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When determining who’s fat, is BMI bunk?

A surprising new study finds that some people with a body mass index that indicates they're overweight may actually have little body fat — and some folks whose BMI categorizes them as skinny actually are way too flabby.

Are you fat? The answer may depend on which test you take.

If you’re going by your body mass index, or BMI, a measure that factors in your weight and height, you are considered overweight if that score is 25 to 29, and obese if it’s 30 or higher. But a surprising new study finds that some people with a BMI pushing 28 actually have little body fat — and some folks with a BMI as low as 24 have too much.

The results question the validity of BMI, the most common measure for determining who needs to shed some pounds, says study author James Pivarnik, a professor of kinesiology and epidemiology at Michigan State University in East Lansing.

“If you’re going to classify a person as overweight by BMI, depending on who you’re working with, that may not be the best way to do it,” he says.

While prior research has found that BMI isn’t always an accurate indicator of fatness in athletes, who may be more muscle than fat, the new study is one of the first to show that BMI may not necessarily work for the general population either.

Other experts say they’ve seen this firsthand with clients, and that clearly BMI isn’t the best test for everyone. “I don’t think it’s accurate enough,” says Dr. Kenneth Cooper, founder of the Cooper Aerobics Center in Dallas.

Pinch an inch?
At his facility, trainers prefer to rely on skin-fold tests that use fat calipers to measure body fat at various points, such as the back of the arm, abdomen and thighs. In addition, they use underwater weighing, a common lab test that determines how much of a person’s body is fat and how much is muscle. Other centers also use a measure called the waist-to-hip ratio, which assesses abdominal fat. Some fat is worse than others, and that around the middle is among the deadliest.

But researchers studying large populations of people rely on BMI because it’s an easy figure to calculate — they just ask people how much they weigh and how tall they are, and then do the math (weight in kilograms divided by height in meters squared). Averaged across many people, BMI is a good indicator of morbid obesity, Pivarnik says. People with a BMI pushing 40, for instance, are bound to be carrying too much around the middle, and elsewhere.

But when you look at certain individuals, BMI may be way off the mark.

In the new study, published in Medicine & Science in Sports & Exercise, the researchers calculated both BMI and body fat percentage for 439 college students.

To determine body fat, they used a BOD POD — a laboratory test that mimics underwater weighing but requires subjects to sit in a dry chamber rather than getting in water.

Study results showed that male and female college athletes, as well as male non-athletes, could have a BMI suggesting they were overweight yet still have healthy levels of body fat, defined as less than 20 percent fat in men and 33 percent in women. On the other hand, non-athlete women with a BMI indicating a normal weight could have too much body fat.

Pivarnik says large amounts of heavy muscle mass in the athletes accounts for the higher BMI, yet the athletes had low body fat because they were in shape. Even young non-athlete men could be muscular and fit yet not overly fat.

For women, the study shows, thin isn’t everything. Those who were slim yet didn’t work out to build muscle still could be quite fatty.

Pivarnik says he worries that some people, particularly young women, may find that pumping iron puts them into the overweight category per BMI, so they skip weight training altogether.

“Don’t worry about the thinness,” he says. “Worry about the working out part.”

Model thin but flabby
Indeed, many experts say that even if you can’t be model thin, it’s important to exercise.

“It’s better to be fat and fit than skinny and unfit,” says Cooper. Research at his center and elsewhere has shown this to be true.

And if you get a BMI suggesting you’re overweight, don’t freak out, says exercise physiologist Gerald Endress, fitness manager at Duke Diet and Fitness Center in Durham, N.C.

Instead, he says, consider getting another test, such as a skin-fold, an underwater weighing or waist-to-hip ratio to more accurately determine your body fat percentage.

Ultimately, it’s more important to strive for a healthy lifestyle than to obsess about any particular number.

“I actually prefer to have much less focus on BMI, body composition or body weight,” says Steven Blair, a professor of exercise science at the University of South Carolina in Columbia, “and instead focus on healthful behaviors — at least 30 minutes of moderate intensity activity at least five days a week, and a diet that is focused on fruit, vegetables, whole grains and limited amounts of saturated fat and highly processed food.”

Smart Fitness appears every other Tuesday.