The goal of this randomised control trial is to measure the gut microbiome response to long-haul travel alongside the demands of pre-season amongst elite football players. The efficacy of a probiotic in modulating the gut microbiome to mitigate the detrimental impacts of long-haul travel as well as training in the heat will also be explored.

The main questions this study aims to answer are:

• How is the gut microbiome of the professional football player impacted by long-haul travel during the pre-season period?
• How is immune function impacted by the demands of pre-season?
• Can a probiotic dietary maintain gut microbial stability in response to long-haul travel? Researchers will compare the effects of the probiotic supplement with a placebo.

Participants will be required to:

• Provide a faecal sample at multiple points throughout the pre-season
• Provide a saliva sample at multiple points throughout the season
• Consume a daily supplement which will either be a placebo or probiotic capsule

Study Overview During the pre-season period it has become increasingly popular for professional football teams to travel to locations such as North America, Asia and Australia in attempts to maximise commercial revenue. The gut microbiome, the diverse bacterial community known to influence key physiological processes, has been shown to be negatively impacted by long-haul travel. As a result, elite athletes may benefit from dietary interventions to mitigate these negative impacts especially when considering the intense heat and physical demands of a pre-season camp conditions which have been shown to induce gut dysbiosis.

This study will firstly attempt to highlight the impact of long-haul travel on the gut microbiome of professional football players. It will also attempt to determine the efficacy of a probiotic dietary intervention in maintaining gut microbial stability in response to long-haul travel. A randomised, placebo-controlled trial will be utilised whereby players will consume either a daily probiotic or placebo. In addition, players will provide multiple faecal samples at specific time points; 1) arrival from the off-season break, 2) prior to the pre-season tour flight, 3) upon arrival to the pre-season tour destination, and 4) within five days of arriving back in UK. It is predicted that the probiotic intervention will result in greater microbial diversity and lead to a reduction in abundance of pathogenic bacterial strains typically seen following long-haul travel.

Background & Rationale The human gut microbiome is a sophisticated eco-system of microorganisms comprised of bacteria, archaea, fungi and viruses. This rich and diverse microbial community actively contributes to physiological processes such as energy metabolism, nutrient uptake and immune response. It is accepted that both the intensity and pattern of physical activity alters the composition of the gut microbiome and there is growing interest regarding the bidirectional association between exercise and the gut microbiome.

One sporting context which has seen a growing body of literature in this area is professional football. The professional football player is required to play up to 50 matches a season with regular periods of congested fixtures often played every 4 days for as long as 3 consecutive weeks. Given that the gut microbiota is heavily involved in the production and storage of energy, processes which are vital to meet the demands of physical performance, a suboptimal gut microbiota configuration may have a significant influence over players performance. As a result of the demands of the professional football player it is vital that such implications are avoided. Various authors have investigated the gut microbiome of the professional footballer across a range of study designs within both male and female cohorts. The results of these studies appear promising with findings suggesting that certain dietary strategies can maintain gut microbial stability and that elite players demonstrate higher prevalence of beneficial microbial populations compared with less active subjects.

When considering microbial stability several factors have a drastic impact on functional richness and microbial diversity. One such factor is travel, with short-term travel having been found to increase intra-individual gut microbiota fluctuations in non-athlete populations. These changes appear to be influenced by the composition of the gut microbiota prior to travel. In addition, international travel combined with dietary changes has been shown to negatively impact the gut microbiome of elite athletes, with reductions in microbial diversity and shifts in taxonomic profiles. However, to date no studies have explored the impact of long-haul travel on the gut microbiome of the professional footballer, highlighting an immediate gap in the literature.

In attempts to maximise commercial opportunities, modern football pre-season tours now regularly involve trips to locations such as North America or Asia. Pre-season tours outside of Europe have also become increasingly important for Premier League clubs as they serve as a vehicle for creating and maintaining strategic partnerships. As a result, players face the prospect of an annual long-haul trip in the pre-season period emphasising the importance of strategies to mitigate negative impacts on player well-being.

The pre-season period is characterised by high intensity tactical and technical training sessions to re-establish fitness following the off-season. Players often complete multiple training sessions per day and are therefore subjected to greater training loads compared to the in-season period. Pre-season tour destinations also typically present hot and humid environmental conditions, increasing exposure to heat stress which can impair performance and pose health risks. Exposure to these stimuli may have a detrimental effect on gut health by reducing functionality and causing gut dysbiosis. This combined with the requirement of long-haul travel presents an opportunity to explore the resilience and sensitivity of the gut microbiome during this period.

