The aim of the study is to evaluate the effectiveness and safety of fecal microbiota transplantation (FMT) in reducing gastrointestinal (GI) and behavioral symptoms in children with autism spectrum disorder (ASD).

Children with ASD often experience GI problems such as constipation, diarrhea, and abdominal pain. These symptoms can negatively affect their daily life and behavior. Recent research suggests that the gut microbiota-the community of bacteria and other microorganisms living in the intestines-plays an important role in digestion, immunity, and communication with the brain through the gut-brain axis. Modifying the gut microbiota may help improve GI symptoms and possibly behavioral functioning.

FMT involves giving a preparation containing gut microbiota from a healthy donor after bowel cleansing. The product used in this study is MBiotix HBI Caps, produced by Human Biome Institute. A placebo (inactive substance) will also be used for comparison. Both will be given as frozen oral capsules that look identical.

20 children aged 6-12 years will take part. Participants will be randomly assigned to receive either the microbiota preparation or placebo. The study includes several visits over about 6 months. Before the first dose, every child will undergo bowel cleansing with a special preparation (Polyethylene Glycol, PEG). During the study, participants will be asked to keep a symptom diary, complete questionnaires, and record child's diet. Biological samples (stool, urine, saliva) will be collected at specific time points for analysis. Every child will also be assessed by a psychologist before the study begins and again during the study using standardized tools (Autism Diagnostic Observation Schedule, Second Edition, ADOS-2) to evaluate behavioral functioning and quality of life.

Inclusion Criteria:

• Providing informed consent for participation in the study by the child's legal representative - parent or legal guardian.
• Diagnosis of autism spectrum disorder according to DSM-5 or ICD-11 criteria.
• Ability to swallow an empty test capsule identical in shape and size to the investigational product
• Abnormal stool consistency observed by parents for at least 2 months prior to study inclusion.

Patients with an average stool consistency greater than 5 on the Bristol Stool Form Scale (BSFS) during the baseline period will be classified as having diarrhea.

Patients with an average stool consistency less than 3 on the Bristol Stool Form Scale during the baseline period will be classified as having constipation.

Exclusion Criteria:

• Presence of a gastrointestinal disease such as celiac disease, food allergy, inflammatory bowel disease, pancreatitis, chronic liver disease, eosinophilic esophagitis, peptic ulcer disease of the stomach or duodenum, Hirschsprung's disease.
• Primary or secondary immunodeficiency, including absolute neutrophil count in peripheral blood <1500 measured within 28 days prior to planned FMT.
• History of surgery involving disruption of intestinal continuity within 3 months prior to study inclusion.
• Lactose or fructose intolerance. Patients with lactose intolerance who follow an elimination diet and still experience symptoms may be included in the study.
• Malnutrition defined as Body Mass Index (BMI) below the 3rd percentile based on Polish growth charts from 2010.
• Overweight or obesity defined as BMI above the 85th percentile based on Polish growth charts from 2010.
• Active infection requiring antibiotic therapy.
• Expected use of antibiotics during participation in the study.
• Use of probiotics during participation in the study.
• Inability to undergo bowel cleansing therapy (Dicopeg Endo).
• Inability to swallow an empty test capsule.
• Lack of consent from the child's legal representative - parent or legal guardian - for participation in the study.

IPD used in the results publication