The general objective of this clinical trial is to assess the physical fitness and gut microbiota of older adults, thereby highlighting the importance of focusing on health maintenance. By making use of available resources and fostering collaboration among the elements involved in the study, more specific objectives can be defined:

1. To determine the state of the gut flora in older adults through the analysis of the intestinal microbiota.
2. To identify changes in the intestinal microbiota resulting from the intake of probiotics in older adults.
3. To evaluate the effects of the intestinal microbiota in older adults participating in a training programme.
4. To assess the physical fitness and perceived quality of life in older adults who engage in sport activities supported by technology.

Participants will:

• Perform physical tests such as the 6-Minute Walk Test (6MWT), the Timed Up and Go (TUG), and the Sit-to-Stand test.
• Complete wellbeing questionnaires to assess their perceived quality of life and health status.
• Complete dietary habit questionnaires.
• Provide stool samples for microbiota analysis.
• Take part in exercise interventions or placebo exercise sessions, depending on their assigned group.

Inclusion Criteria:

• Adults aged 60 years or older.
• Residents of Castilla-La Mancha or Madrid.
• Able to provide informed consent.
• For the intervention group: enrolled in a municipal physical activity program with a minimum frequency of 2 sessions per week.
• Willing to participate in all assessments and follow study instructions.
• Consent: Participants must provide written informed consent to participate in the study.

Exclusion Criteria:

• Recent serious musculoskeletal injuries (e.g., muscle tears, ligament ruptures, fractures).
• Current symptoms during exercise such as chest pain, dizziness, or joint pain.
• Uncontrolled hypertension (≥160/100 mmHg).
• Recent cardiac failure or infectious disease (e.g., meningitis, hepatitis, flu).
• Chronic gastrointestinal diseases (e.g., Crohn's disease, ulcerative colitis, IBS).
• Regular use of medications affecting gut motility or microbiota (e.g., laxatives >3 times/week).
• Immunodeficiency (e.g., HIV, active cancer).
• Use of antibiotics (oral or parenteral) within 4 weeks prior to the study.
• Regular intake of probiotic supplements or foods (e.g., Actimel) within 2 weeks prior to the study.

Individual participant data (IPD) will not be shared due to privacy concerns and the nature of the study being part of a doctoral thesis. The data are collected under strict confidentiality agreements and are intended solely for internal analysis and academic purposes within the research team. Any future data sharing would require additional ethical approvals and participant consent.

In this double-blind study, both participants and outcomes assessors are masked to the intervention assignment. The probiotic and placebo capsules are identical in appearance, taste, and packaging, ensuring that participants cannot distinguish between them. Outcomes assessors responsible for evaluating physical condition, microbiota composition, and quality of life are also blinded to group allocation to minimize assessment bias. The investigators administering the interventions are not blinded due to logistical requirements in distributing the correct supplement.