An analysis comparing the BMI to a blood test and body scan found that the BMI was wrong for half of women and 1 in 4 men.
It's commonly acknowledged that the BMI can produce inaccurate results for athletes and others with a lot of muscle mass. But this research illuminates a problem on the other end of the scale: people who think they're fine but are carrying a lot of fat and little muscle.
"Particularly in women, as they age, their muscles become inserted with fat, even though they stay thin and beautiful in a dress," said Dr. Eric Braverman, a New York physician who co-wrote the study with New York State Health Commissioner Nirav R. Shah. Their work was published in the journal PLoS ONE.
The researchers propose revisions to the cutoff for determining who is obese — to a BMI of 24 or over for women and 28 or over for men, down from 30 or over for both. They also say that doctors should consider blood tests to measure leptin, a hormone that goes up with increased body fat, and DXA scans. The scans are commonly used to assess bone density but can be used to analyze body fat.
Experts agree that the BMI is imperfect but say it's a valuable tool to help guide doctors' conversations with their patients.
It boasts another big benefit: It's free. Scans can cost several hundred dollars, and leptin tests can be more than $100, said Dr. Patricia Choban, medical director of the bariatric-surgery program at Mount Carmel West hospital.
"You have to understand that BMI is an estimate of fatness, it's not a measure of fatness. But there are very few screening-tool estimates we have that are as simple to use," she said.
Carmen Swain, program director of Health and Exercise Science at Ohio State University, said some tests, including DXA scans and skin-fold measurements, do produce more-accurate results when done right. But BMI remains an easy and important tool, she said.
Swain has done research looking at BMI and body-fat percentage in boys and found a good correlation between the measure and actual body composition. There's little research on the reliability of BMI in adults, she said, and it's good that this study offers more insight.
Dr. Darrin Bright, who practices with OhioHealth's Sports Medicine Institute and MAX Sports Medicine, said the BMI "has its flaws; we've known it has flaws since we first started using it."
"It's a piece of the puzzle. It's a screening test," he said, "and we need to understand the limits of a screening test."
BMI should be a starting point for conversations between doctors and patients, Choban and Bright said.
"As a physician, I'm going to measure BMI, then I'm going to ask them about what their activity is," Choban said. "If you look at their fitness level, it's going to give you a pretty good clinical guess. Unfortunately, many people aren't even calculating BMIs."
Bright said he sometimes orders leptin tests, but he doesn't think they or DXA scans are practical for population-wide obesity screening.
Braverman said that as more of these tests are done, the price will come down. Leptin testing should be as routine as cholesterol tests, he said, calling BMI the "baloney mass index."
Even if the new cutoffs are used, the calculation still will miss many people who could have weight-related health problems because of their body fat, he said.
http://www.dispatch.com/content/stories/local/2012/04/04/study-bmi-misses-mark-on-obesity.html
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