Melissa Healy, Los Angeles Times
6:30 AM AKST, March 3, 2011
Move over, BMI. Researchers led by a University of Southern California physician have proposed "a better index of adiposity." This one would require a tape measure and a calculator, but none of the stepping-up-on-the-scale drama featured on NBC's The Biggest Loser and replayed so often in the privacy of our own bathrooms.
The new fat metric is called the Body Adiposity Index, and it's introduced in a study released Thursday in the journal Obesity. It uses a person's height and hip circumference to give a accurate reading of how much of a person's body is made up of fat. That's a measure that physical trainers and some physicians can get by ordering a dual-energy X-ray absorption test (also used to detect osteoporosis) or a hydrostatic underwater weigh-in. But the cost and complexity of taking such measures have made them a rarely-used alternative to good-old BMI.
Dr. Richard N. Bergman, the lead author of the article and a professor of physiology and biophysics at Keck School of Medicine, says the Body Adiposity Index, or BAI, is easy to compute, provides a more precise measure of a patient's ratio of fat-to-lean tissue than the body-mass index (BMI), and offers a standard that researchers could use to make apples-to-apples comparisons of men and women and of people across different ethnic groups.
In an interview, Bergman said he did not think the BAI would topple the BMI from its pedestal, but that, as researchers find links between BAI and health outcomes (such as diabetes, certain cancers, heart disease and stroke), the new measure of fat may become a better way to identify which patients truly have body compositions that put them at risk. In the meantime, he said, it could be a useful measure for doctors to use alongside BMI to counsel their patients.
What prompted him and researchers from the National Institute of Diabetes and Digestive and Kidney health (NIDDK) to explore better ways to measure body fat? "I was upset that BMI is used clinically when in fact, for an individual, it’s rather inaccurate," said Bergman.
Body-Mass Index is a simple but very rough gauge of how fat we are. Calculating BMI requires only a person's height and weight, so researchers have records going back almost 200 years that they can use to plot the steady and, most recently, steep rise in our avoir-du-poids. No surprise, then, that BMI has become the most widely-embraced tool in research on health and nutrition. A search of medical research databases shows that some 45,000 published research studies have used BMI, among other things, to establish links between excess fat and increased health risks such as cancer, cardiovascular disease and diabetes.
As a result, Americans concerned about their weight are as likely to know their BMI number as they do their shoe size--though the weight/height measurement is likely a touchier subject.
The problem is that BMI, while a great tool for charting population trends, is not, in fact, a very good measure of how fat contributes to an individual person's body composition. That shortcoming may be important because advancing research suggests it is body fat--not lean muscle, not healthy bones, and certainly not a BMI number--that increases one's propensity to develop heart disease, type-2 diabetes and other ills. (Much of that research has suggested that visceral fat--blubber that accumulates around the belly--is the most harmful. In light of those findings, Bergman said he and his colleagues were "surprised" to find hip circumference a better way to measure body fat.)
So, it might be more helpful if physicians and public health experts had an easy way to measure just how much fat each of us carries around, and whether that fat plays a distant second fiddle to strong bones and lean muscle, or whether it's become a major player in a person's body composition. Without scales, water tanks or X-ray machines, BAI could provide that.
Bergman and his colleagues settled on the measure using height and hip circumference (throw that tape measure around the hips "at the level of the maximum extension of the buttocks posteriorly in a horizontal plane") after finding that in two studies, it correlated closest with the body composition results gleaned from the more costly X-ray absorption test or the underwater weigh-in tests. The studies looked at large populations of African-Americans and Mexican-Americans, as well as some white European-Americans, and found the BAI consistently correlated with other measures of body composition in all three groups.