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'BMI' a Bust for Predicting Heart Risk

Body Mass Index May Not Be Useful in Predicting Risk From Heart Disease
WebMD Health News
Reviewed by Louise Chang, MD

Aug. 17, 2006 -- Obesity is a strong risk factor for heart disease, but the test most often used to measure obesity may be of little value in determining outcomes among heart patients, new research finds.

Body mass index (BMI) -- a ratio of weight to height -- proved to be a bust for predicting death from heart disease in an analysis of 40 previously reported studies involving 250,000 patients with heart disease followed for an average of four years.

The low-weight patients in the studies -- those with the lowest BMIs -- had the highest rates of death from heart disease and all other causes. Patients considered to be overweight, but not obese, had lower risk for death from any cause than patients whose BMIs fell in the normal range.

The seemingly paradoxical findings do not mean that carrying excess weight is good for heart patients, researchers say. But they do suggest that better ways of measuring obesity are needed.

The analysis is published in the Aug. 19 issue of the journal The Lancet.

"For many years we have used BMI to determine how fat people are," says researcher Francisco Lopez-Jimenez, MD, of the Mayo Clinic College of Medicine. "But it is increasingly clear that this measurement does not tell the whole story for patients with heart disease."

How to Calculate BMI

To understand why, it helps to understand BMI. A person’s body mass index is a comparison of a person’s height to weight. It is calculated by dividing weight (in kilograms) by height (in meters squared). But whether the weight is fat or muscle mass is not part of the equation.

Being underweight was strongly associated with an increased risk of death from heart disease or any cause. This was not a big surprise, Lopez-Jimenez says, because heart patients with the lowest BMIs tend to be older and frailer than patients who are heavier.

"Underweight patients often have very little muscle mass and they often have other health problems," he says.

The finding that overweight patients did not die as often and had fewer heart-related problems than normal-weight patients was more surprising. But the Mayo researchers say the answer may lie in muscle mass.

Since muscle weighs more than fat, it is possible that many of the people in the study who were considered overweight, with BMIs between 25 and 29.9, were really fitter with more muscle than the patients with lower BMIs. If this was the case, it would stand to reason that they would have fewer heart problems.

"I think the inability of the BMI measure to distinguish muscle weight from fat weight is an important reason for this finding," Lopez-Jimenez tells WebMD.

"Rather than proving that obesity is harmless, our data suggest that alternative methods might be needed to better characterize individuals who truly have excess body fat, compared to those in whom BMI is raised because of preserved muscle mass."

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