Anorexia nervosa (AN) is a serious mental disorder occurring mainly in women. AN is characterized by severely restricted food-intake and subsequent low weight. The disease burden for the individual is high with medical complications and psychiatric comorbidities. Despite decades of research, there are large gaps in the understanding of the biological aspects of AN and lack of effective interventions. Current clinical treatment is associated with gastrointestinal problems, high rates of relapse and poor outcome causing long-term sickness absence and disability. During the COVID19 pandemic the prevalence and severity of AN has spiked. Therefore, there is great need of novel strategies for AN treatment, that can be easily implemented in the clinic without adding complexity to the standard care of treatment. During the resent years it has been proposed that mental disorders might be treated via manipulating the composition and function of the microbes that live in the gut (the microbiota) by adding or restricting fermentable nutrients (prebiotics) in the diet. However, in order to use prebiotics to treat the microbiota in AN patients, more knowledge is needed on how the AN microbiota is affected by the current standard care treatment. Whether prebiotics can be useful for normalizing AN microbiota remains to be established. The overall aim of the "Norwegian study of Microbiota in Anorexia Nervosa" (NORMA) is to join forces of researchers, clinical health care services and voluntary sector in a transdiciplinary approach to improve the understanding of the role of the gut microbiota in AN patients. The current project will include a clinical trial in AN patients and experimental studies to screen novel prebiotics for their ability to modify and normalize AN derived microbiota. The long-term goal of the project is to pave the way for a targeted and clinically feasible individualized treatment for better tolerable weight-restoration and improved health in AN patients.

Inclusion Criteria AN group

1. Sex: Female
2. Age: 16-50 years
3. BMI: <18.5 kg/m2
4. fulfilling ICD-10 criteria for AN
5. currently referred to specialized inpatient nutritional treatment for AN
6. able to understand the Norwegian questionnaires.

Exclusion Criteria AND group:

1. history of inflammatory bowel disease, celiac disease, or GI tract surgery;
2. treatment with oral antibiotics the past two months
3. high intake of probiotic supplements over the past two months.

Inclusion criteria control group

1. Sex: Female
2. Age: 16-50 years
3. BMI: >= 18.5 & < 27
4. able to understand the Norwegian questionnaires.

Exclusion Criteria AND group:

1. history of inflammatory bowel disease, celiac disease, or GI tract surgery;
2. treatment with oral antibiotics the past two months
3. high intake of probiotic supplements over the past two months.

The female patients with anorexia nervosa (AN) are undergoing specialized nutritional rehabilitation treatment and will be recruited from five Norwegian specialized inpatient treatment units for eating disorders. The standard care treatment consists of a program of psychotherapy and nutritional rehabilitation to achieve normalization of food-intake and weight gain. Although the clinical strategy of AN treatment is standardized, both the psychotherapy and dietary advices are individualized. The dietary advice during standard care treatment is based on clinical experience, as specific guidelines are lacking. Diets are adapted to each individual in aspects of tolerability and preferences, but typically comprise a high-energy diets aiming for a weekly weight gain of 0.5-1.5 kilograms. Often enteral nutrition (i.e., tube feeding) is required. The control group will be aged-matched female subjects with normal weight - mildly overweight.