Study Challenges Standard Measurements for Body Mass in Adolescents

A recent study found that tri-ponderal mass index (TMI) estimated body fat percentage in adolescents more effectively than body mass index (BMI) z scores.

TMI is calculated as mass divided by height cubed, which was determined as a more effective method for calculating adiposity after regression models showed that the percent body fat scales to height had an exponent closer to 3.
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The cross-sectional study used data from the US National Health and Nutrition Examination Survey from 1999 to 2006 and included 2285 non-Hispanic white participants from 8 to 29 years of age. To determine the effectiveness of TMI and BMI z scores, researchers assessed each adiposity index based on its stability with age, its accuracy for estimating percentage of body fat, and its accuracy in classifying adolescents as overweight or normal weight.

Overall, TMI was found to have greater stability with age and better estimated percent body fat for children 8 to 17 years of age than BMI. In addition, TMI misclassified adolescents as overweight less often than BMI z scores.

“TMI is more accurate than BMI z scores at classifying overweight status and is as accurate as up-to-date BMI percentiles. Yet TMI requires only a single threshold for each sex, instead of the multiple complicated age- and sex-specific thresholds needed for BMI to work in adolescents,” the researchers concluded.

“While TMI was superior to BMI during adolescent development, the ramifications of recommending a bold change to clinical practice and to public health research have not escaped us. Therefore, our work, including our 3-criteria strategy for assessing the merits of obesity, needs to be extended to other racial/ethnic groups and then replicated in large cross-sectional and longitudinal studies.”

—Melissa Weiss

Reference:

Peterson CM, Su H, Thomas DM, et al. Tri-ponderal mass index vs body mass index in estimating body fat during adolescence [Published online May 15, 2017]. JAMA Pediatr. doi:10.1001/jamapediatrics.2017.0460