Anorexia Nervosa (AN) is a severe, debilitating and potentially life threatening illness that is difficult to treat. A cardinal symptom of AN is the mistaken belief on the part of the individuals that they are overweight and must continue to restrict intake. This fixed false belief is a detrimental factor to recovery. It is known that AN involves disturbance in the gut microbiome (GM; the microbes that live in the lower intestinal tract). The GM also affects how one thinks and makes food choices - there appears to be a direct link between the GM and how the brain functions. This connection is thought to occur through chemical processes that convey information from the gut to the brain. It is known that fecal microbiome transplant (FMT) has been useful in treating several illnesses, including several mental illnesses. The investigators intend to deliver FMT to individuals with AN to determine the extent to which this modifies their GM, their biochemistry, their thinking processes and their moods and emotions. The investigators believe this will illuminate important aspects of AN that keep the illness in place, and that this will uncover useful approaches to better treat it.

The overall objective is to determine FMT's acceptability and the extent and means by which it helps patients with AN, restricting type. The investigators hypothesize that FMT will lead to diversification of the GM, improved metabolic and immunological status and reduced cognitive and psychiatric symptoms in patients with AN, details never investigated before. This will be due to FMT's ability to impart healthy microorganisms into the lower intestines and, thereby, improve GBM-axis signaling that may contribute to maintaining AN. The investigators propose that this will result in measurable improvement in cognitive distortions about weight, leading to reduced AN symptomatology, breaking the weight loss/AN cycle.

Specific Aims & Hypotheses

This is a one-group, pre-/post-intervention trial of FMT in AN-restricting type, with 1-week, 3-week and 3-month follow up, administered before specialized eating disorders treatment. The hypotheses are:

1. FMT will result in a sustained increase diversity of microbes with the metabolomic characteristics of increased short-chain fatty acids and bacteriophage composition compared with pre-treatment, as determined by targeted metabolomic approaches.
2. FMT will improve immune profiles, reducing IL6, IL1b and the proportion of Th17 cells, along with an increase in repair cytokines such as TARC, IL4, IL13 and BDNF and Th2 cells.
3. FMT will significantly improve cognitive and affective functioning, including task-switching efficiency, food aversion and obsessional thinking about weight, punishment and reward sensitivity, depression and anxiety symptomatology, and AN symptom severity.

This is a prospective, longitudinal, single-arm, pre-post intervention. Variables will be compared using repeated-measures analyses with each participant as their own comparator.

Inclusion Criteria:

• meet DSM 5 criteria for AN, restricting type, moderate or higher severity, as indicated by a BMI<17
• Participants must we willing and able to swallow FMT capsules without vomiting
• Able to read and understand conversational English

Exclusion Criteria:

1. Medical or psychiatric instability needing hospitalization
2. Patients with AN binge/purge type
3. Use of antibiotics or probiotics in the month prior to treatment
4. Regular oral steroid use, or potent topical steroid use on large sections of skin
5. Chronic immune compromise and chronic illness affecting the intestinal tract or metabolic health
6. Pregnancy or intended pregnancy over the time of study
7. Patients enrolled in any treatment program that involves refeeding

Participants in this study will be awaiting treatment for anorexia nervosa at London Health Sciences Centre's (LHSC, Ontario Canada) Adult Eating Disorders Service (AEDS).

This is a small feasibility study. While we would not make personally identifiable information public, even the basic demographic information could be used to re-identify a participant. In light of the nature of this study we do not feel it is in the interest of our participants to share data on a repository.