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20 Desember, 2009

About your BMI

For years, you've used 2 numbers to manage your weight — the fluctuating one on your bathroom scale and the one on the tag of your favorite pair of jeans.

And then there's the body mass index (BMI). Although much emphasis has been put on this number, the question is: How meaningful is it, really?

The truth is, BMI — a statistical measure of the relationship between a person's weight and height — is useful when generalizing about groups of people. It has been used to classify people as underweight, normal weight, overweight and obese.

An adult with a BMI between 18.5 and 24.9 is considered within the normal range, according to the National Institutes of Health (NIH) in Bethesda, Md.

The NIH says a BMI between 25 and 29.9 is considered overweight, and a BMI of 30 and above is considered obese. (Calculate your BMI here. You'll need to know your height and weight.)

Generally speaking, groups of people with BMIs of 25 and above face a higher risk of serious health problems such as type 2 diabetes, cardiovascular disease, certain cancers and many other conditions. Risks are compounded as BMI increases.

But this number is not always accurate when applied to individuals, especially athletes, children, people who have lost muscle mass and the elderly. The biggest criticism of BMI is this: It fails to distinguish between muscle and fat. What's more, it doesn't measure body fat.

"BMI is a way of estimating your density," but it doesn't consider how your body's put together," says Stephen G. Rice, M.D., Ph.D., director of the Jersey Shore Sports Medicine Center in Neptune, N.J. "If you had huge muscles, you'd weigh more since muscles are dense," he says. Therefore, it's important to look at a person rather than just at his or her BMI number.

"Muscle is 15% heavier than fat," says Robert Girandola, Ph.D., an associate professor of kinesiology at the University of Southern California (USC) and a nutrition and obesity expert. "So if you do resistance training and are very active in sports, it's possible your BMI may be high while your body fat is low."

Therefore, based solely on his BMI, a muscular athlete like Arnold Schwarzenegger in his prime would be defined as obese.

Looking just at BMI is problematic for another group as well — postmenopausal women who lose muscle mass as they age. In those cases, a low BMI may underestimate a woman's body fat. "I have a patient whose BMI is 23" — well within the normal range, says Zhaoping Li, M.D., an associate professor of clinical medicine at the University of California, Los Angeles Center for Human Nutrition and its director for clinical research.

"But her body fat may be 39 [%]. She has no muscle." For women, the average body-fat percentage — remember, that's a different measure from BMI — is 25% to 31%. Thirty-two percent and higher is considered obese.

BMI also fails to consider where fat is deposited, which determines whether someone's body shape is identified as an apple or a pear. Generally, apples — those with excess abdominal fat — are more at risk for obesity- and heart-related health problems than those who are pear-shaped. Conversely, people who carry their fat on their thighs and buttocks have a lower incidence of health risks.

Ethnicity also must be taken into account. For instance, when compared with whites, Asians have lower BMIs but higher percentages of body fat, according to The American Journal of Clinical Nutrition. This means that Asians may be considered overweight with a BMI over 22.9

A better measure of fat

"We need some way of measuring body fat independent of BMI," Rice says. "The number alone without context is just a number." He points to more accurate tests of body composition like being weighed underwater and using calipers to measure skin fold thickness on various parts of the body.

Rather than simply estimating health risks related to body weight and shape, there is a simpler calculation that combines 2 easy-to-get numbers.

"For adults, a better tool is a combination of BMI and waist circumference," says Michael L.Goran, Ph.D., a professor of preventive medicine, physiology and biophysics at USC's Keck School of Medicine and associate director at its Institute for Health Promotion and Disease Prevention Research.

According to the Mayo Clinic's guidelines, women's waist measurements should be less than 35 inches, while men's should be less than 40 inches. If these figures are combined with BMI, together they can be a more accurate predictor of disease risk.

Taking action

To get a more accurate picture of your disease risk than BMI alone, follow these steps:

  • Measure the circumference of your waist at its widest part with your belly relaxed.
  • Use the 2 figures of BMI and waist circumference to find a more accurate estimation of your disease risk by consulting the chart created by the National Heart, Lung and Blood Institute, a part of NIH. According to the chart, a man considered "overweight" by virtue of a BMI between 25 and 29.9 and a waist size of 38 inches is at "increased" risk of disease. But if the same man's waist is 40 inches, he moves into the "high" risk group.

Reviwed by: Stacy Beller Stryer, M.D.


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