Children who follow higher trajectories of body mass index (BMI) and fat mass index across childhood report greater body dissatisfaction at age 14, in both boys and girls, according to a new study published October 14, 2026, in the open access journal PLOS One by Dr. Madelaine Davies Kellock, a Research Fellow at University College London, UK, and colleagues.
Body dissatisfaction has been rising among young people over recent decades and is linked to later depression and eating disorders. Prior research using BMI has suggested weight relates to body dissatisfaction differently in girls and boys, but BMI cannot distinguish fat mass from fat-free mass, which includes muscle, bone and other tissues. Fat mass and fat-free mass may relate to dissatisfaction differently than BMI, particularly given the social desirability of muscularity in boys.
Researchers used data from 13,025 participants in the UK Millennium Cohort Study, which provided measurements of height and weight from age 3, and body composition from ages 7 to 14. For the new analysis, children were grouped into low, moderate and high trajectories of BMI, fat mass index, and fat-free mass index based on that data. At age 14, body dissatisfaction was measured using a self-reported questionnaire.
Children with moderate or high BMI trajectories had higher dissatisfaction scores than those with low trajectories (adjusted mean difference 0.32 and 0.71, on a 0-6 scale), with similar results for fat mass index (0.33 and 0.73), and no evidence these associations differed by sex. The researchers expected higher fat-free mass index, which includes muscle as well as other non-fat tissue, to relate to lower dissatisfaction in boys; instead, boys with high fat-free mass had greater dissatisfaction than those with low fat-free mass (adjusted mean difference 0.31), and the association was larger in girls (0.36 moderate, 0.79 high).
Because the study was observational, it could not establish that body composition causes body dissatisfaction. It was also limited by the single question measurement of body dissatisfaction and the fact that children from more disadvantaged families were more likely to have missing data.
The authors conclude that both girls and boys with higher childhood adiposity are at risk of body dissatisfaction in adolescence and say the findings could help shape ways to target body dissatisfaction.
The authors add: “Body dissatisfaction is often thought of as something that mainly affects girls, but our findings suggest that both girls and boys with higher BMI or fat mass index trajectories in childhood have higher levels of body dissatisfaction in adolescence.
Our findings for fat-free mass index were quite surprising. We did not find that higher fat-free mass index was associated with lower body dissatisfaction in boys, but as we found a smaller association compared to girls, this might suggest that some aspects of fat-free mass are less of a risk factor for body dissatisfaction in boys. There are lots of reasons why someone might have high fat-free mass, so it’s likely that this group includes people with quite different physical characteristics, including high levels of lean muscle mass, as well as those with increased bone and muscle mass partly as a consequence of having high fat mass. This could mask the role that aspects such as muscle mass might play in how young people experience their bodies.
There is now good evidence that body dissatisfaction in adolescence is associated with subsequent poorer mental health. The association between adiposity and body dissatisfaction might emerge because young people with higher weight experience more negative events, such as stigmatising comments and bullying. Therefore, it is important that we think carefully about how we talk about weight and body size with young people, including in schools.”
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Citation: 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 21(10): e0339555. https://doi.org/10.1371/journal.pone.0339555
Author countries: UK.
Funding: MDK was supported by the Economic and Social Research Council - Biotechnology and Biological Sciences Research Council Soc-B Centre for Doctoral Training (ES/P000347/1). FS was supported by a Sir Henry Wellcome Fellowship (209196/Z/17/Z) and a Wellcome Career Development Award (225993/Z/22/Z). AM was supported by the Economic and Social Research Council (ES/W001454/1), and AM and YK were supported by the Equalise: Economic and Social Research Council Centre for Lifecourse Health Equity (ES/Z504270/1). The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.
PLOS One
Observational study
People
Associations of childhood body composition trajectories and adolescent body dissatisfaction: Longitudinal evidence from the UK Millennium Cohort Study
14-Oct-2026
The authors have declared that no competing interests exist.