Nutritional and environmental exposures in athletes: Implications on the epithelial barrier function
Authors: Alba Angelina, Mario Pérez‐Diego, Oscar Palomares
Journal: Allergy (2024)

Over the last decades, industrialization, and technological advances associated with a modern lifestyle have been accompanied by an alarming rise in the prevalence of chronic noncommunicable diseases.1 The epithelial barrier theory attributes this phenomenon to exposure to newly introduced toxic substances that are capable of disrupting the protective epithelial barriers since the 1960s.1 Pollutants such as particulate matter, ozone, diesel exhaust particles, nano- and/or micro-plastics, surfactants, and detergents included in cleaning products, or several additives, emulsifiers and flavor enhancers used in the food industry can compromise the epithelial barriers in the skin, airways, and the gut.1, 2 Genetic factors and exposure to epithelial barrier-damaging agents promote leaky barriers, leading to microbial dysbiosis, bacterial translocation, and perpetuation of local and systemic inflammation.1, 2 In this elegant review article, Kistler W. et al. discuss the impact of a hazardous exposome and physical exertion typically endured by elite athletes. These factors may counteract the health benefits of regular physical activity by disrupting the epithelial barrier function. In this context, experimental evidence supporting the epithelial barrier theory can be applied to sports medicine.3 There is a social and scientific consensus on the health benefits of regular, mild to moderate physical activity. However, epidemiological studies indicate that athletes undergoing excessive training are not in optimal overall health, which can negatively impact their sport performance. For instance, gastrointestinal disorders are very common among elite athletes.4 Likewise, they show a higher risk of infections and noninfectious respiratory diseases, such as exercise-induced asthma, rhinitis, and laryngeal disorders.5 In the context of the epithelial barrier, the benefits of exercise may display a J-shaped relationship. Moderate exercise improves the integrity of the intestinal barrier and in turn, promotes the growth of healthy microbiota populations and ameliorates a proinflammatory immune state.6 On the other hand, compelling evidence shows that prolonged intense physical activity impairs the intestinal epithelial tight junctions, compromising its barrier integrity and allowing the entry of microbes and pollutants that can trigger an inflammatory cascade.7 In some cases, it may also aggravate an existing pathology; for example, a leaky gut facilitates the passage of food allergens and reduces the threshold for food-dependent anaphylaxis.8 Similarly, excessive training has been closely associated with airway epithelial barrier damage and local inflammation, especially in endurance sports, as epithelial injury is a key factor in the development of exercise-induced bronchoconstriction.9 Another facet highlighted by the authors is that athletes are at higher risk of exposure to toxic substances. During intense exercise, hyperpnea and an increase in respiratory rate occur to satisfy oxygen demands, resulting in increased inhalation of toxic airborne substances that may damage the airway epithelial barrier and promote airway inflammation.10 Endurance runners and swimmers are some of the most susceptible athletes due to their high exposure to pollution in urban areas and chlorine-based disinfectants present in swimming pools, respectively.9, 10 The nutrition of athletes should be carefully monitored. Athletes requiring a high-calorie diet may have a higher consumption of processed food products marketed for athletes. Sports drinks and dietary supplements may contain emulsifiers, preservatives, flavor enhancers, and other artificial additives that can disrupt the intestinal epithelial barrier and imbalance microbiota populations, eventually aggravating the well-known gastrointestinal symptoms associated with high-impact exercise such as vomiting and diarrhea. Moreover, it is important to point out that the good hygienic conditions required during elite sports practice can increase the athletes' exposure to chemical disinfectants and detergents, such as those used to clean the changing rooms, towels, and training gear. Altogether, these routines and activities associated with vigorous exercise make athletes more susceptible to potentially harmful substances compared to less active individuals. Considering all of the above-mentioned aspects, the authors conclude that a call for action is necessary to implement strategies that mitigate the environmental burden continuously faced by athletes, including the identification and avoidance of epithelial barrier-damaging agents, the development of safer products, the discovery of novel biomarkers of epithelial barrier disruption, the implementation of therapeutic approaches based on improving barrier function, and further research3 (Figure 1). In summary, the article by Kistler W. et al. provides a comprehensive review of the potential mechanisms underlying the harmful effects of intense physical training in the context of the epithelial barrier theory. The physical exertion reached during intense exercise and the elevated risk of exposure to airborne pollutants and toxic substances might contribute to an impaired epithelial barrier in athletes and physically active individuals. Consequently, athletes are more prone to infections and chronic inflammatory diseases, thereby compromising their sports performance. Future research in sports medicine should include novel strategies to mitigate the deleterious exposome faced by athletes as well as the repair of the epithelial barrier tight junctions. This work was supported by grant PID2020-114396RB-I00 to O.P. from MICINN, Spain. MP-D is a recipient of an FPI (SAF-20217-84978-R) predoctoral fellowship from Ministerio de Economía y Competitividad. A.A. is recipient of a Margarita Salas contract (Reference: UCM-CT18/22) cofinanced by the Ministerio de Universidades, the "Plan de Recuperación, Transformación y Resilencia" and the Complutense University of Madrid. Dr. Oscar Palomares declares research grants from MINECO, Ministerio de Ciencia e Innovación, CAM, Inmunotek S.L., Novartis, and AstraZeneca and receiving fees for presenting scientific lectures or participation in advisory boards from AstraZeneca, Pfizer, GlaxoSmithKline, Inmunotek S.L, Novartis and Sanofi-Genzyme. Data sharing not applicable to this article as no datasets were generated or analysed during the current study.