a nice slideshow at Shapely Prose entitled the BMI Project, intended to show just how flawed the measure is as an assessment of fatness and/or obesity.
While the problems with BMI, then, are hardly unknown, these difficulties have not generally translated into scholarship (aside from critical theory) or into media reporting. In this context comes a strongly worded article in the newest Journal of Health Economics:
Richard V. Burkhauser, John Cawley
Virtually all social science research related to obesity studies a person's body mass index (BMI). Yet there is wide agreement in the medical literature that BMI is seriously flawed because it does not distinguish fat from fat-free mass such as muscle and bone. This paper studies data that include multiple measures of fatness and finds that many important patterns, such as who is classified as obese, group rates of obesity, and correlations of obesity with social science outcomes, are all sensitive to the measure of fatness and obesity used.
We show that, relative to percent body fat, BMI misclassifies substantial fractions of individuals as obese or non-obese; in general, BMI is less accurate classifying men than women. Furthermore, when percent body fat instead of BMI is used to define obesity, the gap in obesity between white and African American men increases substantially but the gap in obesity between African American and white women is cut in half. Finally, total body fat is negatively correlated with employment for some groups and fat-free mass is not significantly correlated with employment for any group, a difference that was obscured in previous research that studied BMI.
In the long run, social science datasets should include more accurate measures of fatness. In the short run, estimating more accurate measures of fatness using height and weight is not possible except by making unattractive assumptions, but there is also no reason to adhere uncritically to BMI as a measure of fatness. Social science research on obesity would be enriched by greater consideration of alternate specifications of weight and height and more accurate measures of fatness.
It is particularly interesting to note how the social construction of BMI affects prevalence and incidence of obesity among racial groups. This in turn shows that prevalence and incidence are themselves irreducibly social concepts, and that our beliefs about who has what health status is irretrievably informed by all manner of social, political, and cultural factors.