It is now well accepted that gut complaints during and after exercise are a common issues amongst athletes (Read more here). The primary causes are changes in blood flow to the gut, stress responses, and mechanical impacts on the gut, in response to exercise per se. In addition, these exercise associated gut complications can be made worse by a wide array of factors within the exercise scenario (Read part 1 here).
Strategic pathway to identify causal factors and manage gut complication and complaints during and after exercise in athletes.
Gastro-intestinal problems need thorough assessment
A substantial amount of anecdotal evidence from sport and exercise nutrition practitioners report athletes’ frustrations in attempting multiple prevention and management strategies (e.g., food and fluid quantity and quality, manoeuvre techniques, hydration, food restriction or avoidance, supplementation, etc…), in a ‘trial and error’ fashion, without any consistent positive results and/or sporadic improvement across a large number of events. Considering the diverse factors that will cause gut issues in athletes, it is clear that ‘one size’ does not fit all, and that an athlete tailored intervention (i.e., applying appropriate and effective strategies to reduce gut complications and complains during and after exercise) is warranted after thorough assessment. This may include understanding the background history of an athlete’s recurrent gut issues, undertaking a gut assessment during exercise to identify the cause and factors that worsen the gut complications and complaints during exercise, blood tests, or more specialised diagnostic testing such as gut motility, gut microbiota composition, food allergy or intolerance investigation.
In the last 5 years there has been an exponential increase in research into prevention and management strategies of gut complications and complaints in athletes. The most promising strategies have included:
Carbohydrate consumption before and during exercise, within individually tolerable limits, has been shown to prevented exercise causing damage to gut cells, and does not cause any accentuated gut discomfort above water (2). Protein (i.e., whey isolate) at the same rate results in similar gut damage prevention benefits, but results in substantially more gut discomfort (2).
Gut-training by challenging the gut with high content and volume of carbohydrates has been shown to improved carbohydrate malabsorption, improved glucose availability into the blood, and substantially reduced gut discomfort, which translated into improve running performance, (1). (Link)
The role of fermentable oligo- di- mono-saccharides and polyols (FODMAP) in managing athletes gut complications during around exercise has been previously covered (link). Recent research revealed that a 24h low FODMAP (2g/day), compared with a high FODMAP (47g), energy and nutrient matched diet, was sufficient to reduced gut complications during endurance running in the heat, by reducing the degree of carbohydrate malabsorption before starting exercise. An interesting finding was that the low FODMAP diet also induced greater gut cell injury and greater disturbance to the gut-blood barrier compared with the high FODMAP diet. The protective effect of a high FODMAP diet on gut cells and gut-blood barrier may lay with its influence on changing the gut microbiota composition (e.g., commensal vs. pathogenic bacteria profile) (3).
Other potential beneficial strategies:
Starting and maintained hydration throughout exercise may protect against gut cell damage and carbohydrate malabsorption compared with dehydration. But forcing fluid consumption to maintain hydration may lead to exaggerated gut complications due to stomach overload. A balance is needed between hydrating and tolerance (4).
Despite further research needed in this area, consistently cooling the body internally (e.g., cold drinks) and externally (e.g., cold shower) during exercise, and/or following heat acclimatisation/acclimation protocols, seem promising in reducing gut complications and complaints during exercise, especially when performed in the heat (5).
Pharmaceutical approaches: Avoid of NSAIDS. Anti-emetics may have a role to play in reducing nausea (5).
Strategies that have shown no substantial effect or need further exploration:
A wide array of nutritional supplementations have been proposed to reduced gut complications and complains during and after exercise (e.g., biotics, glutamine, arginine, L-citralline, anti-oxidants, curcumin, nitrate, bovine colostrum, etc). However, a large proportion of research is riddled with methodological issues that has previously been identified and explored (5,6). Base on the current evidence, there is no justification for advising the use of nutritional supplementation for prevention or management of gut issues during exercise.
Gluten avoidance by athletes as previously been covered in mysportscience (Link). In short, unless an athlete is diagnosed with Coeliac disease, there is no justification for dietary application.
