Associations of childhood body composition trajectories and adolescent body dissatisfaction: Longitudinal evidence from the UK Millennium Cohort Study

Body dissatisfaction is common, particularly among those with higher BMIs. Issues with BMI as an adiposity measure may limit our understanding of how body weight is associated with body dissatisfaction, particularly given gendered body ideals. We investigated the association of childhood trajectories of BMI, fat mass index (FMI), and fat-free mass index (FFMI) with body dissatisfaction at age 14, and sex differences in these associations. This was a secondary analysis of UK Millennium Cohort Study data. Our exposures were trajectories of BMI (3-14 years), FMI and FFMI (7-14 years), modelled using latent class growth analysis. Our outcome was body dissatisfaction at 14 years. We used univariable and multivariable linear regression models to investigate associations and interaction terms to test for sex differences. Among 13,025 participants (49.2% female; 84.4% White), we found three BMI, FMI, and FFMI trajectories (low, moderate, high). Participants with moderate (adjusted mean difference [aMD] 0.32, 95% confidence interval [CI] 0.24, 0.39) and high (aMD 0.71, 95% CI 0.55, 0.86) BMI trajectories and those with moderate (aMD 0.33, 95% CI 0.25, 0.41) and high (aMD 0.73, 95% CI 0.57, 0.89) FMI trajectories had higher body dissatisfaction scores than those with low trajectories. We did not find evidence that these associations differed by sex. For FFMI trajectories, boys with high (aMD 0.31, 95% CI 0.17, 0.46), but not moderate (aMD 0.07, 95% CI -0.03, 0.17) trajectories had greater body dissatisfaction. In girls, mean differences by FFMI trajectory were consistent with adiposity results. BMI and FMI trajectories are similarly associated with body dissatisfaction when FMI is estimated using bioelectrical impedance analysis. Both boys and girls may be susceptible to weight-related body dissatisfaction in adolescence. Further research is needed to understand how muscularity is associated with body dissatisfaction.