Recent news stories have cast doubt on the validity of a widely used index — Body Mass Index, or BMI — as an indicator of overweight, obesity, and associated health risks. It's really kind of fun to read some of these headlines and stories, like the ones that point out how members of leading college basketball teams would qualify as overweight if their BMIs were evaluated according to current government standards. So, if these active, fit, and presumably healthy young men would register as overweight on the Body Mass Index, the index must not be too accurate, right? I don't think so, because in order for such an index to be useful, it should be used in an appropriate manner for the people for whom it is intended — and that doesn't include very fit young athletes.
BMI is calculated by dividing a person's body weight in kilograms by their height in meters squared. BMI has been used as an estimator of body fatness by researchers for a number of years. While there are several ways to actually measure how much fat a person is carrying, these typically involve the use of specialized equipment not easily accessible to most medical practitioners or their patients. Instruments or equipment for underwater weighing, measurement of electrical impedance, or CT scans aren't found in most GPs' offices. Thus, the BMI has come to be used as a surrogate — a marker, if you will — for body fat. And for certain populations, it correlates well with actual measurements of body fat. And there's the rub — for certain populations, like fit young athletes — BMI isn't much use.
BMI correlates well with body fat in sedentary populations. That is, it's great for couch potatoes. But a simple measurement of body weight (which is the basis for BMI) can't distinguish between fat and lean tissue like muscle. Thus, it also can't distinguish people whose body weight is high because they're dedicated body builders or play competitive sports on a regular basis. And people who have and maintain a muscular physique through regular activity are not likely to be prone to increased risk for the types of chronic diseases to which excess body fat predisposes one.
So, who should be concerned about their BMI? If that figure is 30 or greater, an inactive person would be considered obese. And obesity is known to increase the risk of a host of unpleasant conditions: gallbladder disease, arthritis, type 2 diabetes, high blood pressure, and heart disease.
Until 1998, a person whose BMI was between 27 and 29.9 was considered to be overweight — but that year a panel of experts at the National Institutes of Health (NIH) increased the range of overweight to BMIs between 25 and 29.9, because even the lower figures seemed to be correlated with increased health risks.
Whether or not this change in the BMI standards for overweight was a good thing or not (and there are arguments on both sides of the issue), one thing is certainly clear. The prevalence of obesity itself has been increasing rapidly in the United States. In national surveys in 1960-62, about 13 percent of adults 20-74 years old had BMIs of 30 or greater. In the survey spanning 1988-94, that figure had risen to over 22 percent. And once people have become obese, it is very difficult to lose the excess fat and maintain that loss.
So if learning that your BMI of 26 puts you in an "overweight" category and motivates you to instigate lifestyle changes to help prevent obesity itself, the BMI will have served a useful function. But using it for inappropriate comparisons and then decrying its utility sends the wrong public health message and serves only to amuse and confuse consumers.
