Does Body-Mass Index Really Predict Heart Risk?

The widely used body-mass index (BMI) is not an accurate indicator of cardiovascular risk for people with heart disease because it does not distinguish between muscle and fat, a major review of the data contends.

BMI is determined by dividing a person's weight in kilograms by height in meters squared. Doctors have typically gauged a person's weight-linked risk for heart trouble by using their BMI.

However, a new analysis of 40 studies involving over 250,000 patients showed that those with a BMI under 20 were at higher risk of death from cardiovascular disease than those with "obese" BMIs of 30 to 35.

"An explanation for the lack of a positive association with BMI and mortality in older ages is that, in older persons, BMI is a poor measure of body fat," concluded a team of researchers from the Mayo Clinic, who published their findings in the Aug. 19 issue of The Lancet. "The measurement of weight does not differentiate between fat and fat-free mass, and fat-free mass (especially muscle) is progressively lost with increasing age."

Lead researcher Dr. Francisco Lopez-Jimenez, a consultant in the Mayo cardiovascular division, said his suspicions regarding the fallibility of BMI began a while ago. "One of the studies [included in this review] was ours, which we did about four years ago," he said. "At that time, we had an idea that the BMI was not the best way to assess cardiovascular problems. We faced resistance from publishers at that time, but we found other studies confirming it."

Next, he said, "We presented a report at an American Heart Association meeting a few months ago showing that BMI did not correlate with fat. A better way to distinguish between fat and muscle is to take a cross-sectional view of the abdomen, to focus on the waist-hip ratio."

A British study published this month in the American Journal of Clinical Nutrition gave further evidence for that view. Researchers at the London School of Hygiene and Tropical Medicine studied nearly 15,000 patients over age 75 and concluded that it is not appropriate to use BMI to calculate death risk in this age group. They noted that the health risks associated with a high BMI are known to decline with age.

By contrast, the researchers said waist-hip ratio "is a measure of body shape and, to some extent, of lower trunk adiposity [abdominal fat]."

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Bonnie - I have never used BMI in my practice and will not do so in the future.