Secular trends in the fat and fat-free components of body mass index in children aged 8–18 years born 1958–1995

Background: It is unknown whether the secular trend in childhood BMI reflects increases in fat-free mass as well as fat mass. Methods: This study decomposed BMI trends in 488 participants in the Fels Longitudinal Study born between 1958–1995 and aged 8–17.99 years into their fat and fat-free components. Generalized estimating equations estimated birth year cohort (1958–1970, 1971–1983, 1984–1995) effects on 2208 observations of BMI, fat mass index (FMI = fat mass (kg)/height (m)2) and fat-free mass index (FFMI = fat-free mass (kg)/height (m)2). Results: BMI in boys increased across cohorts, with those born between 1984–1995 being 2 kg/m2 larger than those born between 1958–1970 (p = 0.001) and increases in FMI were highly significant (p-values < 0.001). FFMI did not differ by cohort. In girls, there was a significant advantage in BMI (1.2 kg/m2) and FFMI (0.8 kg/m2) of the 1984–1995 cohort compared to the 1971–1983 cohort (p-values < 0.05). Conclusions: Because the long term trend in childhood BMI in boys...