Almost every endurance athlete has suffered from gastrointestinal symptoms (e.g., abdominal cramps, nausea) during a training session or even worse, during a competition. For instance, in a recent study published in Nutrients (Pugh, 2018) the researchers observed that about one every four athletes in different marathon races presented with gastrointestinal symptoms. Moreover, half of the participants also had discomfort during the training sessions. Is there any solution to this problem?

Controlling carbohydrates intake

Carbohydrates are essential to attain an optimal performance in endurance sport, particularly on long-distance disciplines such as the marathon. In these events, the organism consumes glycogen and, if these stores get emptied, we can suffer a hypoglycemia. For this reason, carbohydrates should be regularly consumed during these races in form of gels, drinks or bars. However, both the defect and excess of carbohydrate intake can be detrimental.

Our cells can only transport a given quantity of carbohydrates to be used as fuel, being this limit about 60g/h in the case of some of the most popular sources such as glucose, sucrose or maltodextrins. Thus, if we consume larger doses of carbohydrates, the exceeding part will not be used as a fuel, and will in turn increase the odds of suffering from gastrointestinal issues. However, this limit (60 g/h) is only applicable when we take carbohydrates using the same type of transporter. Therefore, if we consume two carbohydrates that use a different transporter, such as glucose and fructose, we can provide 60 g/h of glucose and a further quantity of fructose (up to 30 g/h), which will enable us to have more energy without increasing the risk of gastrointestinal symptoms.  This has been demonstrated by studies that observed that the combined intake of glucose and fructose (about 108 g/h) resulted in a greater performance (8% higher) during a cycling time trial compared to the intake of the same quantity of carbohydrates but coming just from glucose (Currell, 2008).

Training the gut

It is important to note that, although the tolerance to carbohydrate intake seems to be greater when combining two sources that use different transporters, the gut should be trained to tolerate these quantities. Just as we plan our training, progressively increasing loads so that the organism adapts to them, we must also progressively increase the intake of carbohydrates (and all types of food/liquid) to induce adaptations and promote a greater tolerance. As Dr. Jeukendrup summarized in a review published in Sports Medicine (Jeukendrup, 2017), the gut must be trained. Progressively increasing the intake of carbohydrates will result in an increased tolerance and absorption of this nutrient, which will eventually lead to an enhanced performance and reduced gastrointestinal symptoms. Thus, the author suggests that strategies such as exercising with relatively large volumes of food/fluid on the stomach (i.e., training after lunch) or training with a high carbohydrate intake might increase the tolerance on the day of competition.

Figure 1. Methods proposed by Jeukendrup (2017) to induce adaptations that eventually attenuate gastrointestional symptoms during exercise, with subsequent benefits on performance.

Microbiota, a weapon against gastrointestinal symptoms?

The importance of a ‘healthy’ microbiota is being consistently proven, particularly regarding its role in the prevention of metabolic and digestive disorders. Moreover, recent studies suggest that microbiota might play a role in the prevention of gastrointestinal symptoms during exercise. A recent study analyzed 24 marathon runners that received either a probiotic supplement (which promote the development of microbiota) or a placebo during 4 weeks. The researchers observed that the group that took probiotics had less gastrointestinal symptoms during training sessions. Moreover, after 4 weeks of supplementation all participants ran a marathon, and the group that consumed probiotics had less gastrointestinal symptoms than the group that took the placebo. Thus, probiotic supplementation (or probiotic aliments such as yogurt or kefir) might be an effective strategy against gastrointestinal issues.

In summary, several factors can increase the incidence of gastrointestinal symptoms during exercise, including nutritional variables but also psychological (e.g., stress) and even environmental ones (e.g., temperature). However, some strategies such as controlling the intake of carbohydrates, taking probiotic supplements, or planning the nutritional intake the day of the competition can be determinants to achieve an optimal performance.


Pugh JN, et al. (2018) Prevalence, Severity and Potential Nutritional Causes of Gastrointestinal Symptoms during a Marathon in Recreational Runners. Nutrients. 10(7), pii: E811. doi: 10.3390/nu10070811.

Jeukendrup, AE. (2017) Training the Gut for Athletes. Sports Medicine. 47 (Suppl 1): 101-110.

Currell K, and Jeukendrup AE (2008) Superior endurance performance with ingestion of multiple transportable carbohydrates. Med Sci Sport Exerc. 40(2):275-81. doi: 10.1249/mss.0b013e31815adf19.

Pugh JN, et al. (2019) Four weeks of probiotic supplementation reduces GI symptoms during a marathon race. Eur J Appl Physiol. In press (Apr 13). doi: 10.1007/s00421-019-04136-3.


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