'BMI' a Bust for Predicting Heart Risk
Body Mass Index May Not Be Useful in Predicting Risk From Heart Disease
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.
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
"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
"I think the inability of the BMI measure to distinguish muscle weight
from fat weight is an important reason for this finding," Lopez-Jimenez
"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."