This study aims to evaluate whether a fermented milk drink containing the probiotic strain Lacticaseibacillus paracasei strain Shirota (LCS), commonly found in Yakult®, can help reduce upper respiratory symptoms in triathletes before and after competition.

Athletes often experience cold-like symptoms due to physical stress, intense training, and immune system challenges. The study investigates whether daily consumption of this probiotic drink can help improve immune response and reduce the incidence or severity of symptoms such as sore throat, nasal congestion, or coughing.

This is a randomized, double-blind, placebo-controlled, parallel-group study involving healthy adult triathletes. Participants will be randomly assigned to receive either the probiotic drink or a placebo for a specific period before and after a triathlon event. Symptoms and health markers will be tracked through questionnaires and biological samples.

The goal is to explore whether probiotic supplementation can provide practical, non-pharmacological support for athletes' immune health and well-being during intense physical activity.

Background:

Endurance athletes are frequently exposed to high physical stress, which may lead to temporary immune suppression and increased susceptibility to upper respiratory tract infections (URTIs). Previous studies suggest that probiotics may play a role in supporting the immune system.

Objective:

To assess the efficacy of a fermented milk drink containing Lacticaseibacillus paracasei strain Shirota (LCS) in reducing upper airway symptoms in triathletes before and after a competitive event.

Study Design:

This is a multi-center, double-blind, randomized, placebo-controlled, parallel-group study. Eligible participants will be healthy adult triathletes preparing for a scheduled race. They will be randomized to consume either the probiotic drink or a placebo daily, starting several weeks before the race and continuing for a defined recovery period afterward.

Primary Outcome:

-Salivary IgA Concentration The primary outcome is the change in salivary immunoglobulin A (IgA) concentration before and after the competition, assessed at five time points. This measure will reflect mucosal immune function and the potential modulatory effect of daily intake of fermented milk containing Lacticaseibacillus paracasei strain Shirota.

Secondary Outcomes:

• Systemic Immune and Inflammatory Response Blood samples will be analyzed for markers of systemic immune activation and inflammation (e.g., cytokines), collected at five time points to evaluate the broader immunomodulatory effects of the probiotic intervention.
• Upper Airway Symptoms Incidence, severity, and duration of upper airway symptoms (e.g., sore throat, cough, nasal congestion) will be assessed using the Wisconsin Upper Respiratory Symptom Survey-21 (WURSS-21). Higher scores indicate more severe symptoms and a greater impact on daily functioning.
• Mood State Mood state changes will be evaluated using the Brunel Mood Scale (BRUMS) across six subscales. Total Mood Disturbance (TMD) scores will reflect the psychological response to training and competition. Higher scores indicate worse mood states.

Inclusion Criteria:

• Aged between 25 and 45 years.
• Amateur triathletes with regular endurance training and prior competition experience.
• In good general health, with no history of chronic diseases or conditions affecting immune or inflammatory responses.
• Willing and able to comply with all study procedures, including sample collections and completion of validated questionnaires at five time points.
• Agree to consume a daily dose of fermented milk containing Lacticaseibacillus paracasei strain Shirota (LcS) or placebo from 30 days before until 14 days after the competition.
• No use of probiotics, antibiotics, or immunomodulatory supplements in the 30 days prior to enrollment

Exclusion Criteria:

• Known allergy or intolerance to dairy products, including lactose.
• Use of probiotics or antibiotics within 30 days prior to enrollment.
• Current use of any pharmacological treatment.
• History of chronic rhinopathy.
• Diagnosis of autoimmune disease.
• History of recurrent infections not involving the upper airway.
• Presence of major medical conditions or chronic diseases, including but not limited to respiratory or gastrointestinal disorders.
• Incomplete or missing data necessary for determining eligibility or for study outcomes.

Individual participant data (IPD) that underlie the results reported in the publication will be shared, after de-identification. This includes salivary IgA concentrations, data on immune and inflammatory markers, upper airway symptoms, and mood state scores (e.g., BRUMS questionnaire).

IPD and supporting documents will be available beginning 6 months after publication of the study results and will remain available for 3 years.

Researchers who provide a methodologically sound proposal will be able to access the IPD and supporting information. Requests should be directed to the principal investigator by email. A data access agreement will be required prior to sharing.

This is a double-blind study. Participants, care providers, investigators, and outcomes assessors will be blinded to group assignments to prevent bias.