Study: BMI Linked to Fat Percentage

First, they told us to weigh ourselves on scales.

Then, fitness experts said we should start calculating our “body mass index,” or a ratio of weight to height, to better measure our chunkiness quotient.

And now, just when you thought you had the math down, some physicians say a new measure, called body fat percentage, is a better indicator of health risks that may be associated with your flab.

Fat Better Than BMI For Health Risk

Shape-Up America, a non-profit organization started by former Surgeon General C. Everett Koop in 1994 to educate the public about healthy weight, is starting a campaign to promote the use of body fat percentage as a new marker for obesity risk. Their campaign coincides with the publication of a research article about the relationship of fat percentage to BMI in the Journal of Clinical Nutrition.

To calculate BMI, you multiply your weight in pounds by 703, then divide that number by your height in inches squared. For a person who weighs 149 pounds and is 5 feet 5 inches tall, his weight calculation, 104,747, divided by his height number, 4,225, gives a BMI of 24.8, a value associated with minimal health risk.

An increased BMI, however, is associated with an increased risk for heart disease, stroke, type-2 diabetes and several cancers, research has shown. In 1998, the World Health Organization and the National Institutes of Health declared BMI as the standard for obesity risk assessment.

But according to Shape Up America and study author Steven B. Heymsfield, deputy director of the Obesity Research Center at New York’s St. Lukes-Roosevelt Hospital, a BMI can misclassify someone’s health risk one out of four times, while fat percentage is a more direct measurement of your risk for disease.

Not All BMIs Are Equal

BMI does not, for example, take into consideration the differences in weight between lean muscle mass and body fat. Someone can have a high BMI, but his or her fat percentage could be low. Heymsfield says athletes, firefighters and members of the military often fail BMI and are deemed “overfat,” but actually are in excellent health and have a very low percentage of body fat.

Conversely, someone who looks skinny and has a so-called “healthy BMI” but a high fat percentage, might actually be at risk for disease and could be unaware he or she faces an increased risk.

When Heymsfield checked the medical literature to see if there had been any data to correlate BMI, body fat percentage and health risk, he could not find it. So he studied 1,626 healthy people from Japan, England and the United States and looked at their BMI and body fat percentage by sophisticated medical X-ray equipment.

“Many studies have related BMI to disease risk,” Heymsfield says. “What we did was correlate body fat percentage to BMI, allowing us to take the first big step toward linking body fat percentage directly to disease risk.”

People can be at risk if their BMI and fat percentage are too low, as with anorexics, or too high, as with the obese, the data shows.

While Heymsfield says obesity centers and some fitness facilities have high technology methods to measure body fat percentage, lower tech options exist. Fitness experts use either bioimpedance machines, which measure the resistance of an electric current in fluid, or calipers to measure body fat throughout the body and then place the numbers in equations that determine percentage. Several scales, he says, also are on the market that measure body fat percentage and can cost as little as $100.

In the interest of full disclosure, Heymsfield says he sits on the medical advisory board of Tanita Corp., a fat percentage scale manufacturer headquartered in Tokyo. His research, however, was funded by the NIH.

Harold Kohl, director of physical activity and nutrition at the International Life Sciences Institute in Atlanta, Ga., a sports medicine facility, agrees BMI is an imperfect measurement, but, he says, it remains the best highly correlative indicator of body fat and disease prediction.

“I am not sure that the scales available to consumers are as accurate as BMI for predicting risk,” Kohl says. “BMI is still a great predictor for the majority of the population, but not for all individuals. There is still a limit to fat percentage measures that the mass of the population can do without incurring great cost.”

As more technology does become available, Heymsfield predicts the direct measurement of body fat percentage will replace BMI as a health indicator because it is more accurate.