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Despite the limitations of body mass index (BMI), the measure effectively screened children and adolescents for elevated adiposity, both as expressed by fat mass index (FMI) and percentage of body fat, researchers found.

Among 8- to 19-year-olds, those with a high BMI were 29 times more likely to have a high FMI than those with lower BMIs, reported David Freedman, PhD, now retired from the CDC, and colleagues.

A high BMI was also predictive, though less so, of high levels of percentage of body fat as well as lean mass index, for which relative risks were 12 and 15, respectively, the researchers noted in .

"Although the prediction was not perfect," Freedman and colleagues noted of their work, "a BMI ≥ CDC 95th percentile was a very good indicator of a high FMI and a good indicator of high [percentage of body fat]."

That wasn't a given.

"BMI is based only on weight and height, which can be a poor indicator of adiposity among those with normal or relatively low adiposity," Freedman and colleagues noted. BMI doesn't do a good job distinguishing between fat and lean mass or at characterizing body fat distribution, they added, and some prior research suggested low sensitivity for detecting high adiposity.

Though prior studies have shown that "children with high BMI are likely to have high adiposity, they did not address the agreement between BMI and FMI across the full range of adiposity, nor did they evaluate the relation of BMI to adiposity among children with very high BMIs (e.g., ≥97 percentile)," they added.

With these new findings, "pediatricians can feel confident that identification of elevated BMI (≥95th percentile) continues to be an effective way to screen for increased adiposity," Jaime Moore, MD, MPH, and Stephen Daniels, MD, PhD, MPH, both of Children's Hospital Colorado and the University of Colorado in Aurora, wrote accompanying the study.

"Elevated BMI in childhood persists into adulthood in 4 out of 5 children and is strongly linked to the development of numerous, life-limiting cardiometabolic complications," they noted. "In fact, improved standardization for the identification of pediatric obesity and severe obesity using BMI, when paired with equitable delivery of treatment, could help to counteract weight bias and reduce disparities in obesity-related health outcomes."

Some research has suggested BMI doesn't have the same relation to adiposity across racial and ethnic identity, but Freedman and colleagues noted that a examining national data found no consistent differences in BMI's performance for Hispanic and Black youth versus others.

Despite the reassurances on BMI's performance for screening youth, "the standardized use of BMI to identify patients with obesity is a first step," Moore and Daniels emphasized. "Additional risk stratification for how a specific BMI affects an individual's health should be person-centered and include additional data from the history and diagnostic testing. Further, treatment goals should be focused on health and quality-of-life outcomes, as opposed to a number or percentile on a growth chart."

In their study, Freeman and colleagues reported that BMI predicted high FMI with a kappa statistic of 0.85 and positive predictive value and the sensitivity both at 88%. BMI had kappa statistics of about 0.70 for high levels of percentage of body fat and lean mass index (LMI).

Age by itself, they noted, accounted for little -- 1% to 11% -- of the variability in either FMI or percentage of body fat, but accounted for 47% and 26% of the variability in LMI in boys and girls, respectively. However, the addition of BMI "substantially increased their explained variance," with the combination accounting for 90-94% of variability in LMI and FMI and 68-71% of variability in percentage of body fat.

Further findings included that, for kids and adolescents with a BMI at the 95th percentile, the probability of a high FMI or percentage of body fat was approximately 60%, they noted. The likelihood of a high FMI approached 100% at about the 98th percentile for boys and the 97th percentile for girls. And the probability of high percentage of body fat did not approach 100% until BMI was ≥99.5th percentile. Almost all participants with a very high BMI had high levels of both FMI and LMI, they added.

To complete their study, Freedman and colleagues examined the cross-sectional relation of BMI to adiposity among 6,923 children and adolescents in the National Health and Nutrition Survey from 2011 through 2018. Participants were scanned with dual-energy x-ray absorptiometry (DXA).

Limitations of the study included that some 15.6% of participants with weight and height data did not have valid DXA information, and that there can be "substantial differences in DXA estimates of adiposity compared with more complex body composition methods," Freedman and colleagues noted.

Additionally, "because BMI cannot characterize body fat distribution, several alternatives have been proposed, and many focus on waist circumference," they added.

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