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Background: A dual-energy x-ray absorptiometry (DXA)-derived phenotype classification based on fat mass and muscle mass has been developed for adults. We extended this to a paediatric population.

Methods: Children's (= 17 years) DXA data in NHANES (n = 6120) were used to generate sex- and age-specific deciles of appendicular skeletal muscle mass index and fat mass index with the Lambda Mu Sigma method. Four phenotypes (high [H] or low [L], adiposity [A] and muscle mass [M]: HA-HM, HA-LM, LA-HM, LA-LM) were identified based on being above/below the median compared to same-sex and same-age peers. These reference curves were applied to the QUALITY cohort (n = 630, 8-10 years of age in 2005) to assess whether the phenotypes correctly identified cardiometabolic risk at baseline, follow-up (2008-2010), and their longitudinal changes. Multiple linear regression models were adjusted for age, sex, and Tanner's stage.

Results: Compared to the LA-HM reference group, the HA-HM phenotype was associated with less favourable HDL, triglycerides, and HOMA-IR at baseline and first follow-up, but not in their changes. The HA-LM phenotype was associated with less favourable HOMA-IR at baseline and first follow-up.

Conclusions: Results suggest that phenotypes based on fat and muscle mass may have clinical utility in children and should be further investigated.