The Buzz: BMI

Some of my clients have asked me my opinion about BMI, or Body Mass Index. I don't mention it often or use it to measure their progress, but it's still making headlines in the news and magazines, nonetheless.

Fitness Matters, the American Council on Exercise's professional journal, does a great job of
describing the limitations of using BMI in its May / June issue.

BMI has several significant drawbacks. It can give a skewed result for some individuals and subgroups of the population. Jay Hoffman, a professor of health and exercise science at the College of New Jersey, says that BMI's main drawback is that "it just indexes height versus weight and doesn't take into account body composition."

Bodybuilders and elite athletes end up with a high BMI score because of the extra weight associated with muscle. So do fit civilians. (The article's author, Jim Gerard notes that while he is) hardly a body builder, but (he) work(s) out every day and at 6'2" and 210 pounds, (he has) a BMI result that puts (him) in the "overweight" category despite the fact that he has a 34" waist and a flat stomach. Conversely, taller people carrying a lot of fat may have a body mass index score that falls within the acceptable range.
BMI can also skew results in the opposite direction, as it does in the elderly, who tend to have scores that underestimate their excess weight or obesity because they have much less muscle than the average person.
Miller adds that BMI is not accurate when measuring the body composition of children or some small-boned women who, despite being petite, may have a small bone and muscle mass and a high percentage of body fat.
Another limitation of BMI is that it doesn't address the location of one's body fat, which is arguably a more telling indicator of overall disease risk. Abdominal fat in particular is linked o heart disease, poor lipid profiles and type 2 diabetes. And it's more likely to lead to changes in hormone levels that cause inflamed and eventually clogged arteries. Miller says, "The theory is that abdominal fat is more mobile and that it enters the circulation more readily to form arterial plaques."
Therefore, it's quite possible to have an acceptable BMI while carrying a potentially dangerous spare tire. And certain groups of people--those from Japan and south Asia, for example--that tend to stay relatively slender can still have an increased risk of heart disease from storing unhealthy amounts of abdominal fat.