Will Body Mass Index (BMI) Be Replaced by New Fat Test?

April 8, 2011 — -- A new way to tell how fat a person is without having to step on a scale may replace Body Mass Index, a measure that hasn't been updated for nearly 200 years.

Body Mass Index (BMI) uses height and weight to estimate body fat. The new index, known as Body Adiposity Index (BAI), uses hip-width and height measurements. Scientists hope it will be a simpler and more accurate assessment.

BMI long has been considered useful but flawed. A high BMI is one of several risk factors used to evaluate obesity-related diseases, but for many people, including athletes, BMI is unreliable.

For example, under the current BMI guidelines, a rock solid body builder who is 6-foot-2 and 257 pounds has a BMI of 33. That is considered obese -- the same as a rotund, chip-eating couch spud of similar height and weight.

BAI offers a potential advantage over BMI because it gives a clearer snapshot of how much unhealthy flab a person carries on their body and eliminates much of the guesswork of whether or not a person is truly carrying too much excess weight. It seems to be able to differentiate how much of a person's weight is fat and how much is muscle and fat-free mass -- although like BMI, it still doesn't reveal anything about where an individual's fat is deposited.

Richard Bergman and a team of researchers at the University of Southern California in Los Angeles derived their BAI calculations from a database of about 1,700 Mexican-Americans. When they analyzed an extensive series of physical characteristics they found that hip circumference and height correlated strongly with body fat percentage as measured by a highly reliable but expensive scanning method known as DEXA, or dual-energy X-ray absorptiometry. They reported their findings in the March issue of the journal Obesity.

Bergman said that as with any other new scientific measure, BAI still needs fine tuning such additional testing on other ethnicities and with children. But so far, results have been promising. Recent studies with groups of African Americans also have produced good results.

BAI Is Easy, Cheap, Accurate

If the accuracy of BAI holds up as Bergman believes it will, Bergman said it will prove useful especially in remote areas where scales are both scarce and inaccurate and which have populations such as women who sometimes prefer not to be weighed.

"All it requires is a simple calculation that can be done by any health professional including a doctor, nurse, dietitian or personal trainer," Bergman said.

It's a simple calculation -- if you know what you're doing.

Determining your body fat percentage with BAI involves measuring your height in meters and the widest part of your hips (while your feet are placed together) in centimeters, then plugging those numbers into a complex formula. For the math-challenged, there are already on line BAI calculators that Bergman said are reliable.

Body fat percentage is just one number medical experts use to evaluate a person's overall health and level of "fatness." There are no universally accepted guidelines for the ideal percentage of body fat.

The American Council on Exercise recommends that women strive for between 16 percent and 26 percent body fat and men between 12 percent and 22 percent. Athletic men and women often have lower body fat percentages than that. Those above 38 percent body fat generally are considered to be obese.