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Original Investigation
May 15, 2017

Tri-Ponderal Mass Index vs Body Mass Index in Estimating Body Fat During Adolescence

Author Affiliations
  • 1University of Alabama at Birmingham
  • 2Pennington Biomedical Research Center, Louisiana State University System, Baton Rouge
  • 3Montclair State University, Montclair, New Jersey
  • 4United States Military Academy, West Point, New York
  • 5Albert Einstein College of Medicine, Bronx, New York
  • 6Verona University Medical School, Verona, Italy
JAMA Pediatr. Published online May 15, 2017. doi:10.1001/jamapediatrics.2017.0460
Key Points

Question  Is there a better screening tool than body mass index z scores to estimate percent body fat in children and adolescents aged 8 to 17 years?

Findings  In this cross-sectional study, using data from the 1999 to 2006 National Health and Nutrition Examination Survey, the tri-ponderal mass index (mass divided by heightcubed) estimated body fat percentage better than body mass index. Moreover, the tri-ponderal mass index more effectively diagnoses overweight vs normal weight status than the body mass index z scores.

Meaning  It is worth considering replacing body mass index z scores with the more accurate and easier-to-use tri-ponderal mass index to screen for obesity and overweight status in children and adolescents.

Abstract

Importance  Body mass index (BMI) is used to diagnose obesity in adolescents worldwide, despite evidence that weight does not scale with height squared in adolescents. To account for this, health care providers diagnose obesity using BMI percentiles for each age (BMI z scores), but this does not ensure that BMI is accurate in adolescents.

Objective  To compare the accuracy of BMI vs other body fat indices of the form body mass divided by heightn in estimating body fat levels in adolescents.

Design, Setting, and Participants  Cross-sectional data from the 1999 to 2006 US National Health and Nutrition Examination Survey were analyzed between September 2015 and December 2016.

Main Outcomes and Measures  Dual-energy x-ray absorptiometry and anthropometric data were used to determine changes in body fat levels, body proportions, and the scaling relationships among body mass, height, and percent body fat. To assess the merits of each adiposity index, 3 criteria were used: stability with age, accuracy in estimating percent body fat, and accuracy in classifying adolescents as overweight vs normal weight.

Results  Participants included 2285 non-Hispanic white participants aged 8 to 29 years. Percent body fat varied with both age and height during adolescence, invalidating the standard weight-to-height regression as the way of finding the optimal body fat index. Because the correct regression model (percent body fat is proportional to mass divided by heightn) suggested that percent body fat scales to height with an exponent closer to 3, we therefore focused on the tri-ponderal mass index (TMI; mass divided by height cubed) as an alternative to BMI z scores. For ages 8 to 17 years, TMI yielded greater stability with age and estimated percent body fat better than BMI (R2 = 0.64 vs 0.38 in boys and R2 = 0.72 vs 0.66 in girls). Moreover, TMI misclassified adolescents as overweight vs normal weight less often than BMI z scores (TMI, 8.4%; 95% CI, 7.3%-9.5% vs BMI, 19.4%; 95% CI, 17.8%-20.0%; P < .001) and performed equally as well as updated BMI percentiles derived from the same data set (TMI, 8.4%; 95% CI, 7.3%-9.5% vs BMI, 8.0%; 95% CI, 6.9%-9.1%; P = .62).

Conclusions and Relevance  The tri-ponderal mass index estimates body fat levels more accurately than BMI in non-Hispanic white adolescents aged 8 to 17 years. 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.

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