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:
|Underweight Less than 18.5|
Recommended 18.6 to 24.9
Overweight 25.0 to 29.9
Obese 30 or greater
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. (To figure your BMI, use our calculator.)
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.
"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:
"What we want people to shoot for is a range rather than a magic number," says Barbara J. Moore, PhD, president of Shape Up! America. "It's comforting to know that women can be and should be fatter than men. They have a totally different reproduction function and the higher fat in women supports that reproductive function."
But not all measures of fat percentage are equal. Some methods have high error rates. The two most common methods used are skin-fold measurement and bioelectrical impedence analysis
In skin-fold measurement, a trained specialist uses calipers to measure specific spots on the body. These measurements are compared to a chart that estimates fat percentage. You may have seen this used in your gym or doctor's office. These skin-fold devices can also be purchased and used at home. However, the accuracy of this method varies greatly based on the user's abilities. Bioelectrical impedance analysis, the other common method, is the technology behind the many fat percentage scales sold for home use.
"The error rates for these can be as high as 8%, plus or minus," says DuVal. "Other methods are highly accurate but much more complicated, like X-ray analysis, water displacement, and others. That's why BMI has its place in weight management. It may be crude, but it does give you a good idea of risk quickly and easily."
DuVal says that fat-percentage measurement, despite the inaccuracies, can be useful.
"In terms of ease-of-use and usefulness, the BMI can't be beat," he says. "But if a home, fat-measurement device helps someone stay focused on their diet and exercise level and motivated to stay healthy, then I think the device has a place in weight management."