Which matters more: body fat or BMI? Which is the more important gauge for slimming down: body mass index or percentage of body fat? We answer three of your body-measurement questions here

Suzanne Schlosberg

Q What is the difference between BMI and body-fat percentage?

A Although both are ways of estimating how “fat” a person is body-fat percentage is a more useful tool because it distinguishes between the weight of fat and that of your lean body mass–bones, organs, muscle and connective tissue. But there are no hard and fast rules on the ideal percentage of fat, says exercise scientist Larry Tucker, Ph.D., director of the Body Composition Laboratory at Brigham Young University in Provo, Utah: Some say the upper limit of healthy for women is 32 percent; others argue for 35 percent.

However, the location of the fat is probably more important than the amount of fat, Tucker says. Most methods of body-fat testing don’t reveal how much fat is located in the abdominal area and how much is located elsewhere. Excess deep abdominal fat, clumped around your organs, is linked to increased risk of diabetes, heart disease and other serious conditions, whereas ample hips and thighs likely pose fewer health risks.

As a measure of disease risk, your waist circumference may be more useful than either BMI or body-fat percentage. Women with a waist measurement greater than 35 inches are at higher risk for the diseases above, according to the National Institutes of Health.

If you’re trying to slim down, body-fat percentage can be a helpful way to gauge your progress; however, Tucker emphasizes, “there’s a lot of potential error in body-fat measurement.” Even the most accurate methods, including underwater weighing and a DEXA body scan, have a margin of error of about 1-3 percent when performed by even the most experienced technicians. Less-accurate methods, such as skin-fold calipers and bioelectrical impedance (see question on next page), can have a margin of error of 4 percent–even greater if the tester doesn’t do everything precisely right.

Q I just got a fancy scale that gives your body mass index. Being 5-foot-5 and 140 pounds, I was expecting my BMI to be in the low 20s. To my surprise, it turned out to be 29.6, as opposed to the 23.3 I got using Shape.com’s BMI tool. According to one reading I am healthy; the other says I’m overweight. Which should I believe?

A “The 23.3 is correct,” says body-fat expert Larry Tucker. One way to verify the number is to do the math yourself, plugging your height and weight into the formula for BMI: weight in pounds X 704.5; divided by [height in inches X height in inches]. In your case that would be: 140 pounds X 704.5 = 98,630, which, divided by 65 X 65, comes to 23.3.

However, Tucker notes, don’t rely on your BMI results to determine whether you are healthy. It’s important to consider these numbers in conjunction with other health factors such as your levels of activity, blood pressure and cholesterol, and how much body fat you have and where it’s located. Because BMI takes into consideration only height and weight, it can be inaccurate if a person has a lot of muscle mass, which weighs more than fat. “A lean, muscular athlete may register as obese on the BMI chart,” Tucker says. (Someone with a BMI of 25 or greater is considered overweight, and a BMI of 30 or more is considered obese.)

BMI also may be inaccurate for people who have little muscle. “As people get older, their muscle mass tends to decline,” Tucker says. So a person’s weight–and BMI–may stay the same even though a greater proportion of the person’s weight is fat.

Q This morning I got a body-fat analysis done at my gym. I stood barefoot on a high-tech scale and received a quick printout, but nobody explained the results. What are TBW, FFM, BMR and impedance?

A TBW is an acronym for “total body weight” and FFM is short for “fat-free mass,” the weight of everything in your body that is not fat. BMR means “basal metabolic rate” (aka RMR, or resting metabolic rate)–in other words, “how much energy [in calories] your body would expend if you were just lying still all day and hadn’t eaten for at least 12 hours,” says Tony G. Babb, Ph.D., a research scientist at the Institute for Exercise and Environmental Medicine at the University of Texas Southwestern Medical Center in Dallas.

The BMR number that appeared on your body-fat test results was predicted from such measures as your age, gender, total body weight and body composition; however, Babb emphasizes, it’s only a rough estimate. “Measuring oxygen consumption is more accurate, but you’d have to breathe through a special mouthpiece in a lab,” he adds.

Impedance is the term for the method used by the body-fat scale you stood on. Also known as bio-impedance analysis, it sends an alternating current through your body from one foot to the other. The faster the electrical signal travels, the more muscular you are. That’s because water conducts electricity, and muscle contains significant amounts of water; fat contains virtually no water, so it impedes the signal. The scale uses the speed of the signal to calculate your body-fat percentage.

This method of testing is very sensitive: How well-hydrated you are when the test is done, and your temperature, prior exercise and meals, all can affect results. Also, impedance devices are typically less accurate for people who are obese or very lean. For best results, get tested first thing in the morning, before you’ve eaten or exercised but about a half-hour to an hour after you’ve had a glass of water. Also, avoid testing during menstruation because some women retain water during that time. Even then, Babb says, “take the results as a gross estimate.”

Send your questions to Shape, Weight-Loss Q & A, 21100 Erwin St., Woodland Hills, CA 91367; fax: (818) 704-7620; e-mail: WeightLossQ&A@Shape.com.

Suzanne Schlosberg is the author of The Curse of the Singles Table (Warner Books, 2004).

COPYRIGHT 2004 Weider Publications

COPYRIGHT 2004 Gale Group

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