While the association between exercise and the gut microbiome is well established, understanding of dietary interventions to modulate the gut microbiome is still evolving. One commonly used approach involves providing live microorganisms to manipulate gut microbiome composition and correct perturbations. Probiotics are defined as live microorganisms that, when administered in adequate amounts, confer a health benefit on the host. However, there remains a lack of research investigating probiotic interventions in elite football populations. Existing evidence suggests probiotic-rich foods may increase microbial diversity in professional players, but further investigation is required.

Considering the above, it is reasonable to postulate that a dietary intervention aimed at modulating the gut microbiome may mitigate disturbances associated with long-haul travel and intensive pre-season training in hot and humid conditions. Therefore, this study will explore the impact of long-haul travel on the gut microbiome composition of elite football players and assess whether a probiotic intervention helps maintain gut microbial stability during a Premier League pre-season schedule.

Main Research Questions

• How does long-haul travel impact the gut microbiome composition of elite football players?
• Does a probiotic intervention offset gut microbiome disruptions associated with long-haul travel?
• Is there an added impact of environmental factors and training load on gut microbiome composition?

Methods Study Design This study will be designed as a randomised control trial to investigate the impact of long-haul travel on the gut microbiome of professional football players and explore whether probiotic supplementation offsets observed disruptions. Sample size will be calculated using GPower based on previous research demonstrating significant changes in microbial diversity following dietary interventions. A large expected effect size and statistical power of 80% will be used for an a priori sample size calculation.

Players will be randomly divided into two groups: 1) intervention (daily probiotic) and 2) control (daily placebo). Randomisation will be conducted using an online computer-generated tool. The intervention group will consume a probiotic capsule daily with their first meal, beginning five days prior to departure. Prior to travel, players will follow a structured training schedule with controlled nutrition provided by club staff. This approach aims to minimise inter-individual variability and establish a stable baseline gut microbiome prior to travel.

Dietary Assessment Direct dietary recording throughout the study period will not be feasible. However, dietary intake will be highly controlled as players will consume most meals at the training ground or team hotel, prepared from pre-planned menus by the club's Head Chef. This provides substantial control over food quality and nutrient intake and limits variability seen in free-living environments.

Salivary Immunoglobin A Long-haul travel and environmental stressors may disrupt immune homeostasis and increase susceptibility to illness. Salivary immunoglobin A will be monitored as a biomarker of immune function. Samples will be collected at baseline and multiple time points during travel using a standardised collection protocol. Samples will be analysed using a lateral flow device, with results recorded and stored by the lead researcher.

Gastrointestinal Symptoms Players will complete a daily gastrointestinal symptom questionnaire where possible to capture subjective responses to long-haul travel. The questionnaire assesses symptoms across multiple domains including reflux, abdominal pain, indigestion, diarrhoea and constipation using a 7-point Likert scale. This information will provide important contextual insight alongside microbiome data.

Temperature & Humidity Environmental conditions will be monitored using a heat stress tracker to measure temperature, humidity and heat stress indices. Measurements will be taken before and after each training session to quantify environmental exposure.

Faecal Sample Collection & Analysis Players will provide four faecal samples across the study period to assess changes in gut microbiome composition. Samples will be collected using validated self-collection methods and preserved for transport and storage. Samples collected in the UK will be frozen and processed within one month.

Faecal samples will be analysed using 16S rRNA sequencing. Alpha and beta diversity metrics will be used to assess microbial diversity and community composition. Statistical analyses will evaluate within- and between-group changes across time points. Differential abundance analysis and correlation testing will be performed using appropriate statistical software, with adjustments made for multiple comparisons.

Inclusion Criteria:

• Male professional footballers who are involved in regular training during the pre-season period.
• Generally healthy, with no acute or chronic illnesses that could interfere with the study intervention or outcomes.
• Not currently consuming probiotics, prebiotics, or antibiotics within 4 weeks prior to enrolment.
• Willing to refrain from starting any new dietary supplements or probiotics during the study period.
• Able and willing to provide written informed consent.

Exclusion Criteria:

• History of gastrointestinal disorders (e.g., Crohn's disease, ulcerative colitis, irritable bowel syndrome) or chronic diseases that could interfere with probiotic metabolism or study outcomes.
• Acute illness (e.g., infection) within 2 weeks prior to enrollment.
• Known allergy or hypersensitivity to components of the probiotic or placebo. - - - Use of antibiotics, probiotics, prebiotics, or synbiotics within 4 weeks prior to study start.
• Use of immunosuppressive medications or other supplements likely to impact gut microbiota.
• Regular use of alcohol or drugs at levels that could interfere with study participation or outcomes.
• Participation in another clinical trial within the past 3 months.
• Any injury or medical condition that limits participation in full training or matches during the study period.

Individual participant data (IPD) will not be shared because the study involves a small population of elite athletes, and sharing detailed participant data could risk breaching confidentiality and anonymity. In addition, there are no plans for secondary analyses requiring IPD sharing.