*1.1. Anorexia Nervosa*

Anorexia nervosa (AN) is a serious illness leading to high morbidity and mortality [1–4]. It is characterised by a restriction of energy intake, weight loss, fear of weight gain and distorted body image. According to the diagnostic criteria of the International Classification of Diseases, 11th Revision (ICD-11) [5] and the Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5) [6], the resulting malnutrition and low body weight may result in massive impairment to health. Often it takes years for patients with AN to achieve a first remission or to recover permanently. A quarter of adult patients go on to develop an enduring form of the disorder, and one-third of patients continue to suffer from residual symptoms in the long-term. The long-term outcome of adolescent-onset AN is more favourable [7]. Because of its severe and protracted course, AN represents a high emotional and economic burden for sufferers, carers and the society in general [8,9]. Age of onset peaks in middle

to late adolescence, which affects educational and professional development. The consequences of starvation can have a negative impact on bone density, growth, and brain maturation, especially in children and adolescents. Many patients are affected by comorbid psychological diseases, such as depression, anxiety or obsessive–compulsive disorder. Additionally, the ego-syntonic nature of AN leads to a strong ambivalence regarding weight gain and recovery, which complicates and often slows down the recovery process. In light of these factors, treatment of AN remains challenging. To improve patients' chances of recovery, all individuals dealing with this illness should be well informed about the nature and challenges of treating AN.
