The investigators hypothesize that prolonged undernutrition in anorexia nervosa alters the microbiome to a different steady-state (dysbiotic) composition that sustains the disease, even after returning to normal diet. The investigators propose that transplanting a fully ecologically functioning GM from a healthy donor, through a FMT, can reboot the gut-brain-axis, ameliorate symptoms and improve clinical outcomes.

To approach this, in the challenging AN patient group, the investigators want to conduct a FMT feasibility/pilot study.

Anorexia nervosa (AN) still carries the highest fatality rate of any psychiatric disease, and less than half of the patients recover, completely refractory to any treatment. The etiology remains unknown and evidence for treatment is lacking. We have an established collaboration bridging several disciplines and people in medicine, bioinformatics and biology with a focus on the pathological role of gut microbiome in the psychiatric disorder Anorexia nervosa. The GM in healthy people has been shown to affect weight gain, appetite and behavior through the gut-brain axis. We hypothesize that prolonged undernutrition in AN alters the microbiome to a different steady-state (dysbiotic) composition that sustains the disease, even after returning to normal diet. AN patients enter voluntarily into programs of therapeutic weight gain and reintroduction to normal food intake. The investigators propose that transplanting a fully ecologically functioning GM from a healthy donor, through a FMT, can reboot the gut-brain-axis, ameliorate symptoms and improve clinical outcomes. To approach this, in the challenging AN patient group, we want to conduct a FMT feasibility/pilot study. The investigators want approach the FMT intervention, with 20 AN patients in sequence before any attempt at more complex blinded /placebo clinical trials. The investigators will collect GM samples and clinical data from these patients and describe the altered composition and diversity of the AN-GM and how it changes with FMT.

Ultimately, the investigators aim to provide evidence that GM modulation with FMT can ameliorateclinical outcomes of AN. The investigators will investigate the feasibility of using FMT in supportive treatment of AN and by identifying specific bacterial and viral changes with FMT, we will lay the groundwork for randomized placebo-controlledstudies, to develop new GM interventions to complement current treatment of AN.

Inclusion: Women referred to the participating specialized centers or by advertising who fulfill the of a diagnosis of Anorexia Nervosa according to the criteria as specified in the Diagnostic and Statistical Manual of Mental Disorders(DSM) version 5.

Exclusion:

Age <18 years. Known ongoing abuse of laxatives.

Antibiotic treatment within the last three months.

Probiotics within the last month

Inflammatory bowel disease, colorectal cancer or other known chronic bowel disorder