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."
Fat vs. Fit
There is intriguing evidence that two alternative tests -- measuring waist circumference or waist-to-hip ratio -- may be better ways to distinguish between the fit and the fat.
Although BMI has been used in most studies, Lopez-Jimenez says the few studies that have calculated obesity using waist circumference or waist-to-hip ratio suggest that these measures are more predictive of poor health outcomes.
American Heart Association former president Robert Eckel, MD, tells WebMD that he considers waist circumference measurement a routine part of a patient exam.
"I still calculate BMI," he says. "But waist circumference may be a better indicator of cardiovascular disease risk that goes beyond BMI."
A study published late last year found waist-to-hip ratio to be a much better predictor of heart attack risk than BMI in many different ethnic groups.
Researcher Salim Yusuf, MD, and colleagues from the Population Health Research Institute at Ontario’s McMaster University concluded that BMI is a weak predictor of heart attack risk.
But Eckel says measuring BMI may be more beneficial than the studies suggest if patients who are considered overweight or obese end up being treated more aggressively with heart-protecting therapies.
"Someone with a BMI of 30 or above is likely to have other risk factors," he says. "It may be that the better outcomes among heavier people in this study could be explained by more aggressive treatment to control hypertension, LDL cholesterol, triglycerides [blood fats] and glucose [blood sugar]."