This study is a pre-post, open-label cohort study designed to investigate how a food-grade probiotic supplement affects biological measurements and wellbeing in healthy adults. Participants will take one capsule daily for either 1 month or 6 months.

During the study, participants will complete online cognitive tasks and provide blood and stool samples collected during home visits by trained staff. The samples will be analysed to explore changes in gut bacteria and other biological markers.

This study aims to understand whether the supplement is well tolerated and whether measurable biological changes occur. The study does not involve any experimental drugs or invasive procedures beyond blood sampling and stool collection, and participants will not be asked to change any current prescribed medications or treatments; with eligibility exclusions applying for recent antibiotics or immunosuppressants. The supplement is being studied for research purposes only and is not intended to diagnose, treat, or prevent disease. Participants will be invited to participate in a follow-up visit to assess long-term effects.

Inclusion Criteria:

• Aged 18 years or older at the time of providing informed consent.
• In generally good health, with no known medical conditions that could interfere with the aims of the study or participant safety, as judged by self-report and screening questionnaire.
• No history of significant psychiatric or neurological illness
• Ability to complete study specific tasks, including:
• Daily oral intake of a probiotic capsule
• Self-collection of stool samples
• Completion of online questionnaires and simple cognitive tasks
• Able to read, understand, and communicate in English, in order to give informed consent and follow study instructions.
• Access to the internet and a suitable device (e.g., smartphone, tablet, or computer) for completing online aspects of the study.

Exclusion Criteria:

• Participants will be excluded from the study if any of the following criteria apply:
• Use of any systemic antibiotic or systemic immunosuppressive medication within the past 8 weeks, including but not limited to:
• β-lactam antibiotics, macrolides, tetracyclines, fluoroquinolones, aminoglycosides, glycopeptides, sulfonamides, antimycobacterial agents, systemic antifungals or antivirals
• Use of any systemic immunosuppressants within the past 16 weeks, including, but not limited to:
• Systemic corticosteroids, conventional immunosuppressants, biologic immunomodulators, targeted synthetic immunosuppressants, cytotoxic chemotherapy, or transplant-related immunosuppression.
• A longer exclusion window is applied for systemic immunosuppressive therapies due to their prolonged and potentially lasting effects on immune function and gut microbial composition compared with systemic antibiotics.
• Participants who commence systemic antibiotics or immunosuppressive therapy after enrolment will not be withdrawn from safety monitoring but will stop the supplement and will not continue the intervention phase of the study. With participant consent, data collected may still be used.
• Major gastrointestinal disease or surgery:
• Any history of significant gastrointestinal disease or surgery that may alter gut microbiome composition, immune function, or nutrient absorption, including but not limited to inflammatory bowel disease (Crohn's disease, ulcerative colitis, indeterminate colitis); coeliac disease or other chronic malabsorptive disorders; chronic liver, biliary, or pancreatic disease; moderate-to-severe or unstable irritable bowel syndrome; gastrointestinal malignancy; chronic gastrointestinal motility disorders; recurrent Clostridioides difficile infection; or any major gastrointestinal surgery such as bowel resection, bariatric surgery, colectomy, ileostomy or colostomy. Prior appendectomy or cholecystectomy alone is not exclusionary.
• Current pregnancy or development of pregnancy, breastfeeding
• Known allergy to probiotic or compounding ingredients:
• Vitamin D3
• Isomaltooligosaccharide
• Yeast extract

All personal data collected during the study will be pseudonymised at the earliest possible stage. Each participant will be assigned a unique study ID number, which will be used on all samples, questionnaires, and datasets. A separate, password-protected file will securely store the link between participant names and study IDs. This file will be accessible only to authorised study personnel.

No identifiable personal information will be included in the research database or shared with external analysts. All electronic data will be stored on secure, encrypted servers in compliance with UK GDPR and the Data Protection Act 2018. Access will be limited to members of the research team through password-protected accounts and role-based permissions.

Hard copies of personal data will not be collected or stored. Any correspondence containing identifiable information (e.g. email addresses) will be deleted or anonymised once no longer needed.