Longitudinal association of infant and early childhood body mass index with childhood and adolescent mental health: a Mendelian randomization study

BackgroundMental health issue during childhood and adolescence could have a lifelong influence on the quality of life, but its early-life risk factors are unclear. This study aims to explore the association of early childhood body mass index (BMI) on childhood and adolescent mental health. MethodsWe conducted a bi-directional two-sample Mendelian randomization (MR) study to investigate the association between infant and early childhood BMI and childhood and adolescence mental health disorders. Genetic instruments for BMI of 12 childhood age groups (from birth to 8 years old) were extracted from genome-wide association studies of up-to 28,681 European participants, and were used to proxy the primary childhood BMI exposures. Overall childhood BMI from an independent cohort was used as the validation exposure. Genetic associations with four childhood mental health disorders, including behavioural, emotional and social functioning disorders, aggression, and internalizing problems, were obtained from FinnGen and EAGLE consortia. The inverse-variance weighted or Wald ratio method was used as the discovery method, where MR-RAPS, dIVW, MR_cML were used as validation methods. ResultsIn the primary analysis, tthe 1-year-old and 2-year-old BMI were robustly associated with behavioural and emotional disorders onset during childhood and adolescence (OR=1.10, 95%CI=1.01 to 1.19, P=0.024; OR=1.12, 95%CI=1.002 to 1.24, P=0.046; respectively). These findings were replicated for emotional and social functioning disorders onset during childhood. BMI at 2 years old was robustly associated with aggression during childhood (OR=1.02, 95%CI=1.002 to 1.04, P=0.029). The analysis using independent childhood BMI data validated results for aggression. The bi-directional MR showed that none of the childhood mental health disorders had a reverse association with childhood BMI at any timepoint. ConclusionsThis study shows that BMI between 1-2 years old, and between 1.5-2 years old were robustly associated with behavioural and emotional disorder, and aggression, respectively. More attention is needed for early childhood weight control to prevent mental health disorders during childhood.