Associations of Body-Size and Estimated Body-Composition Indicators with Body-Shape Concerns and Eating Disorder-Related Traits in Adolescents: A School-Based Cross-Sectional Study

Background/Objectives: Adolescence is a critical period for the development of body-shape concerns and eating disorder-related traits. This study examined sex differences and associations among anthropometric and estimated body-composition indicators, body-shape concerns, and eating disorder-related traits in adolescents. Methods: This school-based cross-sectional study included a convenience sample of 723 adolescents aged 11–17 years (404 boys and 319 girls) from three secondary schools in Valencia, Spain. Anthropometric and skinfold measurements were obtained, and participants completed the Body Shape Questionnaire (BSQ) and the Drive for Thinness, Bulimia, and Body Dissatisfaction subscales of the Eating Disorder Inventory-3 Referral Form. Between-sex comparisons, sex-stratified Spearman correlations, and multivariable linear regression models were performed. Results: Girls had higher estimated body-fat percentages than boys (19.29% vs. 11.95%; p < 0.001), as well as higher BSQ (62.23 ± 26.71 vs. 56.62 ± 27.19; p < 0.001) and Drive for Thinness scores (11.43 ± 6.55 vs. 8.36 ± 6.26; p < 0.001). No significant sex differences were observed for Bulimia or Body Dissatisfaction. Sex-stratified correlations between anthropometric or estimated body-composition indicators and the psychological outcomes were weak. Although several associations reached nominal significance, none remained statistically significant after false-discovery-rate correction for multiple testing. Nevertheless, the adjusted multivariable analysis continued to show an independent association between body-fat percentage and Drive for Thinness. The sex-stratified correlations should therefore be regarded as exploratory findings requiring replication. Conclusions: Girls showed greater body-shape concerns and Drive for Thinness. Body-fat percentage was independently associated with EDI-DT scores after adjustment for age and sex; its association with BSQ scores was of borderline significance with robust standard errors. These cross-sectional findings do not support causal interpretations.