Not all fat people are metabolically equal. And a simple number like the body mass index (or BMI) does not take that key fact into account. A growing number of studies over the past few years have made the pitfalls of relying solely on the BMI to make health predictions increasingly clear. The latest—and in some ways most comprehensive—of these reports was published today in the journal Science.
But there’s no need to abandon the BMI, which is the calculated by taking your weight in kilograms and dividing it by the square of your height in meters. The formula, which was developed back in the 1800s by a Belgian statistician and sociologist, is still a good first-order approximation of health risk for most people. And there are ways of determining if you fall into one of the groups for which it is not relevant.
The BMI’s flaws as a health indicator first attained widespread popular attention when a scientist with the U.S. Centers for Disease Control reported seemingly counterintuitive results based on national mortality statistics. In studies published in 2005 and again in 2013, Katherine Flegal and her co-authors found that people who fall into the overweight but not obese category as measured by the BMI tend to live longer, as a group, than folks who fall into the normal weight range.
Flegal herself cautioned against over-interpreting her results. Once you start delving deeper into the data, you understand why. There is so much variation between individuals in each of the half-dozen usual groupings of BMI numbers that it can mask a wide range of problems. The standard BMI groupings are underweight (less than 18.5), normal (18.5 to 24.9), overweight (25 to 29.9), obesity class I 30 to 34.9, obesity class 2 35 to 39.9 and obesity class 3 (greater than 40).
As the Science paper points out, it is possible to have a normal BMI and be metabolically abnormal, meaning the body has trouble processing various nutrients and other compounds found in the blood. Increasingly, researchers are documenting the many ways that insulin resistance, for example, increases the risk for heart disease, Alzheimer’s disease and other life-shortening ailments. People with a normal BMI who are also alcoholic with fatty liver disease or cirrhosis are simply not healthy by anyone’s definition. According to one analysis from 2008, nearly one in four people with a normal BMI are metabolically unhealthy while about half of overweight individuals are metabolically normal. (Melinda Wenner Moyer covers the Science article and the so-called "obesity paradox" in greater depth here.)
Where your body stores its excess fat also turns out to be important. Fatty deposits under the skin of the arms or legs, for example, apparently just sort of sits there (it may also trap certain toxic metals and keep them away from vital organs) and doesn’t cause much harm. But fatty deposits around the abdomen, or vital organs (like the liver), are much more hazardous to your health.
Rather than throw out the BMI altogether, some researchers think it makes sense to add another measurement—waist circumference—to the mix. This is easier said than done. Basically, you need to place the tape measure around your middle so that it lines up horizontally with the top of your hip bone (the iliac crest) on either side of your body. Then take that measurement (plus your height, weight, sex and age) and plug them into one of several so-called ABSI (A Body Shape Index) calculators that are available on the web.
Other calculations, such as the one that looks at the ratio between your height and waist circumference, try to get at the same thing—do you have more fat distributed around your middle (bad) or beneath the skin on your legs and buttocks (no harm)?
One of the most accurate ways to measure—as opposed to just estimating—the amount of fat in a person’s body is to calculate the amount of water that is displaced when you fully submerge a person underwater. But don’t try this at home, or you may wind up with a flood on your bathroom floor. Shades of Archimedes!
But even percentage body fat is not the answer to everything. As an article in the August issue of Scientific American by JoAnn E. Manson and colleagues describes in great detail, whether or not you are physically fit also plays an important role in how healthy your metabolism is.
The point is good health depends on a lot of things—starting with wether or not you smoke, how physically fit you are, the kinds and amounts of food you put in your body, your age, surroundings and the company you keep. And sometimes you can do everything right and still become seriously ill. Changing your lifestyle, behavior, weight or body shape can help to reduce the chances that you will die prematurely, but they will never eliminate the risk entirely.