BMI doesn’t tell all health risks

Tue., March 14, 2006

Andy Dworkin The Oregonian

Running the numbers

BMI

Multiply your weight in pounds by 703. Divide that number by your height in inches, squared (height in inches times itself). Or visit http://nhlbisupport.com/bmi/ to have the math done for you.

The number links to six weight classes. Disease and death risk rise notably from the high 20s. Less than 18.5: underweight; 18.5-24.9: normal; 25-29.9: overweight; 30-34.9: obesity I; 35-39.9: obesity II; 40-plus — extreme obesity

Waist size

Find a point just above the top of the hip bone. Exhale, and circle a tape measure around your waist at this height, pulling snugly without compressing the skin.

A waist size more than 40 inches in men and 35 inches in women means high risk for disease. This measure does not work well in people under 5 feet tall or with a BMI over 35.

Waist-to-hip ratio

Measure waist as described above. Measure your hips with a tape measure at the widest point around the buttocks. Divide the waist number by the hip number.

What it means: Heart disease risk rises with higher numbers, starting significantly at a ratio of 0.9 for men and 0.85 for women.

As the nation waddles through its obesity epidemic, doctors and dieters increasingly talk BMI like so many hippies discussing astrological signs.

A complex little calculation, body mass index sorts people into categories of underweight (BMI less than 18.5), normal (18.5-24.9), overweight (25-29.9) and obese (30 and up). By that definition, about two-thirds of U.S. adults are overweight or obese. That one number leaves many of them fretting about dieting, disease or death.

But many scientists question whether BMI is really the powerful predictor of health it’s made out to be. While many diseases seem linked to BMI, researchers have wondered whether other measures, such as waist size, may better predict who is really at risk, especially for heart disease. A big 2005 study that found lower death rates among BMI-defined overweight adults further fueled the debate.

In fact, many health experts now recommend considering waist size, or the ratio of waist to hip size, alongside BMI as a way to judge obesity. The good news is that combo seems better able to fix one’s risk of death, at least from heart disease. The bad news is it may put even more people in the risky category.

“It means that people at BMIs of 23, who may not think they have a weight problem, may actually have a weight problem,” said Dr. Arya Sharma, a professor of medicine at McMaster University in Hamilton, Ontario, who thinks that waist-to-hip ratio better predicts heart attacks.

Doctors now worry more that the amount of fat, and where you carry it, affects disease more than bulk. Fat carried in the abdomen seems especially risky, at least for diabetes, heart disease and stroke.

“Unfortunately, we don’t have a simple instrument that will actually measure a person’s body fat” or belly fat, Sharma said. While CT scans and MRIs can do the trick, they are complex and costly. In their place, scientists think they can get a good sense of abdominal fat from good, old-fashioned waist size.

In a study of more than 27,000 people worldwide, Sharma and others found that growing waist size significantly predicted increasing heart attack risk. Surprisingly, that held even for people with BMIs below 23, he said.

Another surprise was that heart attack risk fell as hip size got bigger. Scientists aren’t sure why that happened, Sharma said, but rising hip size may reflect more muscle, or the hips could be a safer place to store fat than the waist.

Whatever the cause, Sharma said, the waist-to-hip ratio seemed the most powerful predictor of heart disease. When the ratio tops 0.85 for women, and 0.9 for men, cardiac risk rises. And that measure seemed to be more predictive than BMI across a range of body sizes, age and nationalities, he said.

None of that means BMI is meaningless. It remains a good, cheap and fairly easy estimate of obesity. But people worried about their health should consider more than BMI alone.

The NIH suggests considering waist size along with BMI. Men with a waist over 40 inches and women with a waist over 35 inches have added disease risk at any BMI below 35, the NIH says. Sharma suggests a similar approach using waist-to-hip ratio instead of waist size.

A final step considers other risk factors: Smoking, diabetes, sleep apnea or a history of heart disease all toss a person into a higher-risk category.