The goal of this clinical trial is to determine if Fecal Microbiota Transplantation (FMT) is effective in treating Anorexia Nervosa (AN). The study will also explore how FMT works. The main questions it aims to answer are:

1. What is the efficacy of combined FMT treatment in patients with AN?
2. What is the relationship between treatment efficacy and changes in gut microbiota, and what mechanisms underlie the effects of FMT in treating AN? Researchers will compare FMT to a placebo (a look-alike substance that contains no active ingredients) to see if FMT is effective in treating AN.

Participants will:

• Be hospitalized
• Take FMT or placebo daily for 6 days
• Provide stool and blood samples before and after FMT
• Visit the clinic every 4 weeks for check-ups and tests

Anorexia Nervosa (AN) is a chronic, challenging psychiatric disorder characterized by an intense fear of gaining weight, extreme weight loss, severe malnutrition, and physical wasting. The core behavioral trait of AN is a "persistent restriction of energy intake," which results in low body weight, malnutrition, and, in severe cases, death due to multiple organ failure. Consequently, AN is considered the most fatal mental disorder. With globalization and rapid economic development in China, dieting and weight loss have become fashionable. A domestic survey found that 49.9% to 55.7% of female college students engage in dieting and weight loss behaviors, and the prevalence of AN in China has been increasing rapidly. Thus, AN is becoming a significant public health issue in China.

More than 90% of AN patients experience functional gastrointestinal symptoms (FGS), including bloating, constipation, and difficulty swallowing. These FGS are largely associated with disordered eating behaviors. For example, patients with restrictive eating often prefer high-fiber, low-fat, and low-carbohydrate foods, which can lead to symptoms such as early satiety, constipation, and flatulence. Therefore, disordered eating behavior can contribute to abnormal gastrointestinal function, while, conversely, disordered GI function can exacerbate the severity of AN, hinder patients from resuming normal eating habits, and impact the effectiveness of clinical treatment. Recent studies increasingly suggest that gut microbes play an essential role in influencing GI symptoms in psychiatric disorders.

The human gastrointestinal tract hosts billions of microorganisms, with a total gene count about 150 times greater than that of the human genome. These microorganisms are closely linked to the regulation of gastrointestinal function and have also been shown to influence mood, eating behavior, appetite, and nutrient metabolism. Researchers have discovered that Nod2 receptors expressed by inhibitory neurons in the hypothalamus and arcuate nucleus can directly sense cytosolic peptides produced by bacteria, regulating feeding behavior and suggesting that gut microbes may influence appetite through the brain-gut axis. Cross-sectional studies have found abnormalities in gut flora composition or metabolites in AN patients, while longitudinal studies have shown that the abundance of gut flora increases after weight regain in AN patients; however, flora diversity and gastrointestinal symptoms do not fully return to normal. This suggests that, while body weight may normalize, the gut microbiome remains "unhealthy."

Fecal Microbiota Transplantation (FMT) involves transplanting healthy intestinal flora into a patient's gut. FMT primarily treats digestive diseases like intestinal infections, irritable bowel syndrome, Crohn's disease, and chronic diarrhea, effectively relieving symptoms such as constipation and diarrhea. FMT has also been used to treat neuropsychiatric disorders like autism, mood disorders, ADHD, and sleep disorders. Studies have shown that FMT can alleviate symptoms of depression, anxiety, and sleep disturbances caused by chronic stress, reduce inflammatory responses to slow the progression of Alzheimer's disease, and improve symptoms in patients with autism spectrum disorder (ASD). Although some case reports describe FMT's application in AN patients, there is still a lack of extensive randomized controlled trials to validate its efficacy.

Currently, no studies have explored the role of FMT combined with conventional treatments in improving AN's clinical symptoms. Given that mood and GI symptoms are common comorbidities of AN, and there is no effective pharmacologic treatment for these symptoms, FMT offers a promising new strategy to enhance clinical outcomes as a supportive treatment for AN.

Inclusion Criteria:

• Female, aged 12-35 years, meets DSM-5 diagnostic criteria for anorexia nervosa.
• BMI > 14 kg/m²; for children/adolescents, BMI > the 0.3 percentile for their age.
• Presence of digestive symptoms (e.g., constipation, bloating, diarrhea).
• Duration of the disease (since the onset of dieting and weight loss) is less than two years.
• Each patient must understand the nature of this study and sign an informed consent form.

Exclusion Criteria:

• Co-morbid severe mental disorders such as schizophrenia, bipolar disorder, intellectual disability, persistent delusional disorder, schizoaffective disorder, or organic mental disorders.
• Substance abusers/addicts, individuals with a high suicide risk, or those with strong destructive impulses or antisocial behavior.
• Those with severe low body mass anorexia nervosa (e.g., blood pressure < 80/50 mmHg, heart rate < 40 beats/min, weight loss > 1 kg per week, or those requiring assistance in sitting or squatting in a prone position).
• Patients with severe somatic diseases or comorbidities, including:
• Organic intestinal lesions such as congenital megacolon, intestinal obstruction, or intussusception.
• Previous pathological intestinal inflammatory conditions such as inflammatory bowel disease.
• History of traumatic brain injury, cerebral palsy, encephalitis, or other organic brain disorders.
• Severe hepatic or renal insufficiency (defined as levels greater than three times the upper limit of normal).
• Pregnant or breastfeeding women.
• Use of medications such as probiotics or antibiotics that clearly affect the intestinal flora within 1 months prior to the visit or during the treatment.
• Any other reason that the investigator deems unsuitable for participation in this clinical trial.