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What causes gut issues during and after exercise?

Many competitions take place in hot and challenging conditions. The Tokyo Olympic Games are only an example. The expected hot conditions are likely to affect athletes’ performance, both physically and mentally. This will challenge their ability to tolerate competition in the heat. One under-estimated issue that affects performance and that is highly reported amongst various athlete populations (especially when competing in the heat), is gastrointestinal symptoms i.e. gut complaints.

Gut complaints in athletes

Most competitive recreational or elite athletes experience gut complaints at some point. For some athletes it’s sporadic and has no consequences; but for other it’s repetitive, frustrating, debilitating, and can lead to more serious short- and long-term health complications (1). The incidence, severity, type, and performance outcomes of exercise induced gut complaints has been covered in a previous blog. There are several factors that influence the diversity and severity of gut complaints. These include the event type, duration and intensity of exercise, ambient conditions and methodological aspects (1). The big question is, what is causing these debilitating gut issues in a large number of athletes? This is what we will cover in this blog.

What is causing these debilitating gut issues in a large number of athletes?

What are the causes?

Exercise induced gut issues predominantly arise from normal physiological changes that occur to the gastrointestinal and immune systems in response to exercise (1). There is a large body of evidence linking changes in both blood flow and the stress response when initiating exercise to gut complaints (2). There is also growing evidence that a third physiological change in mechanical (e.g., foot strike, jarring, jolting, and sudden impact) aspects may be linked. Each of these factors is covered in more detail below:

1. Blood diverted away from the gut

When exercising, blood is diverted to the muscles (to provide nutrients and oxygen to the working muscles) and skin (to help keep the body cool). This reduces blood flow to the gut. If this persists for a long time, then the gastrointestinal barrier may become damaged. It also becomes highly permeable to unwanted and pathogenic microorganisms (e.g., bacteria and bacterial endotoxins) that are naturally present along the gastrointestinal tract as part of the gut microbiota. The localised damage along with the permeability of microorganisms into the circulating blood stream prompts local and overall immune responses, characterised as inflammation.

2. Stress response

The stress hormone response of exercise promotes a reduction in overall gut function, including reduced gastrointestinal motility, nutrient digestion and absorption. This subsequently increases the risk of nutrient (e.g., carbohydrate) malabsorption (1,2). Therefore, exercise per se naturally impedes an athlete’s ability to provide nutrients and water at a time of increased requirements (3).

3. Mechanical effects

The mechanical strains of exercise on the gut have been speculated to result in gastrointestinal barrier damage and reduced function (1,2).

The pathways do not occur in isolation...

It is important to note that these different mechanisms that contribute to gut issues are not independent of each other. They interact dynamically with each other. For example, gastrointestinal tissue damage may reduce its function, while reduced gut function may increase tissue damage, and both perturbations may increase risk of nutrient malabsorption. Moreover, the presence of malabsorbed nutrients along the intestinal tract, as a result of tissue damage and/or reduced function, may:

  1. Stimulate gut hormones and contribute to slowing or braking gut motility

  2. Be subject to bacterial fermentation along the intestinal tract that will result in increasing gas and water content.

It is important to note that these different mechanisms that contribute to gut issues are not independent of each other.

What increases gut complaints?

There also exists a wide spectrum of factors that can aggravate or prevent gut complaints during and after exercise, dependant of the magnitude of exposure and how these are managed. These factors include:

  • exercise intensity (e.g., high > low),

  • duration (e.g., long > short),

  • modality (e.g., running > cycling),

  • ambient conditions (e.g., hot > warm > temperate),

  • circadian variation (e.g., nocturnal > diurnal),

  • hydration status (e.g., dehydration > euhydration),

  • pre-exercise diary habits (e.g., macronutrient composition),

  • feeding tolerance (e.g., gut-training < no gut-training),

  • pharmaceuticals (e.g., none < non-steroidal anti-inflammatory drugs),

  • predisposition to gut disease/disorders (e.g., predisposition > no predisposition),

  • and the gut microbiota composition (e.g., α-diversity and relative abundance of commensal bacteria).


All of these pathways, dynamic interactions, and predisposing factors are possible reasons for athletes reporting gut complaints during and after exercise. They also explain why the type, incident, and severity may alter within and between exercise bouts (e.g., training and competition), and within and between individual athletes. It thus shows the importance of individual gut assessment during exercise to diagnose the principal causal and predisposing factors prior to any prevention and/or management strategies are trialled. It clearly shows how a ‘one size’ does not fit all, and that trial-and-error approaches are lengthy and often without closure. Find out about gut assessment during exercise and the evidence-based prevention and management options available in part 2.


  1. 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.

  2. 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.

  3. 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.


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