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
From the WebMD Archives

Trying to get in shape? Then don't depend on your bathroom scales. To get the most accurate measure of your progress, experts say, you need to track your body 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 to strive for," Bryant says. "The only way to know how you're doing is through 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 very good new devices are available to measure your body fat.

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

The BMI Test

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

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

Price: None.

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

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

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

Body Fat-Measuring Scales

"Bioelectrical impedance analysis" has been added to traditional bathroom scales. The scales send a harmless electrical current up through your body 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 most accurate.

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

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

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

DEXA Scanning

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

Price: $200 to $300

The verdict: Looking good.

DEXA is "an emerging technique that holds a lot of promise," Bryant tells WebMD. "It allows us to determine the amount of body fat overall, and to 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 be offering DEXA scanning to assess body fat, Bryant tells WebMD. "If your BMI says you're in the obese category and you have a strong family history of heart disease and diabetes, it might behoove you to get more precise assessment of body composition," says Bryant.

Grade: A. "It's one of the most accurate methods out there," says McCrory. "I haven't heard any news about DEXA in health clubs. 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 narrow tables used for this test.

It's "quite noninvasive," says Kravitz. "Very good technique."

Underwater Testing

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

"This test is considered the gold standard, the most accurate assessment technique," Bryant tells WebMD. Universities use this primarily with athletes, 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 body fat," says Bryant. But going into the water can be a problem. Some find the procedure "disconcerting."

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

Bod Pod

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

Price: $40 to $65 per test.

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

Grade: A. "It's much easier and more convenient than underwater weighing," says McCrory. "It is about as accurate and reliable as DEXA, is much cheaper, and is becoming more widely available."

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 seamstress would," says Bryant. "This is taken at the belly button level."

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

Price: None.

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

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

Skinfold Calipers

Health clubs offer this test; it's the most widely used method for measuring body fat, says Bryant. Basically, it's a "pinch" test using a measuring device 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 person giving you the test. "The skinfold test can be reasonably accurate," Bryant tells WebMD. "But if the tester isn't experienced, or if they're using cheapo plastic calipers, take it with a grain of salt. It will be terribly unreliable."

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

Infrared Light Measuring

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

Here's what happens: A probe is placed on a body site -- the biceps, for instance -- sending an infrared light ray through both fat and muscle. Your height, weight, sex, age, frame size, and activity level are factored in. The final number is a "rough estimate" of your body fat percentage, says Bryant.

Price: $25 to $50 per test.

The verdict: "It hasn't proven to be terribly accurate," 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 many insurance companies. But the experts say they just don't work very well, even if 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 are based on a limited sample of the population and can be misleading."

Grade: F. "These do nothing to help us understand body composition," 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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