In an entry on the Well New York Times blog, Jane Brody cogently describes the origins and uses (and misuses) of the Body Mass Index or BMI (body weight divided by the square of height). She notes that while the original use of the BMI was to describe changes in the physical characteristics of a population, it then was widely considered to be useful as a tool for individuals to measure weight status a use that can lead to numerous inaccuracies and faulty diagnoses.
In particular, BMI does not distinguish between lean and fat tissue, or take into account the distribution of body fat; both of which can be important for characterizing disease risk. As wenoted way back in 02, people have been surprised that elite athletes may have BMIs that slot them solidly into the obese category but this is just a result of the lack of discrimination between muscle (which athletes have a lot of) and body fat.
ACSH s Dr. Ruth Kava commented While BMI is certainly not the last word in defining obesity, the index does have its uses. Certainly it can reliably discern trends in populations over time. And it can be useful as a preliminary index of disease risk for sedentary people, especially if combined with other measures, such as waist circumference. It s not perfect, but it s not a worthless indicator either.