Associations of childhood body composition trajectories and adolescent body dissatisfaction: Longitudinal evidence from the UK Millennium Cohort Study.
Davies Kellock M, Solmi F, McMunn A, Kelly Y • PloS one • 2026
BMI and FMI trajectories are similarly associated with body dissatisfaction when FMI is estimated using bioelectrical impedance analysis, and both boys and girls may be susceptible to weight-related body dissatisfaction in adolescence.
Key Findings
Results
Participants with moderate and high BMI trajectories had significantly greater body dissatisfaction at age 14 compared to those with low BMI trajectories.
Sample included 13,025 participants (49.2% female; 84.4% White) from the UK Millennium Cohort Study.
Moderate BMI trajectory: adjusted mean difference (aMD) 0.32, 95% CI 0.24–0.39.
High BMI trajectory: aMD 0.71, 95% CI 0.55–0.86.
BMI trajectories were modelled from ages 3–14 years using latent class growth analysis, yielding three trajectory groups (low, moderate, high).
Results
Participants with moderate and high fat mass index (FMI) trajectories had greater body dissatisfaction at age 14 compared to those with low FMI trajectories, with effect sizes similar to those for BMI.
Moderate FMI trajectory: aMD 0.33, 95% CI 0.25–0.41.
High FMI trajectory: aMD 0.73, 95% CI 0.57–0.89.
FMI trajectories were modelled from ages 7–14 years using latent class growth analysis.
FMI was estimated using bioelectrical impedance analysis.
The similarity of FMI and BMI effect sizes suggests BMI adequately captures adiposity-related body dissatisfaction in this context.
Results
The associations of BMI and FMI trajectories with body dissatisfaction did not differ significantly by sex.
Interaction terms were used to test for sex differences in the association between BMI/FMI trajectories and body dissatisfaction.
No evidence of sex differences was found for BMI or FMI trajectory associations with body dissatisfaction.
This suggests both boys and girls may be susceptible to weight-related body dissatisfaction in adolescence.
Results
Boys with high fat-free mass index (FFMI) trajectories had greater body dissatisfaction, but those with moderate FFMI trajectories did not differ significantly from those with low trajectories.
High FFMI trajectory in boys: aMD 0.31, 95% CI 0.17–0.46.
Moderate FFMI trajectory in boys: aMD 0.07, 95% CI -0.03–0.17 (not statistically significant).
FFMI trajectories were modelled from ages 7–14 years.
This finding is contrary to what might be expected if higher muscularity were protective against body dissatisfaction in boys.
Results
In girls, associations of FFMI trajectories with body dissatisfaction were consistent with the pattern seen for adiposity measures (BMI and FMI).
Mean differences by FFMI trajectory in girls were described as 'consistent with adiposity results.'
This pattern may reflect that higher fat-free mass in girls is correlated with higher overall body size, which is associated with greater dissatisfaction.
FFMI was estimated using bioelectrical impedance analysis from ages 7–14 years.
Methods
Three distinct trajectory groups (low, moderate, high) were identified for each of BMI, FMI, and FFMI using latent class growth analysis.
BMI trajectories spanned ages 3–14 years; FMI and FFMI trajectories spanned ages 7–14 years.
Latent class growth analysis was the modelling approach used for all three measures.
The outcome, body dissatisfaction, was measured at age 14.
Multivariable linear regression models were used to examine associations after adjustment for potential confounders.
What This Means
This research used data from over 13,000 children in the UK Millennium Cohort Study to examine how patterns of body composition from childhood into adolescence are linked to body dissatisfaction at age 14. The study tracked three measures — body mass index (BMI), fat mass index (FMI, a measure of body fat), and fat-free mass index (FFMI, a measure of lean/muscle mass) — and grouped children into low, moderate, and high trajectory groups based on how these measures changed over time. Children who followed higher BMI or higher fat mass trajectories throughout childhood were significantly more dissatisfied with their bodies at age 14, and this was true for both boys and girls equally.
One notable finding was that BMI and FMI trajectories produced very similar results, suggesting that in this study population, BMI does a reasonably good job of reflecting body fat-related body dissatisfaction despite its known limitations. The results for fat-free mass (FFMI) were more complex: boys with the highest FFMI trajectories reported more body dissatisfaction — not less — which challenges the assumption that higher muscularity would make boys feel better about their bodies. In girls, higher FFMI was similarly linked to more dissatisfaction, possibly because greater muscle mass in girls tends to coincide with larger overall body size.
This research suggests that weight-related body dissatisfaction in adolescence is not just a female issue — boys are also affected. It also raises questions about how muscularity relates to body image in boys, since higher lean mass did not appear protective. Understanding these patterns could help inform body image interventions that account for the different ways body composition affects how young people feel about themselves.
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Davies Kellock M, Solmi F, McMunn A, Kelly Y. (2026). Associations of childhood body composition trajectories and adolescent body dissatisfaction: Longitudinal evidence from the UK Millennium Cohort Study.. PloS one. https://doi.org/10.1371/journal.pone.0339555