Triponderal mass index superior to BMI for evaluating body fat during adolescence
Using the US National Health and Nutrition Examination Survey (1999–2006), researchers analysed cross-sectional dual-energy X-ray absorptiometry and anthropometric data from 2,285 non-Hispanic white participants aged 8–29 years to determine changes in body fat levels, body proportions, and the scaling relationships among body mass, height, and percent body fat. Stability with age, accuracy in estimating percent body fat, and accuracy in classifying adolescents as overweight vs normal weight were used to determine the superior index.
The standard weight-to-height regression was not valid for finding the optimal body fat index since percent body fat varied with both age and height during adolescence. Unlike BMI which increases dramatically with age, necessitating the development of age-specific percentiles, TMI was stable between ages 8 and 17. TMI was also better at estimating percent body fat than BMI (R2=0.64 vs 0.38 for boys and R2=0.72 vs 0.66 for girls), and misclassified adolescents as overweight rather than normal weight less often than BMI z-scores (8.4 percent vs 19.4 percent misclassified, respectively; p<0.001). In addition, the index performed as well as updated BMI percentiles derived from the same data set (TMI 8.4 percent vs BMI 8.0 percent; p=0.62), but was much simpler to use since TMI does not require percentile calculations.