Body fat may be more significant than weight in predicting health risks

Your body’s fat percentage is a better predictor of health risks than the number you see on the scale, confirms a new study.

Body mass index — a ratio of weight to height — is still widely used in doctors’ offices. But it’s an imperfect measure of body fat and, therefore, researchers say, categorizes some muscular people as being overweight.

Now, a new study of more than 50,000 middle-aged and older adults reveals that a high body fat percentage, but not high BMI, is a risk factor for near-term death. Researchers examined the medical charts and X-rays of people in Manitoba, Canada, who had gotten osteoporosis screenings between 1999 and 2013. They calculated BMI based on the patient’s medical chart, and body fat based on the area seen in the scan.

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The study found that high body fat percentage — defined as greater than 38 percent fat in women and greater than 36 percent fat in men — was associated with increased mortality over an average of 4-6 years.

The study also found that people with a low BMI face a greater risk for death. That may be because low muscle mass reflects unhealthy weight loss, or the frailty associated with chronic disease. The results were published Monday in Annals of Internal Medicine.

“Healthy body composition is not just thinness,” said study author Dr. William Leslie at the University of Manitoba.

The study’s wider application is limited in a few ways. Most of the participants in the study were white females, and the study didn’t track participants’ BMI or body fat over time. Some relevant data, such as whether participants smoked, wasn’t collected.

Despite these limitations, Leslie believes the study had general implications for both patients and physicians. “It attunes them to the fact that weight and BMI are not the be-all and end-all,” he said.

Though the average person won’t have access to high-tech X-ray machines, Leslie said assessing body fat composition “can be as simple as just looking at somebody” or measuring waist circumference. Skin tissue folds or low overall fitness would also be signs of more fat.

Dr. Shanna Levine, instructor at the Icahn School of Medicine at Mount Sinai, who was not involved in the study, agrees with its conclusion that BMI is not a good surrogate for fat. When counseling her patients, Levine advises them not to fixate on the bathroom scale but to focus on how their clothes fit because that’s a better indication of their body composition and weight distribution.

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