Fit-Ideal Internalisation and Disordered Eating in Adolescent Girls: Psychometric Evaluation and Incremental Validity of the Fit Internalization Test

Objectives: The present study aimed to validate the Lithuanian translation of the Fit Ideal Internalization Test (FIIT) in adolescent girls. Methods: A total of 1067 girls (mean age = 17.27, SD = 0.61; age range = 15–19 years) participated in the study. FIIT, muscular ideal internalization, body dissatisfaction (BD), disordered eating (DE), body appreciation (BA), and exercise motivation were assessed. Results: Exploratory factor analysis (n = 534) replicated the original three-factor structure, explaining 74.43% of the total variance. Confirmatory factor analysis (n = 533) supported the three-factor model, with strong factor loadings. Composite reliability was excellent (McDonald’s ω = 0.919–0.969). Measurement invariance was established across sport participation groups, and sport-involved girls reported higher Fit Drive, Fit Overvaluation, and FIIT total scores than non-sport-involved girls. All FIIT subscales were positively associated with muscular internalization, BD, and DE and negatively associated with BA, while Fit Idealisation and Fit Overvaluation showed weak negative associations with autonomous exercise motivation, consistent with the hypothesised pattern of associations. The FIIT explained significant additional variance in DE beyond body mass index, BD, and muscular ideal internalization, with Fit Idealisation showing the strongest unique association (β = 0.401, p < 0.001). Conclusions: The Lithuanian FIIT appears to be a psychometrically sound instrument for assessing fit ideal internalization among Lithuanian adolescent girls.