Weighing In on Body Fat

Reviewed by Charlotte E. Grayson Mathis, MD
From the WebMD Archives

Aug. 29, 2000 (Washington) -- The numbers on the scale used to be the gold standard measurement of obesity. Then came the body mass index or BMI -- a ratio of height and weight currently used to determine healthy body weight. But now a new study weighs in and suggests that simple body fat measurements may be best. Some experts believe that body fat percentage may be a more accurate indicator of good health than BMI.

New research published in the September issue of The American Journal of Clinical Nutrition links BMIs to corresponding healthy levels of body fat, apparently for the first time. "We found that based on BMI, that when we measured ... percent body fat, approximately one-fourth of the subjects we evaluated were misclassified," says study coauthor Steven B. Heymsfield, MD, deputy director of the obesity research center at St. Luke's-Roosevelt Hospital Center in New York.

According to Heymsfield's study of over 1,600 patients, a man aged 40 to 59 could have up to 24% body fat and still be healthy while a women of the same age could go all the way 38% body fat without a problem.

There are various ways BMI could lead a doctor to the wrong conclusion about a patient's body fat, says Heymsfield. A healthy BMI is considered somewhere between 18.5 to 24.9. But an athlete with lots of muscle tissue could have a considerably higher number and not be overweight. Others might have an acceptable BMI but still be "overfat."

Too little fat can also be a problem, says Heymsfield. In that case, women may have their menstrual cycles interrupted as well as develop osteoporosis, a dangerous thinning of bone.

The researchers offer up George Washington as an example of BMI misclassification. They point out that the father of our country would have an unhealthy BMI of 25.7 based on his height and weight, but he couldn't be considered obese because of his active lifestyle.

Former U.S. Surgeon General C. Everett Koop agrees, saying that Americans need to find another way of measuring obesity besides using BMI. Instead, he suggests that "every American ... know and monitor his or her percent body fat throughout our life, just as they currently monitor blood pressure readings or their cholesterol level." Koop made this recommendation at a news conference held here Monday.

Koop's nonprofit organization, Shape Up America!, says that being overweight kills some 300,000 Americans annually by contributing to a variety of chronic diseases. However, by recognizing those who are truly at risk, they say the toll could be reduced considerably.

However, Koop insists that he's not giving people an excuse to claim that they're really in better shape than the scale or mirror would indicate by focusing on yet another number. "This is not a game we're playing. ... We are truly interested in the health of America."

Still, it isn't easy to figure out your body fat percentage without seeking professional help. Shape Up America! Offers a "Body Fat Lab" on its web site to identify a "healthy range of body fat," says Koop. A scale-like device also provides what's called a "bio impedance analysis" for those who want to monitor their progress daily. Koop says this is an instance where patients will often learn about body fat percentage before their doctors.

Professional dieticians contacted by WebMD believe the current fat measurement approaches are adequate if used properly. "BMI is a tool, just like the body fat percentage is a tool, just like weight is a tool. None of them are going to be the be all and end all," Cindy Moore, MS, RD, director of nutrition therapy at the Cleveland Clinic Foundation in Ohio, tells WebMD.

"I think BMI is good enough ... It's the best we have without doing anything fancy," says Nelda Mercer, RD, who works with the University of Michigan health program.