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Bye-bye, BMI
Not quite. But now the body mass index is only part of your fitness equation
At 6 feet 3 inches tall and 202 pounds, Chauncey Billups has a body mass index (BMI) of 25.2. Yet, at 5 feet 5 inches and 150 pounds, a sedentary middle-aged female would nearly match the Pistons guard, with a BMI of 25.
On a standard BMI scale, both Billups and the woman fall into the category of “overweight.” But clearly, the two are as different as a muscle car and a minivan when it comes to physique. So what’s the problem?
BMI, or body mass index, is an indicator of body fat determined by a mathematical formula (see Vital Info, below). The resulting number can be used for both adult men and women to assess their risk for obesity-related diseases, according to the National Institutes of Health (NIH).
But because BMI does not measure body fat directly, it cannot discriminate between body fat and lean mass, according to the NIH. And since muscle is denser than fat, BMI may overestimate body fat in athletes or underestimate body fat in older people and others who have lost muscle mass.
MUSCLE MATTERS
Jonathan Ehrman, Ph.D., director of the Weight Management Program at Henry Ford Hospital, says BMI is an integral part of goal-setting for his clients.
“Initially, we talk to them about losing 5 to 10 percent of body weight,” Ehrman says. “Then we talk about the relationship of BMI to disease risk, getting BMI between 30 and 25, or in some cases, just to 35. We go by increments of 5 percent on a BMI scale.”
The program, which served almost 2,000 people in 2006, saw an average BMI of 42, according to Ehrman. “In 99.9 percent of the clients I see, if BMI is high, body fat is high,” he says.
But athletes who boast lean muscle mass may find themselves discouraged and deceived by BMI interpretations.
Lisa MacDonald, a trainer at Oakland Athletic Club, in Birmingham, says she doesn’t typically use BMI with her clients. “It’s not a good classification, because it’s so variable based on your body composition,” she says.
As many trainers, coaches and other fitness professionals do, MacDonald measures body composition using skin fold techniques. With a caliper, she measures subcutaneous fat in seven sites: chest, back of upper arm, shoulder blade, above hip, below navel, upper thigh and calf.
“I think that’s a little better measure for body fat, but I still only use it as a baseline measurement,” says MacDonald.
TOUGH LOVE HANDLES
Another potential drawback of BMI is that it fails to take into account where fat is located – whether it’s concentrated in the abdominal region, or throughout the body. Some health experts criticize BMI’s ability to assess health risks thought to be associated with different types of fat.
Some studies claim that visceral fat (surrounding vital organs in the abdominal region) is linked to greater health risks than subcutaneous fat (directly under the skin throughout the body). Because BMI cannot distinguish between the two, some experts argue that it may not be the best measure of cardiovascular risk. A study published in November 2005 in the British medical journal The Lancet contends that “redefinition of obesity based on waist-hip ratio instead of BMI increases the estimate of heart attacks attributable to obesity in most ethnic groups.”
Shukri David, M.D., chief of cardiology at Providence Hospital in Southfield and physician leader for the St. John Cardiovascular Network, asserts the value of measuring waist-hip ratio and waist circumference, but he maintains that BMI is still a reliable and convenient tool to measure body fat and assess health risk.
While abdominal fat is linked to heart disease, David points out that carrying a heavy load – wherever it is in the body – increases the strain on the heart. Obesity can also lead to sleep apnea, hypertension and dyslipidemia (for example, high LDL cholesterol, low HDL cholesterol or high levels of triglycerides), David says.
Charles Burant, M.D., Ph.D., an endocrinologist and director of the Michigan Metabolomics & Obesity Center at the University of Michigan, agrees. “There are subtleties that may go into waist measurements versus BMI,” he says, “but in general, if you’re overweight, you’re in trouble.”
On its Web site, the Centers for Disease Control and Prevention (CDC) calls BMI one of the best methods for population assessment of overweight and obesity. However, the CDC acknowledges BMI’s limitations: “BMI is used as a screening tool to identify possible weight problems for adults. … However, to determine if excess weight is a health risk, a health-care provider would need to perform further assessments.”