Multi-Strain Probiotic Supplementation, Salivary Immunity, and Upper Respiratory Symptoms in Trained Athletes

A Randomized, Double-Blind, Placebo-Controlled Study Exploring the Impact of Multi-Strain Probiotic Supplementation on Mucosal Immunity and Upper Respiratory Symptom Episodes in Team-Sport Athletes

This completed external pilot study assessed the feasibility of conducting a randomised, double-blind, placebo-controlled trial of four-week multi-strain probiotic supplementation in trained athletes. Participants were allocated to receive either one probiotic capsule containing 20 billion colony-forming units or a matched rice-flour placebo capsule daily for 28 days. Feasibility measures included recruitment, retention, supplementation adherence and completion of daily monitoring records. Preliminary biological and symptom outcomes included salivary secretory immunoglobulin A and upper respiratory symptom burden. The findings were intended to inform the design of a future definitive randomised controlled trial and were not intended to provide a definitive assessment of efficacy.

Inclusion Criteria:

• Male trained athletes aged 18-40 years.
• Regularly participating in structured training and/or competitive sport.
• Apparently healthy at enrolment, with no acute illness or injury that would prevent completion of the exercise testing protocol.
• Able to complete high-intensity treadmill exercise testing.
• Willing to consume the assigned supplement daily for the full intervention period.
• Willing to avoid probiotic supplements, prebiotic supplements, and other new immune- or gut-health-related supplements during the study period.
• Willing and able to provide saliva samples and complete daily upper respiratory symptom monitoring.
• Able to provide written informed consent.

Exclusion Criteria:

• Current acute respiratory, gastrointestinal, or systemic illness at enrolment.
• Use of antibiotics within the previous 4 weeks.
• Regular use of probiotic or prebiotic supplements before enrolment, unless willing to stop before and during the study period.
• Use of medication or supplements likely to affect immune, inflammatory, gastrointestinal, or salivary biomarker outcomes.
• Known chronic gastrointestinal disease, immune disorder, metabolic disease, cardiovascular disease, or other medical condition that could affect study outcomes or exercise safety.
• Known allergy or intolerance to any ingredient in the probiotic or placebo capsule.
• Current smoker or regular nicotine user.
• Any musculoskeletal injury or health condition limiting maximal or high-intensity treadmill exercise.
• Participation in another intervention study during the study period.
• Inability or unwillingness to comply with the study procedures.