To minimize energy deficit and limit the use of glycogen reserves during ultra-endurance events, adequate, sufficient, and regular hydration and nutrition throughout the effort will be essential and decisive elements.
In ultra-endurance athletes, gastrointestinal problems (GIP) are extremely common and represent the leading cause of withdrawal from races.
On average, in marathons, between 30% and 50% of runners are affected by digestive problems, a prevalence that climbs to 83% for 60 km races and even reaches 96% for 161 km ultra-marathons (Source). These problems include both upper digestive issues like heartburn, nausea, vomiting, and lower digestive issues like abdominal pain, urgent urges, or diarrhea. These symptoms, in most cases, will increase with the duration of the effort and depending on certain particularly difficult conditions: high temperatures, high humidity, high altitude. In the most severe cases, this can lead to hospitalization.
Origins and explanations of Gastrointestinal Problems:
Firstly, mechanical constraints related to the activities performed (for example, repeated impacts in disciplines like running, trail running...) can lead to the release of prostaglandins and intestinal peptides, which accelerate intestinal motility.
In addition to mechanical factors, circulatory factors can also contribute to GIP. The reduction in splanchnic blood flow during exercise leads to ischemia (lack of oxygenation) of the digestive tract, thus limiting digestive function. It is important to remember that this phenomenon is exacerbated by dehydration. During intense efforts, the reduction in blood flow to the digestive system can reach up to 80%, which severely limits the ease of assimilation of ingested nutrients.
The lack of oxygenation of the digestive system, as well as the secretion of certain anorexigenic hormones, completely inhibit the feeling of hunger in most athletes.
Above 70% of VO₂ max, abdominal blood flow significantly decreases to be redirected to the muscles requiring more oxygen supply. Depending on the intensity, duration, and effort management, solid foods can be extremely difficult, if not impossible, for some athletes to chew and swallow, with the risk of causing or aggravating digestive problems. In cases where solids are not tolerated, athletes should opt for liquid or semi-liquid solutions to ensure adequate nutrition. Gels are therefore good alternatives to provide sufficient carbohydrates. With appropriate testing and a progressive increase in carbohydrate intake during exercise, high doses of carbohydrates (up to 1.4 g/min) are generally well tolerated.
Strategies such as the simultaneous ingestion of glucose and fructose (each molecule using different transporters in the intestine) can increase carbohydrate oxidation from 1 to 1.26 g/min due to better bioavailability. Thus, glucose or glucose and fructose-based gels are generally well tolerated, however, for some athletes, digestive symptoms may still appear. This is why multiple tests during intensive training and in conditions similar to those of races will be essential to train the digestive system.
Finally, it is essential to remember that taking non-steroidal anti-inflammatory drugs is strongly discouraged, as it aggravates symptoms in the majority of cases by reducing the mucoprotective film and increasing acid secretions, leading to greater intestinal permeability.
Nutritional strategies to prevent GIP:
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Limit fiber and fat intake: Foods high in fat and dietary fiber take much longer to digest, increasing the risk of GIP during prolonged exertion. Therefore, around and during long events, it will be essential to choose highly digestible foods primarily rich in carbohydrates (compote, banana, white starches...) and lean proteins.
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Adjust carbohydrate intake: Carbohydrate intakes between 60 and 90 g per hour are recommended for events longer than three hours, with a glucose:fructose ratio of 2:1. The combination of the two carbohydrates uses different transporters in the intestine, allowing for more efficient absorption and reducing the risk of digestive problems related to transporter saturation.
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Test strategies beforehand: Since each athlete has a different digestive tolerance, it is recommended to test nutritional strategies multiple times during training to avoid any surprises on race day. "Gut training" (or intestinal training), consisting of progressively increasing carbohydrate intake during efforts of varying intensity and in all scenarios, should be implemented to help the digestive system adapt to high carbohydrate and fluid volumes under physical stress. Although evidence is still developing, it has been shown that intestinal carbohydrate transporters can be modulated by training, and gastric emptying also improves with training. Discover Fondation, the first supplement dedicated to the intestinal health of endurance athletes.
Conclusion
GIP are very common performance barriers for ultra-endurance athletes. The causes are varied, ranging from dehydration to poor carbohydrate intake management, as well as mechanical and circulatory constraints. However, the risks of digestive problems can be minimized through proper adjustment of fluid and nutritional intake around and during efforts, as well as through nutritional tests conducted during training. "Gut training" appears to be a promising solution, offering athletes better tolerance to energy intake and optimizing performance during the most demanding events.
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