Changes in and Remission of Body Weight and Eating Disorder Psychopathology in Adolescents with Anorexia Nervosa During and Four Weeks Post Inpatient Treatment

Objectives: To assess associations between body weight metrics and eating disorder (ED)-psychopathology in adolescents with anorexia nervosa (AN) at baseline and four weeks post-discharge (4-week follow-up) from inpatient psychiatric multimodal treatment (IMT), calculating full and partial body mass index (BMI) percentile/ED-psychopathology remission rates. Methods: Secondary analysis of a prospective observational cohort study in adolescents (12–18 years) with AN-restricting (AN-R)/-binge–purge (AN-BP), and atypical AN (AAN). Body weight metrics and ED-psychopathology (Eating Disorder Examination Questionnaire, EDE-Q) were assessed at baseline and 4-week follow-up. Remission at 4-week follow-up was calculated by applying German-AN-S3-guidelines-based vs. DSM-5 criteria. Results: In 40 adolescents (mean age = 15.6 ± 1.5 years; females = 90%; BMI z-score = −2.59 ± 1.07) receiving IMT (median duration = 118 (IQR = 90–150) days), BMI z-score increased (−2.61 to −1.04, p < 0.001) and EDE-Q global score decreased (3.26 to 1.81, p < 0.001) from baseline to 4-week follow-up. Greater weight gain/week during IMT had a positive impact on ED-psychopathology at 4-week follow-up. In multivariable analyses, greater BMI z-score improvement was independently predicted by lower baseline BMI z-score (p < 0.001) and fewer baseline comorbid psychiatric diagnoses (p = 0.034) (R2Adjusted = 0.545). Greater EDE-Q global score improvement was independently predicted by typical vs. atypical AN (p = 0.005), higher baseline BMI z-score (p = 0.012), and higher baseline EDE-Q restraint (p = 0.048) (R2Adjusted = 0.376). By applying stricter S3-guideline-based vs. DSM-5 BMI percentile criteria, full BMI percentile/ED-psychopathology remission was lower (12.5% vs. 50.0%, p = 0.001), while non-remission was higher (25.0% vs. 5.0%, p = 0.001). Conclusions: The complex relationships between body weight metrics, ED-psychopathology, and treatment outcome trajectories in AN, including treatment response and remission criteria, require further investigation.