Is infant body mass index associated with adulthood body composition trajectories? An exploratory analysis.

Background: Infant body mass index (BMI) is increasingly used as a marker of obesity risk based on its association with young-adulthood BMI.

Objectives: The aim of this study is to test the association of infant BMI with young-adulthood fat mass and fat-free mass, and how this association changes during advancing adulthood.

Methods: Body mass index Z-score at age 9 months was measured in 350 White, non-Hispanic Fels Longitudinal Study participants. This exposure was entered into multilevel models to test its association with trajectories describing 2665 BMI observations and 1388 observations of fat mass index (FMI, kg m ) and fat-free mass index (FFMI, kg m ) between ages 20 and 60 years.

Results: Partitioning young-adulthood BMI into its fat and fat-free components, infant BMI Z-score was associated with FFMI (β = 0.745; 95% confidence interval = 0.367 to 1.124) but not FMI (0.528; -0.055 to 1.110) at age 20 years. Greater infant BMI Z-score was associated with slower age-related increases in all outcomes, such that (looking at 10-year intervals) only FFMI at age 30 years was related to infant BMI Z-score (0.338; 0.119, 0.557).

Conclusions: Focus on infant BMI reduction for adulthood obesity prevention warrants caution as high infant BMI values are associated with greater lean mass, which is protective against ageing changes.