Predictors of change: Does a higher body weight at the beginning of treatment actually relate to a better treatment outcome in persons with anorexia nervosa?

Objective: It is often stated that higher body weight at the beginning of treatment predicts a better outcome at the end of treatment in persons with anorexia nervosa. However, this interpretation is usually based on the between-persons relationship of body mass index at admission and discharge, which primarily reflects the fact that the rank-ordering of persons with anorexia nervosa according to their body weight is quite similar at different measurements. Method: Changes of body mass index from admission to discharge were analyzed based on 4863 persons with anorexia nervosa (97% female) who received inpatient treatment between 2015 and 2024 at the Schoen Clinic Roseneck (Prien am Chiemsee, Germany). Results: Body mass index at admission positively related to body mass index at discharge (r = .6) but negatively related to body mass index change from admission to discharge (r = .4), that is, lower body mass index at admission predicted larger weight gain. This relationship remained when controlling for age, sex, length of stay, and comorbid mental disorders. Discussion: While it is true that higher body weight at the beginning of treatment is associated with higher body weight at the end of treatment in persons with anorexia nervosa, lower body weight at the beginning of treatment actually relates to a larger weight gain during treatment.