Secondary objectives are to (1) prospectively identify baseline predictors of treatment outcome and dropout, (2) determine variables related to the treatment process and patient engagement as predictors of the outcome and/or treatment dropout, and (3) in a multidisciplinary approach, focus on selected pathophysiological mechanisms including changes in the gut microbiota as well as immunological measures in patients with severe AN in different stages of the disease, and determine to what extent they are related to treatment outcome.