The most commonly used tool to determine overweight and obesity appears to give some racial groups a false sense of security and others unnecessary cause for concern.
The body mass index (BMI) uses height and weight to produce a number that enables scientists to gauge risk of weight-related illnesses, such as heart disease, high blood pressure and type 2 diabetes. (A BMI of 18.5 to 24.9 is considered healthy; 25 to 29.9 is overweight, and 30 or higher, which equals 186 pounds or more for someone 5 feet 6 inches tall, is obese.)
In June, a study of nearly 1,200 people became the latest to show that the current BMI categories significantly underestimate health risks in many non-Europeans. Even when researchers took into account waist size and waist-to-hip ratios, they found that people in South Asian, Chinese and aboriginal groups (such as the Maori in New Zealand) had a high risk of weight-related health problems at BMI levels considered healthy.
BMI and other body measurement thresholds "do not apply to non-European ethnic groups and warrant revision," concluded Salim Yusuf, director of the Population Health Research Institute at McMaster University in Hamilton, Ontario, and his colleagues in the International Journal of Obesity and Related Metabolic Disorders.
This isn't the first time the BMI has come into question. In 2000, a group advising the World Health Organization (WHO) suggested setting a BMI of 23 as the threshold for overweight and 25 for obesity in Asian populations. That recommendation has not been officially adopted by either the WHO or the National Institutes of Health, which sets BMI categories in the United States in conjunction with outside experts.
Other research has found that BMI often overestimates risk. African Americans and others of African descent as well as Polynesians generally have less body fat and more lean muscle mass than do people of European descent. For this reason, some scientists believe that for these groups a BMI of 26 should be the threshold for overweight and 32 for obesity.
The National Heart, Lung, and Blood Institute also cautions at its Web-based BMI calculator (www.nhlbisupport.com/bmi/) that the measurement is less accurate in pinpointing overweight and obesity among muscular people, such as athletes who have a BMI of 27 to 30 and are generally leaner than the rest of the population. In the elderly, the BMI can underestimate risk, since muscle is lost with aging.
"Many of us have begun to realize that body mass index is a very poor measure of adiposity -- how fat you are -- and how much health risk you face, " Yusuf said.
The BMI is frequently misused by both health professionals and the public, scientists say. The index was intended to screen "populations, not individuals," said Wilfred Y. Fujimoto, professor emeritus of medicine at the University of Washington School of Medicine in Seattle. Weight-related health risks are best determined, he said, "not only by looking at body weight, but also at what other tests might show."
Until there's more of a consensus on the value of the BMI, here's what experts recommend to help you determine your weight-related health risks:
Use the BMI as a starting point, not gospel. "It's a difficult message to get across, although we've been trying to convey it for years," said Van Hubbard, director of the National Institutes of Health's Division on Nutrition Research Coordination. So check your BMI, but also ask your doctor or other health professional to look at other measurements to determine your risk. Most important: waist size, waist-to-hip ratio, blood pressure and levels of blood cholesterol, triglycerdies and blood sugar. Don't assume you're at elevated risk -- nor that you're home free -- based on BMI alone.
Watch the trend. If your weight, BMI, waist size and blood cholesterol and other test results are all rising, it's important to lower them by losing some weight, even if the numbers are still in healthy ranges. If your blood tests and blood pressure are stable but your weight and waist are growing, "don't gain any more weight," Hubbard said.
Do a gut check. No matter what your race, sex or age, large abdomens "almost always indicate too much body fat," Yusuf said. To measure your waist, stand with your feet together, abdomen relaxed. Place a soft tape measure at your natural waist, the narrowest part of your upper body, the spot just above your navel. Current guidelines are for women to aim for a waist of 35 inches or less; men, 40 inches or less. But Fujimoto and others believe that those of Asian heritage should aim for 30 inches or less for women and 35 for men. For now, other racial groups should aim for the standard goals.
Decrease risk by staying active throughout the day. Doing so burns calories, and even just taking the stairs can also help boost muscle mass, which seems to help lower risk. At McMaster University, Yusuf meets with colleagues while walking rather than while sitting down over lunch. In helping to design a new research building, he's made sure that the elevators are in obscure corners with signs noting that their use contributes to obesity. Stairs are located in an open area to invite walking. "The key," he said, "is to build more physical activity into everything you do."
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