Inclusion Criteria:
* Aged 50 years or older at the time of providing informed consent.
* Stable medical conditions are permitted provided they are not psychiatric or neurological in nature and are not known to interfere with the aims of the study, participant safety, or patient ability to consent.
* 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:
* Previous use of the study probiotic (1C-01) as individuals who have previously taken 1C-01 are not eligible.
* 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* Participants who commence systemic antibiotics or immunosuppressive therapy after enrolment will not be withdrawn from safety monitoring but will stop the supplement and will not continue the intervention phase of the study.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.
* Women of child-bearing potential (WOCBP). Women of non-child-bearing potential (WONCBP) are eligible; for the purposes of this study, WONCBP is defined as post-menopausal, i.e. no menstrual period for ≥12 consecutive months with no alternative medical cause. Breastfeeding women are also excluded.
* Known allergy to probiotic or compounding ingredients:
* Vitamin D3
* Isomaltooligosaccharide
* Yeast extract
