Does Body-Mass Index Really Predict Heart Risk?
New data suggests BMI can't distinguish between fat, muscle
THURSDAY, Aug. 17, 2006 (HealthDay News) -- 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. A BMI between 18.5 and 24.9 is considered healthy, while over 25 is considered overweight and over 30 is considered obese.
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]."
In other words, waist-hip ratio is better than BMI at spotting the "pot belly," a notoriously unhealthy pattern of fat deposition as it relates to cardiovascular function.
"We conclude that the association observed for waist-hip ratio and mortality is probably explained by abdominal adiposity," the U.K. group added.
But even though waist-hip-ratio may be a better marker than BMI in most cases, BMI is still an important indicator of risk for people who are very obese, Lopez-Jimenez said.
"The group of people with BMI of 35 or higher had a bad outcome," he said. Oftentimes, he said, "patients with BMIs of 35 to 40 naively believe that they have a lot of muscle. In [these] extreme cases, fat determines the measurement."
One expert urged caution in interpreting the results of the new study.
Dr. Robert H. Eckel, professor of medicine at the University of Colorado and immediate past president of the American Heart Association, noted that people in the study were followed for a relatively short time -- a bit less than four years on average.
"You need a much longer period to determine risk," Eckel said. "Four years would not be enough to determine the impact of measures taken, such as controlling high blood pressure. We still want to prevent obesity, despite what this study shows and might not show."
To calculate your own BMI, head to the U.S. Centers for Disease Control and Prevention.