Immune function and gastrointestinal health in elite sport
It is well known that participating in elite sport comes at a high physical cost. For athletes, this often means an increase in the incidence and severity of upper respiratory tract infections (URTI) and gastrointestinal symptoms (GIS) which become more pronounced during heavy training and competition periods (Gleeson and Pyne., 2015; Pugh et al., 2018). Between 30 and 90% of athletes experience GIS (bloating, diarrhoea, nausea, reflux), and 35-65% of athletes are regularly affected by URTI (coughing, sore throat, nasal congestion), making them the most common non-injury related reasons for training absence and failure to reach sports performance goals (Pugh et al., 2018). Given the prevalence of these issues, identifying interventions to minimise the impact these may have on athlete health and performance become increasingly important.
Stress and the gut microbiome
In elite team sports, such as rugby, a physically demanding training and competition schedule combined with international travel and limited recovery time, all impact health and wellbeing (Coughlan et al., 2011). The physical nature of competitive rugby, with repeated bouts of high-intensity exercise and alternating periods of high-strength collisions, increase the risk of exercise-associated physiological stress, with players experiencing prolonged immune suppression, which as a result, increases susceptibility to URTI (Coughlan et al., 2011).
The same physiological stress is also associated with an increased incidence of uncomfortable GIS, by causing changes in GI mucosal activity and increasing GI membrane permeability which worsen with an increased training load (Sivamaruthi et al., 2019; Waterman and Kapur., 2012). The average reported incidence of URTI amongst rugby players is 4 times across one 18-week rugby season, and uncomfortable GIS are experienced more frequently in elite athletes when compared to non-athlete populations (Tiernan et al., 2019). This increased prevalence in athletes may have a negative impact on wellbeing and performance across a competitive season.
Modulating the gut microbiota via probiotic supplementation is a well-known complementary therapeutic approach for supporting GI health in athletes (Sivamaruthi et al., 2019). Prebiotics, whilst lesser known, are rapidly emerging as an effective way of supporting the gut microbiome, GI health and the immune system (Vulevic et al., 2015; Williams et al., 2016). For instance, prebiotics such as Bimuno GOS (galacto-oligosaccharides) have been shown to stimulate the production of bifidobacteria in the gut (Depeint et al., 2008; Vulevic et al., 2008; Vulevic et al., 2015), a microbial species known to have positive immunomodulatory effects; specifically, bifidobacteria have been shown to improve intestinal barrier function, reduce pro-inflammatory cytokines and increase salivary immunoglobulin-A (sIgA) production (Vulevic et al., 2015; Williams et al., 2016). Immunoglobulin A is the primary class of anti-body residing in mucous membranes and is considered to be a key marker of immune status that could be useful in identifying risk of URTI (Trochimiak and Hübner-Woźniak., 2012).
In a recent double-blind parallel-group randomised controlled trial, elite rugby union players were supplemented with the prebiotic, Bimuno GOS, daily for 24 weeks, resulting in a positive impact on immune function (Parker et al., 2023). The results indicated that in comparison with the placebo group, the rugby players consuming Bimuno GOS experienced a shorter duration of upper respiratory symptoms whilst saliva samples revealed a significantly higher concentration and secretion rate of slgA at 24-weeks, when immunity may have been compromised by an increased training load (Parker et al, 2023). Given that immunoglobulin-A plays an important role as the first line of defence against pathogens (Pappa et al., 2018), this suggests that GOS may modulate the immune system to reduce the duration of URTI and minimise days missed from training, which could be of clinical significance to coaches as it suggests a quicker return to competition following a period of respiratory illness.
In the same study, Bimuno GOS also demonstrated positive effects on GIS, with rugby players reporting a reduced incidence (more symptom-free weeks) and severity (lower symptom scores) of total and upper GIS symptoms, after the 24-week intervention, providing additional evidence that supplementation with prebiotics may support athlete wellbeing (Parker et al., 2023). The proposed mechanisms for how prebiotics may reduce GIS are via positive changes to the gut microflora, resulting in the reduction in the relative abundance of pathogenic bacteria, improved gut barrier function and increased production of metabolites such as short chain fatty acids which act as nutrients for colonocytes (Guarino et al., 2020). Increasing levels of bifidobacteria in the gut via GOS has been shown to support the maintenance of healthy intestinal function and gut homeostasis leading to benefits to host health (Cai et al., 2019).
Taken together, the findings of this study in elite rugby union players suggest that prebiotics, such as GOS, may afford immune-modulating benefits by reducing the duration of URTIs and helping to maintain gastrointestinal health by minimising incidence and severity of GIS in elite athletes. Supporting athlete health in this way, may improve the ability of players to train and compete, which could have significant implications for sporting performance across a competitive season.
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