Hey, what is your BMI? If that's a number you'd rather not share -- if, in fact, you can't help thinking there must be something wrong with the BMI calculator -- I feel your pain. And although I can't fix the calculator, I can tell you there's a growing debate over how good the body-mass index is as a predictor of an individual patient's health prospects.

You can read about that whole debate: "BMI may not be telling the whole truth."

There’s an inside joke often told at conferences convened to discuss the nation’s epidemic of obesity: If the 72 million American adults with a body-mass index above 30 -- the demarcation line for obesity -- want to improve their health and avert a plague of weight-related diseases, they have two options: They can lose weight. Or they can get taller.

Why does it get a laugh? Because those attending these conferences know that a patient sets off medical alarm bells not when he or she eats too much of the wrong thing or is genetically disposed to deposit fat in the wrong places, or spends too little time in physical activity. A patient is considered dangerously fat when his or her weight-divided-by-height-squared (in kilograms and centimeters, respectively) equals 30 or above.

In other words, the definition of overweight or obesity -- and all the health consequences that research has attached to those definitions -- rests on a numerical calculation that only loosely approximates how “fat” an individual is. A 5-foot-6 patient weighing 200 pounds could drop from obese to merely overweight by losing 18 pounds. But she could do the same if she could, by some magic, grow 3 inches.

More damning still, the BMI calculation was rigged by its Belgian inventor, mathematician Adolphe Quetelet, to fit existing data on 19th century Europeans. The formula, say experts, fails to take into account a patient’s level of physical fitness or the ratio of fat to muscle and bone in his body -- both factors known to influence health. Finally, the BMI lulls modern physicians into thinking they need no better measure of an individual patient’s health prospects.

“It is mathematical snake oil,” says Stanford mathematics professor Keith Devlin, who chafes that, despite his perfect blood pressure and cholesterol numbers, his intensive routine of competitive bicycling and his 32-inch waist, his BMI number identifies him as “overweight,” (defined as falling between 25 and 29.9), prompting his physician to nudge him to exercise more and eat less.

"The BMI is one of those all-powerful magic entities -- a *number*,” Devlin recently grumped in a blog posting on the Mathematical Assn. of America’s website. “And not just any number, but one that is generated by a *mathematical formula*. So it has to be taken seriously, right?”

To many of the people who fall on the wrong side of the number 30, the joke is more tragic than funny: Losing weight (and maintaining that loss) can seem as impossible a task as getting taller.

Writing in the British Journal of Sports Medicine, Duck Chul-Lee, a research scientist at the University of South Carolina’s School of Public Health, recently suggested that physicians should acknowledge that, for many patients, losing weight and keeping it off is a daunting challenge. Though weight loss is important, Lee said, physicians might do better to shift their principal objective, getting patients to improve their fitness (a goal that could be achieved by increasing physical activity, building muscle and stamina and quitting smoking) as a more sustainable means of lowering their health risks.

For those older than 60, scientists at the University of South Carolina have found that fitness trumps BMI strongly as a predictor of a patient's longevity. And several studies since have found that overweight and obese women (as measured by BMI) who are regular exercisers can drive down their elevated risk for type 2 diabetes or cardiovascular disease substantially.

If you want to calculate your own BMI, remember that the formula above in in metric. To do the calculation in feet, inches and pounds, the formula is: Divide your weight, in pounds, by the following: your height, in inches, times your height in inches again; then, multiply the resulting number by 703.