Natural Weight Control | Natural Health Blog

BAI, Better Body Measurements

The Body Mass Index (BMI) is the primary tool used to judge a healthy weight.  It’s a measurement of your weight in relation to your height, and as such, it correlates strongly (in adults) with your total body fat content. It’s calculated by taking your weight in pounds and dividing it by your height in inches squared, then multiplying by 703. Or you can use one of the many BMI calculators on the Internet.  For many people, it presents an accurate picture of whether one is underweight, overweight, or right where they should be.

However, for many other people, the BMI presents limitations.  Take, for example, an elite athlete with lots of muscle mass (which weighs a great deal) and very little body fat.  His BMI result would be a high number, maybe even above 30, which is considered obese.  But when further body fat testing is performed, it is obvious that isn’t the case.  It would be hard to find many fitter subjects, but you would never know that just to check his BMI.

That’s why researchers at the University of Southern California’s Keck School of Medicine in Los Angeles recently developed a new method of assessing weight that focuses more on the amount of body fat and less on overall size.  Their measurement system, called the Body Adiposity Index (BAI), concentrates on your height and hip circumference instead of your overall weight.  The formula is your hip measurement in centimeters divided by your height in meters multiplied by the square root of your height minus 18.

In a WebMD article1, the researchers provide an example of how much closer to reality the BAI brings us than the BMI.  A female subject who is 5′ 4″ tall, weighs 127 pounds, and has a hip circumference of 37″ has a BMI of 21.7 (putting her right in the middle of the normal weight range) and a BAI of 27.8 (which correlates instead to her body fat percentages).  When a dual-energy X-ray absorptiometry (DXA) scan is used to measure actual body fat versus muscle and bone, it determines that she has 28.4 percent body fat, very close to the BAI measurement.  The BAI presents more of a total representation than the BMI ever did.

In use since the 1800s, the BMI has played an important role in determining healthy weight.  However, for years now, many in the medical community have been questioning its accuracy.  Since the BMI only focuses on height and weight, it does not differentiate between fat and muscle mass, resulting in many fit people — with extremely low body fat percentages — ending up with a BMI over 25 and being deemed overweight.

Another problem with the BMI system is that it makes no difference how fat is distributed around the body.  Many studies have linked fat accumulations in the midsection — giving the person an “apple” shape — with increased risk of heart disease and diabetes.  When researchers at the Mayo Clinic in Rochester, Minnesota analyzed the data from 40 studies involving 250,000 people with heart disease, they discovered that only the extremely obese patients had a higher risk of mortality; those with “overweight” BMIs actually had fewer cardiovascular problems than the participants with normal BMIs.

You can be of normal weight according to your BMI and even look thin, but have little muscle and plenty of fat, upping your risk for all kinds of disease.  That’s why BMI is so deceptive.  Research has confirmed that people who maintain their weight through diet alone rather than diet and exercise together are likely to have major deposits of internal fat, even if they appear slim.  The internal fat surrounds vital organs like the heart, liver, and pancreas, making it far more dangerous than visible fat.  Obese people who exercise are actually at lower risk of mortality than thin people who are sedentary.

The new formula may not have all its kinks worked out yet, though.  So far it’s only been tested on a relatively small population of 1,733 Mexican-Americans and 223 African-Americans, leaving more research necessary with the general populace.  And there were some discrepancies among the thinnest of the subjects’ BAIs as well.  But overall, it’s definitely a step in the right direction.  With obesity such a serious problem all over the world and only getting worse, the time has certainly come to look beyond the BMI and start instituting a method that can accurately assess our fat levels and our health.


1 Doheny, Kathleen. “New Alternative to BMI for Measuring Body Fat.” WebMD. 3 march 2011.  WebMD, LLC. 9 March 2011.