The new fat index

Move over, body mass index. There’s a new kid on the block: the body adiposity index.

The body adiposity index, or BAI, was recently introduced in a study, published in the Obesity journal, as a better and more accurate measure of body fat. Unlike the BMI, which is based on weight and height, the BAI uses height and hip circumference to determine where an individual falls on the weight spectrum.

One of the researchers talked to WebMD about why this method could be more helpful for assessing body fat:

The body mass index (BMI) does not accurately represent the amount of [body] fat,” researcher Richard N. Bergman, Keck Professor of Medicine at the University of California’s Keck School of Medicine, tells WebMD.

The new measure, called body adiposity index (BAI), does, he says. So far, he has validated the new measurement in Hispanic and African-American populations, and says more research is required to confirm how well it works in whites and other ethnic groups.

With BMI, he says, “you get a relative number” assessing body fat. With the new BAI, “you get a number which is the percent fat.” The new method, he says, is more accurate.

Here’s how it works: Take your hip measurement in centimeters and divide it by your height in meters times the square root of your height minus 18. The resulting number is your body adiposity index. The scale is the same as that for the body mass index – 18.4  or lower is underweight. between 18.5 and 24.9 is considered normal, 25 to 29.9 is overweight, anything over 30 is considered obese, and 40-plus is considered very obese.

The mathematics for the BAI seem rather complicated. Square root? I haven’t dealt with square roots since high school and I don’t like them any better now than I did back then. And the formula, or at least this early version of it, requires you to do the calculations in centimeters and meters rather than inches and feet. The body mass index, on the other hand, only requires you to divide your weight in kilograms by your height in meters squared – or cheat with one of the many handy online calculators that do it for you.

It has in fact been an ongoing academic challenge to come up with an accurate, consistent tool for assessing weight against the norm. The body mass index has been widely used for the past couple of decades and many Americans regard it as gospel, but it has its shortcomings. For one thing, it doesn’t account for individual variations in frame size. For another, it doesn’t distinguish between weight that can be attributed to muscularity vs. weight that’s due to fat. Nor does the adult BMI adjust for age.

The cutpoints are somewhat arbitrary as well. It’s not clear, for instance, the point at which women should be considered underweight. Varying expert opinions put the threshold at anywhere from 18.5 to 20. The division between normal and overweight also has been shifting. In 1985, the overweight threshold was 27.8 for men and 27.3 for women. In the late 1980s this was refined to allow slightly higher thresholds for people as they aged. The current cutoff point, however, is 25, which is stricter than the earlier criteria and has placed millions of Americans into the overweight category who weren’t previously considered so.

I suspect the new body adiposity index, while a useful tool, will have shortcomings of its own. Obtaining accurate hip measurements might be tricky, depending on where you hold the tape measure. For some people whose weight is carried primarily in their hips, the results could be somewhat skewed. The BAI also has been validated so far only among 1,700 people of Mexican-American and African-American background, so it’s premature to suggest it’s a good measurement for the population as a whole.

To complicate the picture, other research has found that the waist to hip ratio may be a better indicator of heart disease risk than the body mass index, at least among older adults.

So who’s right? All of these measures seem to be somewhat helpful for those who want to know where they are on the healthy weight spectrum. But it’s important to note that even the experts have yet to reach a consensus on what constitutes the ideal percentage of body fat for maximum health. Health encompasses more than what we weigh, and the numbers on the scale, the body mass index or the body adiposity index are only one piece of the whole story.

Photo: Wikimedia Commons

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