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Bathroom Scales Don't Tell The Whole Story

Experts rate the best and worst in body-fat measurement devices.

Reviewed by Kathleen M. Zelman, MPH, RD, LD on June 09, 2005

Trying to get in shape? Then don't depend on your bathroom scales. To getthe most accurate measure of your progress, experts say, you need to track yourbody fat as well as your weight.

"Most people focus only on losing weight, not on the fat," Cedric X.Bryant, PhD, chief exercise physiologist with the American Council on Exercise,tells WebMD.

"Preserving lean tissue and losing body fat -- that's what you need tostrive for," Bryant says. "The only way to know how you're doing isthrough some form of body-composition assessment."

You know about the old standard measuring tools, like the body mass index(BMI) and the tape measure. And thanks to today's technology wizards, some verygood new devices are available to measure your body fat.

To learn which are worth your time and money, WebMD got ratings from Bryantand from two more top exercise physiologists: Megan McCrory, PhD, an energymetabolism scientist with the Jean Mayer USDA Human Nutrition Research Centeron Aging at Tufts University in Boston; and Len Kravitz, PhD, senior exercisephysiologist for IDEA Health and Fitness Association.

The BMI Test

This is a simple calculation, using the most basic tools -- your height andweight. Plug these numbers into a BMI calculator to learn whether you areobese, overweight, or normal weight.

The BMI was developed using large, population-based studies. Though itdoesn't address percentage of body fat or muscle, it helps health careprofessionals quickly assess which patients may be at risk of health problemslinked to excess weight.

Price: None.

The verdict: Free and readily available; good for assessinghealth risks but doesn't measure body-fat percentage. If you are short, or verymuscular, results tend to be less accurate.

"It's a good starting point, a really good way to get a basic estimateof whether you are overweight or not," says Bryant. "BMI tends tocorrelate pretty closely with health risks associated with being overweight orobese."

The experts' grade: D. "The BMI doesn't give you bodyfat measurement," says McCrory. "But if gives an excellent BMImeasurement!"

Body Fat-Measuring Scales

"Bioelectrical impedance analysis" has been added to traditionalbathroom scales. The scales send a harmless electrical current up through yourbody to "read" the amount of fat body mass and lean body mass --calculating your percentage of body fat.

Price: $50 to $100 per scale.

The verdict: Convenient, but not always the mostaccurate.

"The problem is, these devices are very sensitive to hydration -- howmuch fluid is in your body," Bryant tells WebMD. So it's important tostrictly follow the guidelines for weighing yourself -- time of day, fluid andfood intake. Even your menstrual cycle affects this reading. "However, withall this factored in, the scales are an easy, at-home way to keep track of yourweight and fat-loss progress."

There also are handheld versions that use this same technology. Justremember: You get what you pay for. Higher price equals greater accuracy.

Grade: C+. "Even though they may not be accurate, itmay be good for tracking changes with a diet and exercise program," saysMcCrory. "Just keep in mind that the scales might be off by 5%, plus orminus. Follow the instructions carefully. Taking a shower beforehand reallymakes the reading inaccurate!"

DEXA Scanning

DEXA is "dual energy X-ray absorpitometry" -- the same imagingtechnology doctors use to measure bone density to determine osteoporosis risk,explains Bryant. During the test, you lie on an X-ray table for about 10minutes while the scanner measures your body fat, muscle, and bone mineraldensity.

Price: $200 to $300

The verdict: Looking good.

DEXA is "an emerging technique that holds a lot of promise," Bryanttells WebMD. "It allows us to determine the amount of body fat overall, andto identify fat deposits in specific body regions. That's very important,because stores of body fat can be much more indicative of disease risk."For example, extra abdominal fat increases the risk of heart disease, cancer,and type 2 diabetes.

Primary-care doctors, physical therapists, and health clubs will soon beoffering DEXA scanning to assess body fat, Bryant tells WebMD. "If your BMIsays you're in the obese category and you have a strong family history of heartdisease and diabetes, it might behoove you to get more precise assessment ofbody composition," says Bryant.

Grade: A. "It's one of the most accurate methods outthere," says McCrory. "I haven't heard any news about DEXA in healthclubs. But if you have the opportunity to be tested by DEXA, go for it."She warns, however, that obese people may have a hard time lying on the narrowtables used for this test.

