TMI has edge over BMI for estimating body fat in adolescents
medwireNews: The tri-ponderal mass index (TMI) estimates body fat levels in adolescents more accurately than body mass index (BMI), study results suggest.
The TMI, which is mass divided by height cubed, estimated adiposity more accurately than BMI, especially among boys, in whom TMI explained 64% of the variance in percent body fat, whereas BMI explained only 38%. The corresponding values in girls were 72% versus 66%.
“Body mass index screening in children and adolescents is an important tool to combat obesity worldwide,” with international organizations relying on BMI z scores to assess the prevalence of obesity in these age groups, explain the researchers in JAMA Pediatrics.
“However, weight is not proportional to height squared during adolescence, casting doubt on the accuracy of BMI percentiles in adolescents,” they add.
Courtney Peterson (University of Alabama at Birmingham, USA) and study co-authors analyzed cross-sectional data from the 1999 to 2006 US National Health and Nutrition Examination Survey to compare the accuracy of different body fat indices in 2178 male and 2220 female non-Hispanic White participants.
Using thresholds for overweight of 16.0 kg/m3 for boys and 16.8 kg/m3 for girls aged between 8 and 17 years, they found that TMI misclassified overweight status significantly less often than BMI z scores when compared with dual-energy x-ray absorptiometry measures of body fat, with misclassification rates of 8.4% and 19.4%, respectively.
When TMI thresholds for obesity of 18.8 kg/m3 for boys and 19.7 kg/m3 for girls were used, TMI misclassified fewer adolescents as obese than BMI z scores did, at 8.0% versus 11.3%.
And when the team analyzed changes in the indices with age, TMI was “nearly stable” throughout adolescence at approximately 14.0 kg/m3, whereas BMI increased by at least 7.9 kg/m2 between ages 8–9 and 25–29 years.
Taken together, these results suggest that “[i]t 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,” say Peterson and colleagues.
However, the team cautions that because TMI is based on body fat distribution rather than health risks, it is still an imperfect measure of overweight status. They also note that the study findings may not be generalizable to adolescents of other ethnicities because thresholds for diagnosing overweight and obesity may vary across ethnic groups.
And the researchers conclude that their work “needs to be extended to other racial/ethnic groups and then replicated in large cross-sectional and longitudinal studies.”
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