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Body mass index is a good tool for diagnosing obesity in adults, but researchers believe there's a better way to measure body fat in youngsters.

CONTRIBUTED PHOTO BY METROSOURCE

For decades, doctors worldwide have used the body mass index to estimate a person's body fat, and for most adults, it provides an accurate result. 

But in children, BMI is much less reliable, according to researchers from Baton Rouge's Pennington Biomedical Research Center, who participated in a study that found the calculation problematic in diagnosing patients between the ages of 8 and 17.

"You misclassify some children as being overweight or obese who are actually normal weight," said Dr. Steven Heymsfield, a professor at Pennington.

Another method, the tri-ponderal mass index, proves to be more accurate and easier to use for adolescents, according to a study published this month in the Journal of the American Medical Association Pediatrics. Using this formula may lower the number of obese and overweight children and teens in Louisiana and the United States, said Heymsfield, the senior investigator in the study. 

Initially developed in the 1800s, BMI is a measure of body fat based on height and weight (weight in kilograms divided by the square of height in meters). A person with a BMI above 25 is considered overweight. 

Doctors have long known that it doesn't accurately diagnose children and teens. Instead, doctors use a chart based on BMI that places children into different percentiles.

The alternate measurement, TMI, works similarly to BMI, but it finds that children's weight changes as height cubed instead of squared. A TMI above 16 means a child is overweight. In addition to being more accurate, the measurement is easier to use in schools and in mobile health clinics, Heymsfield said.   

"You don’t need a chart," Heymsfield said. "There’s only one number you need to know if you are overweight or not."

A former assistant professor at Pennington, mathematician Courtney Peterson developed TMI from a century-old equation and proved its accuracy, Heymsfield said. Peterson is now an assistant professor at the University of Alabama at Birmingham.

Using BMI, 20 percent of adolescents are wrongly classified as obese or overweight, Heymsfield said, but TMI has a lower margin of error.

About 1 in 5 American children is considered obese or overweight, and in Louisiana, the statistics are worse — about 1 out of every 3, according to a Pennington study. 

In an effort to combat childhood obesity, some schools monitor their students' height and weight. 

"They are getting their height and weight measured, and if they exceed certain thresholds, they get letters sent home," Heymsfield said. "There is a lot of stigma with being classified as overweight or obese."

If schools and doctors can switch to the TMI measurement, they could diagnose adolescents more accurately. Heymsfield said researchers are already using the alternate equation to analyze Louisiana's children.

"For Louisiana in particular, we have the most obese state or the second," Heymsfield said. "We’re always right up there. This applies to a lot of adolescents in Louisiana. It is very important to get it right to make sure we have the statistics on obesity prevalence rates correct."

Follow Kyle Peveto on Twitter, @kylepeveto.