It's "quite noninvasive," says Kravitz. "Very goodtechnique."

Underwater Testing

Also called hydrodensitometry testing, this involves getting into a tankfilled with water. Based on the amount of water you displace, your body densityand body fat can be calculated.

"This test is considered the gold standard, the most accurate assessmenttechnique," Bryant tells WebMD. Universities use this primarily withathletes, and will likely let you try it, too -- for a small fee.

Price: $25 to $75 per test.

The verdict: "It's a very accurate way to measure bodyfat," says Bryant. But going into the water can be a problem. Some find theprocedure "disconcerting."

Grade: B-. Inconvenience is a big issue here, agreesMcCrory. "My guess is that underwater testing will be a 'has been' in a fewyears."

Bod Pod

The Bod Pod is a new tool that relies on air displacement to determine bodyfat, says Bryant. There's no submersion; you don't get wet. But you have to getinto the Bod Pod chamber, be very still, control your breath ­ all factors thatcan affect the results. Your hydration level before the test can also affectresults. "When all these are controlled pretty well, you'll get a body fatcalculation that's within 3% to 4% accurate ­ not as high as one mightexpect," Bryant says.

Price: $40 to $65 per test.

The verdict: McCrory says she believes it may be the way ofthe future, though Bryant notes that it needs some refinement.

Grade: A. "It's much easier and more convenient thanunderwater weighing," says McCrory. "It is about as accurate andreliable as DEXA, is much cheaper, and is becoming more widelyavailable."

The Tape Measure

It's one of the oldest "obesity tests" known to mankind. However,waist circumference in this context "is not defined as a seamstresswould," says Bryant. "This is taken at the belly button level."

Men with measurements higher than 40, or women with waist measurementshigher than 35, are considered obese, he says.

Price: None.

The verdict: This is a basic indicator of a body fatproblem, says Bryant. "It's a good technique," says Kravitz.

Grade: A. Girth measurement is "accurate andreliable" for assessing your risk of chronic diseases like diabetes, heartdisease, stroke, and cancer, says McCrory. Quite simply: The bigger the girth,the bigger the risk. "However, some new research is coming out suggestingthat there is more risk than previously thought at even lower riskcircumferences."

Skinfold Calipers

Health clubs offer this test; it's the most widely used method for measuringbody fat, says Bryant. Basically, it's a "pinch" test using a measuringdevice at several points on the body, like thighs, hips, and upper arm.

Price: $20 to $40 per test.

The verdict: Much depends on the skills of the persongiving you the test. "The skinfold test can be reasonably accurate,"Bryant tells WebMD. "But if the tester isn't experienced, or if they'reusing cheapo plastic calipers, take it with a grain of salt. It will beterribly unreliable."

Grade: D. "These are rarely done correctly," saysMcCrory. "The technician usually does not grab enough fat so the result isusually a big underestimate of body fat. It's also difficult to grab the fatconsistently."

Infrared Light Measuring

Infrared light measuring is an inexpensive way to measure body fat with asoil-analysis-type device that agronomists use, Bryant explains.

Here's what happens: A probe is placed on a body site -- the biceps, forinstance -- sending an infrared light ray through both fat and muscle. Yourheight, weight, sex, age, frame size, and activity level are factored in. Thefinal number is a "rough estimate" of your body fat percentage, saysBryant.

Price: $25 to $50 per test.

The verdict: "It hasn't proven to be terriblyaccurate," Bryant tells WebMD.

Grade: F. Don't waste your time or money, says McCrory.

Height/Weight Charts

These are the simple height-vs.-weight tables used for years by manyinsurance companies. But the experts say they just don't work very well, evenif they take body frame and sex into account.

Price: None.

The verdict: "These charts have significant limitations,"says Bryant. "They really aren't measuring fat-to-lean tissue. They arebased on a limited sample of the population and can be misleading."

Grade: F. "These do nothing to help us understand bodycomposition," Kravitz says.

Show Sources

SOURCES: Cedric X. Bryant, PhD, chief exercise physiologist, American Council on Exercise. Megan McCrory, PhD, energy metabolism scientist, Jean Mayer USDA Human Nutrition Research Center on Aging, Tufts University, Boston. Len Kravitz, PhD, senior exercise physiologist, IDEA.

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