Although still in its infancy stage of research, the impact of the gut microbiota composition on gut issues during exercise have shown a potential role in modifying the gut microbiota to mitigate some of these issues (7).
It has previously been suggested that dairy milk before and after exercise exacerbated gut complaints around exercise. Recent evidence has shown this is not the case; whereby a high dairy milk content breakfast was given before 2h of endurance running, with no evidence of carbohydrate malabsorption observed in healthy athletes (4). Additionally, malabsorption of a recovery beverage after exercise was similar for dairy milk and a carbohydrate-electrolyte beverage with similar carbohydrate content (8).
Whether the fibre and/or residue content of foods and/or fluids consumed around exercise cause gut complaints that effect performance has yet to be clearly determined using controlled laboratory research trials.
Based on the current evidence with have to date and from a professional practice perspective, the best approach to resolving and management gut issues during exercise in athletes involve a 3-level approach:
Level 1: Individual athlete assessment (infographic).
Level 2: Individualised prevention and management intervention (infographic).
Level 3: General advice, which may include, small and frequent carbohydrate feeding during exercise within tolerance, maintain hydration, cooling strategies, no need for nutritional supplementation, dietary fibre intake within tolerance level, avoidance of NSAIDS and liaise with a sports medic regarding use of anti-emetics.
Further investigation into the role of modification of the gut microbiota for managing gut complications during exercise is currently ongoing. However, increased α-diversity and relative abundance of SCFA commensal bacteria appears to protect against some damaging effects of exercise on the gut; therefore predominate intake of plant based foods, rich in fibre and FODMAP within tolerance levels, outside the exercise periods is warranted; which supports the application of 'Personalised and Periodised Nutrition' (Link).
Costa, R.J.S., Miall, A., Khoo, A., Rauch, C., Snipe, R., Camões-Costa, V., Gibson, P., (2017). Gut-training: The impact of two weeks repetitive gut-challenge during exercise on gastrointestinal status, glucose availability, fuel kinetics, and running performance. Appl.Physiol.Nutri.Metab., 42(5):547-557.
Snipe, R., Khoo, A., Kitic, C., Gibson, P., Costa, R.J.S., (2017). Carbohydrate and protein intake during exertional-heat stress ameliorates intestinal epithelial injury and small intestine permeability. Appl.Physiol.Nutri.Metab., 42(12):1283-1292.
Gaskell, S.K., Taylor, B., Muir, J., Costa, R.J.S., (2020). Impact of 24-hour low and high fermentable oligo- di- mono- saccharide polyol diets on markers of exercise-induced gastrointestinal syndrome in response to exertional-heat stress. Appl.Physiol.Nutri.Metab. 45(6): 569-580.
Costa, R.J.S., Camões-Costa, V., Snipe, R.M.J., Dixon, D., Russo, I., Huschtscha, Z. (2019). The impact of exercise-induced hypohydration on intestinal integrity, function, symptoms, and systemic endotoxin and inflammatory responses. J. Appl. Physiol., 126(5):1281-1291.
Costa, R.J.S., Gaskell, S.K., McCubbin, A.J., Snipe, R.M.J (2020). Exertional-heat stress associated gastrointestinal perturbations- management strategies for athletes preparing for and competing in the 2020 Tokyo Olympic Games. Temp. 7(1):58-88.
Costa, R.J.S., Snipe, R., Kitic, C., Gibson, P., (2017). Systematic review: Exercise-induced gastrointestinal syndrome- Implication for health and disease. Alim.Therap.Pharmacol., 46(3):246-265.
Bennett C., Snipe R., Henry R., Costa R.J.S. (2020). Is the gut microbiota bacterial abundance and composition associated with intestinal epithelial injury, systemic inflammatory profile, and gastrointestinal symptoms in response to exertional-heat stress? J Sci Med Sport. (In press).
Russo, I., Della Gatta, P.A., Garnham, A., Porter, J., Burke, L.M., Costa, R.J.S., (2021). Assessing overall exercise recovery processes using carbohydrate and carbohydrate-protein containing recovery beverages. Front.Physiol. (Revisions).