t’s hard to resist quick-and-dirty medical tests. After all, they’re fast, easy, cheap and noninvasive. The only problem is that their accuracy is limited. For the past several decades, the standard quick-and-dirty test for obesity has been the Body Mass Index (BMI): Multiply your weight in pounds by 703, and divide the result by the square of your height in inches. A normal BMI is 18.5 to 25; anything over that means you’re overweight. Sure it’s easy -- all you need is a calculator -- but is it accurate?
Flaws in the Formula
More and more, experts are acknowledging the limitations of the BMI formula. “When deciding whether someone is obese, there are other considerations besides BMI,” explains obesity specialist Arya M. Sharma, M.D., a professor of medicine at McMaster University School of Medicine in Hamilton, Ontario. These considerations include:
Athletic conditioning Since muscle weighs more than fat, ultra-fit athletes with lots of muscle and very little body fat can get BMI scores in the “overweight” category even though they’re fitter than the average person.
Age As you get older, muscle tends to get replaced by fat, which can cause BMI scores to skew low and understate your fat-related health risks.
Gender Women naturally carry more body fat than men, which can skew BMI scores low, understating a woman’s fat-related health risks.
Fat distribution Most important, BMI says nothing about where your fat is located. Research shows that people with a lot of abdominal fat (aka pot belly or apple shape) have a greater risk of diabetes, high cholesterol, high blood pressure and heart disease than people who collect fat on their hips, thighs and buttocks (aka pear shape).
In a study conducted at Ohio State University Medical Center, researchers studied 21,351 adults and found that almost half (45 percent) of participants with BMI scores in the normal range nevertheless had high blood pressure (hypertension). Measuring waist circumference, on the other hand, was twice as accurate in predicting hypertension, reports study participant Deborah A. Levine, M.D. And at McMaster University School of Medicine, Dr. Sharma surveyed 27,098 people in 52 countries and found that BMI did not predict heart attack risk -- but having a high waist-to-hip ratio did.
So if you take two people with the same BMI, one shaped like an apple, the other shaped like a pear, the apple-shaped person will have a higher risk of heart attack, even though their BMIs are identical.
Better than BMI
Given the limitations of the BMI formula, what’s the best way to determine whether you’re fit or fat?
Hydrostatic weighing This is the gold standard. Using special equipment, you’re weighed while completely submerged under water. The drawback: Hydrostatic weighing is rarely available outside of research institutions.
DEXA (dual energy X-ray absorptiometry) DEXA is the special X-ray used to determine bone density and osteoporosis. It can also be used to estimate body fat. But it requires very expensive equipment and trained technicians.
Bioelectrical impedance analysis This test passes a tiny, safe electrical current through the body. Until recently, this method required expensive equipment and a trained technician. Now, some health clubs offer it, and home test kits have become available, but their reliability is not entirely clear.
Skinfold measurement A tweezer-like caliper is used to measure folds of skin (and the underlying fat) at several places around the body. Like BMI, this test is quick -- but somewhat dirty. For the most accurate results, have it done by a trained doctor, nurse or technician.
Waist circumference This test doesn’t measure body fat, but it does provide an indication of your risk of fat-related health conditions. Simply measure your waist at the navel. “Risk of diabetes, high blood pressure and heart disease begins to rise with a waist measurement above 31 inches in women and 37 inches in men,” explains Harvey Simon, M.D., an associate professor of medicine at Harvard Medical School. “Measurements above 35 inches in women and 40 inches in men are in the danger zone.”
Waist-to-hip ratio This test is almost as quick as waist circumference, but less dirty. Divide your waist measurement by your hip measurement. In women, a waist measurement that’s more than 80 percent of your hips signals too much abdominal fat and an increased risk for health problems associated with obesity. In men, the figure is 95 percent (because men have narrower hips).
The Bottom Line
Today, BMI continues to play a key role in physicians’ diagnoses of obesity. However, more doctors are complementing it with other tests. “Personally, I now pay more attention to waist measurement than BMI,” says Dr. Simon. Dr. Sharma adds, “Not long ago, doctors put people on scales and measured their heights to calculate BMI and their risk of cardiovascular disease. Now we know they should forget the scale and take out a tape measure.”
Michael Castleman has been called "one of the nation's leading health writers" (Library Journal). He is the author of 11 consumer health books and more than 1,500 health articles for magazines and the Web.
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