Over the years, experts have questioned the accuracy of body mass index scores, known as BMI “z-scores,” for estimating body fat percentages in kids (the z is specific to younger age groups and requires complicated calculations to get results). The criticism is that adolescent weight doesn’t scale with height, which can produce faulty data. Now, researchers claim to have discovered a new, more accurate formula for measuring body fat in kids aged 8–17.

Called tri-ponderal mass index (TMI), the new calculation is mass divided by height cubed. This formula was developed as part of a study that compared BMI against other body fat indices in 2,285 adolescent subjects. Subjects also underwent dual-energy x-ray absorptiometry, which gave the scientists an accurate assessment of body fat percentage. When analyzing the data, the scientists found fewer discrepancies in TMI scores than in BMI z-scores. As a result, they are encouraging healthcare practitioners to adopt TMI when assessing body fat in kids.

“The tri-ponderal mass index estimates body fat levels more accurately than BMI in non-Hispanic white adolescents aged 8 to 17 years,” the researchers said. “Moreover, TMI diagnoses adolescents as overweight more accurately than BMI z scores and equally as well as updated BMI percentiles, but is much simpler to use than either because it does not involve complicated percentiles. Taken together, it is worth considering replacing BMI z scores with TMI to estimate body fat levels in adolescents.”

The study appeared in the Journal of the American Medical Association Pediatrics (2017; 171 [7], 629–36).

Ryan Halvorson

Ryan Halvorson is an award-winning writer and editor. He is a long-time author and presenter for IDEA Health & Fitness Association, fitness industry consultant and former director of group training for Bird Rock Fit. He is also a Master Trainer for TriggerPoint.

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