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

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.

Figure 1.. Height (Mean and 95% CI) for the First and Fourth Quartiles of Percent Body Fat as a Function of Age

Figure 2.. Scaling Exponents for Body Fat Regressions

Scaling exponents, n (mean and 95% CI), for the regression model percent body fat are proportional to mass divided by heightⁿ for both nonadult (aged 8-17 years) and adult (aged 18-29 years) participants.

Figure 3.. Mean Values With 95% CIs for Body Mass Index (BMI) and Tri-Ponderal Mass Index (TMI)

Figure 4.. Regressions of Percent Body Fat vs Body Mass Index (BMI) and Tri-Ponderal Mass Index (TMI)

The second-order polynomial regression lines and corresponding best-fit equations are shown in the respective figures for the weighted and imputed data, while the data points shown are for the first imputation. BMI was calculated as weight in kilograms divided by height in meters squared.

Figure 5.. Misclassification Rates for the Tri-Ponderal Mass Index (TMI) and Body Mass Index (BMI)

False-positive rate (FPR), false-negative rate (FNR), and total misclassification rates with 95% CIs for TMI vs BMI z scores (A) and TMI vs BMI-for-age in assessing overweight status in nonadults aged 8 to 17 years (B).