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.
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:
|
|
Women
|
Men |
|
Essential fat
|
10-12% |
2-4% |
|
Athletes |
14-20% |
6-13% |
|
Fitness |
21-24% |
14-17% |
|
Acceptable |
25-31% |
18-25% |
|
Obese |
32% plus |
26% plus |
"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."
SOURCES: Centers for
Disease Control and Prevention • American Council on Exercise •
Tammy Callahan, marketing manager, Life Measurement Inc. • Harry
DuVal, PhD, associate professor of exercise science, University of
Georgia • American Journal of Clinical Nutrition, September
2000 • Barbara J. Moore, PhD, president of Shape Up! America.
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