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During prolonged exercise, the body redirects a significant portion of the blood to the working muscles. At the same time, blood flow to the digestive tract decreases, which impairs digestion and nutrient absorption. If we add pre-competition stress, high temperatures, or dehydration to the mix, the risk of gastrointestinal problems increases significantly.
However, research shows that the digestive tract can adapt to the regular intake of fluids and carbohydrates during exercise. Just as muscles adapt to long-distance running, the intestines can increase their capacity to transport and absorb glucose and fructose. This reduces the risk of gastrointestinal discomfort while improving energy availability during a run.
During runs lasting more than 90 minutes, glycogen stores are gradually depleted. A regular intake of carbohydrates helps maintain blood glucose levels, delays fatigue, and allows you to maintain your target pace.
Current sports nutrition recommendations indicate that:
- During exercise lasting 1–2.5 hours, it is recommended to consume about 30–60 g of carbohydrates per hour,
- During exercise lasting longer than 2.5 hours, you can increase your carbohydrate intake to as much as 60–90 g per hour, preferably using a mixture of glucose (or maltodextrin) and fructose.
However, even the best-planned nutrition strategy won't work if it's only implemented during a competition. Every energy gel, isotonic drink, or snack should be tested beforehand during training sessions.
It is estimated that gastrointestinal problems affect as many as 30–70% of endurance runners, and their prevalence increases with the duration and intensity of exercise.
The most common symptoms are:
- diarrhea,
- a sudden urge to have a bowel movement,
- abdominal pain,
- nausea,
- a feeling of fullness,
- bloating.
Many factors contribute to their occurrence. In addition to reduced blood flow through the intestines, other contributing factors include dehydration, high ambient temperatures, consuming large amounts of fiber or fat before a run, energy gels that haven’t been used before, and excessive consumption of hypertonic beverages.
It's important to take care of your digestive health throughout the entire training period, not just in the week leading up to the race.
The most important rules include:
- regularly practicing carbohydrate intake during long runs,
- testing specific gels, energy bars, and drinks planned for the competition,
- Avoid experimenting with your diet on race day,
- limiting very fatty, hard-to-digest, and high-fiber meals for 24–48 hours before the marathon,
- an adequate daily energy intake, as a chronic calorie deficit can negatively affect bowel function and recovery.
A growing body of research also suggests that a healthy gut microbiota can support immunity, recovery, and exercise tolerance. However, the foundation remains a daily diet rich in a variety of plant-based foods, fermented dairy products (if well tolerated), and an adequate amount of fiber—except during the period immediately preceding a competition.
Dehydration as low as about 2% of body weight can impair performance and exacerbate gastrointestinal problems. Therefore, a hydration plan should be practiced just as much as a nutrition strategy.
During long training sessions, it's a good idea to:
- start your run well-hydrated,
- Drink small amounts of fluids regularly instead of large amounts all at once,
- Adjust your fluid intake based on the temperature, pace, and your individual sweat rate.
Isotonic beverages, which provide both carbohydrates and sodium, are a good choice during exercise lasting more than about 60–90 minutes.
When we run, we lose mainly sodium through sweat, as well as smaller amounts of potassium, magnesium, and calcium. It is sodium that plays a key role in maintaining fluid and electrolyte balance and aids in the absorption of glucose and water in the intestines.
People who sweat heavily or exercise in high temperatures should pay special attention to ensuring an adequate intake of sodium during exercise. In practice, this is most often provided by high-quality isotonic drinks or energy gels containing added electrolytes.
Just as you can’t run a marathon without prior preparation, you shouldn’t expect your digestive system to easily handle large amounts of gels during a race. The best strategy is to gradually increase your carbohydrate intake during long training sessions. You can start with about 30 g per hour and then, if your body tolerates this well, gradually increase your intake according to your race plan. Regular practice helps improve your digestive system’s tolerance, reduce the risk of gastrointestinal issues, and increase your comfort during the race.
Modern preparation for a half-marathon and marathon involves not only running training but also careful planning of nutrition and hydration. Stomach conditioning, an adequate intake of carbohydrates, maintaining proper hydration, and the right amount of electrolytes can significantly improve your running comfort and help you achieve a better result. It’s worth remembering that a nutrition strategy also requires regular practice—ideally, exactly as you plan to implement it on race day.
Bibliography:
1. https://pubmed.ncbi.nlm.nih.gov/21916794/
2. https://pubmed.ncbi.nlm.nih.gov/28589631/
3. https://pubmed.ncbi.nlm.nih.gov/26920240/
4. https://pubmed.ncbi.nlm.nih.gov/21660838/
5. https://pubmed.ncbi.nlm.nih.gov/28919842/
Prepared by Justyna Kurasz
