Body Fat Measurement: Percentage Vs. Body Mass

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What's the best measurement to assess health risks from being overweight? Experts say BMI and body-fat percentage both have their place

By John Casey
WebMD Weight Loss Clinic - Feature

Reviewed By Michael Smith, MD

Weight, body fat, body mass index -- what do all these numbers mean? And what do they really tell you about your health?

Some experts tout BMI, or body mass index, as the most accurate way to determine the effect of weight on your health. In fact, most recent medical research uses BMI as an indicator of someone's health status and disease risk.

The CDC provides the following ranges for BMI values for adults:

But others feel that body-fat percentage is really the way to go.

"The BMI numbers are way too general to be really useful," says Tammy Callahan, marketing manager of Life Measurement Inc., which manufactures a fat analyzer for use in gymnasiums and medical settings. "These numbers were developed using data from enormous numbers of people. They don't tell you anything about your own body composition, how much of your weight is fat, and how much is muscles and tissue."

But don't throw out that BMI chart just yet.

Are You At Risk?

"I'm not against people using devices to figure out fat percentages, but it is a well established fact that your BMI number does tell you a lot about your risk of diseases, especially heart disease and diabetes," says Harry DuVal, PhD, associate professor of exercise science at the University of Georgia in Athens. "Fat percentages just don't have enough research behind them yet to tell you how much risk of disease you face."

You're probably familiar with body mass index. BMI is an equation that gives you a numerical rating of your health based on height and weight. As your BMI goes up, so does your risk of developing weight-related diseases, such as heart disease and diabetes.

But even as more and more people are using their BMI number as an indicator of overall health, research on fat percentage is improving.

In September 2000, the American Journal of Clinical Nutrition published a study showing that body-fat percentage may be a better measure of your risk of weight-related diseases than BMI. Steven Heymsfield, MD, director of the Obesity Research Center at St. Luke's Roosevelt Hospital in New York, and his colleagues evaluated more than 1,600 people from diverse ethnic backgrounds. Researchers took body-fat measurements and studied how their body fat related to disease risk.

"Many studies have related BMI to disease risk," noted Heymsfield. "What we did was correlate body-fat percentage to BMI, allowing us to take the first big step toward linking body-fat percentage to disease risk. This new research reveals the value of assessing body fat more directly using the latest scientific technology to measure body-fat percentage," he added.

Although several research studies have indicated that an elevated BMI is associated with heart disease, stroke, type 2 diabetes and some cancers, BMI does not distinguish fat from muscle.

"If we think of BMI being a rough measure of body fatness, there are people -- especially some highly trained athletes -- who are overweight but not overfat," says Heymsfield. "Likewise, there are people who are of a normal weight according to BMI scales but who are overfat. BMI is a broad, general measure of risk. Body-fat assessment is much more specific to your actual fat content and thus provides a more accurate picture."

How Much Fat is OK?

The American Council on Exercise provides the following ranges for body-fat percentage: