For endurance athletes, performance is influenced by more than muscles and lungs. The digestive system can determine how comfortably an athlete tolerates fluid and carbohydrate during prolonged exercise.
Gastrointestinal symptoms—including cramping, bloating, nausea, reflux and urgent bowel movements—are widely reported during endurance events. They can disrupt pacing, fuelling and fluid intake, so digestive preparation deserves a place in the training plan.
Why does stomach distress happen?
Symptoms rarely have one cause. Exercise intensity, mechanical movement, food choices, hydration status, heat and individual tolerance can interact.
01
Reduced gut blood flow
During exercise, blood flow is redirected towards working muscles and the skin. At higher intensities or in hot conditions, reduced gastrointestinal blood flow may affect digestion and intestinal function.
02
Mechanical stress
Repetitive impact—particularly during running—places physical stress on the digestive organs and may contribute to discomfort during longer events.
03
Nutritional triggers
Large meals and foods high in fat, fibre or poorly tolerated carbohydrates can be harder to manage close to exercise.
04
Fluid-management errors
Both inadequate fluid replacement and excessive drinking can contribute to discomfort. Strategies should account for sweat losses, duration and environmental conditions.
The gut is trainable
The digestive system can adapt to repeated fuelling during exercise. This process is commonly called gut training.
Research suggests that regularly practising carbohydrate intake during training can improve tolerance to a planned race-fuelling strategy. Adaptations may involve gastric comfort, carbohydrate transport and an athlete’s familiarity with consuming food or drink while moving.
The objective is not to force the highest possible intake. It is to establish an appropriate plan and practise it progressively under conditions that resemble the target event.
Nothing new on race day.
Carbohydrate intake and tolerance
Carbohydrate requirements depend on exercise duration, intensity and individual goals. Sports-nutrition guidance commonly discusses approximately 30–60 grams per hour for longer sessions, with higher intakes sometimes used during prolonged endurance events by athletes who have practised using multiple carbohydrate sources.
These figures are not universal targets. Tolerance varies and intake should be developed progressively. Athletes with medical conditions or persistent symptoms should seek individual guidance from a qualified sports dietitian or healthcare professional.
Sodium, fluid and absorption
Sodium helps regulate extracellular fluid balance and participates in intestinal transport mechanisms involving glucose and water. This is one reason endurance-fuelling products often combine carbohydrate, sodium and fluid.
However, more is not automatically better. An individual strategy should consider sweat rate, likely sodium losses, exercise duration, weather and access to fluid. The plan should be rehearsed in training rather than introduced during competition.
A practical gut-training plan
Before the start
The pre-race meal is another part of the plan to rehearse. Many athletes choose a familiar carbohydrate-focused meal around two to four hours before an event, while moderating fat, fibre and foods known to cause personal discomfort.
Common triggers can include:
- Large high-fibre meals
- High-fat foods close to exercise
- Dairy products for people sensitive to lactose
- Large amounts of fructose consumed alone
- Unfamiliar foods, supplements or drink mixes
Triggers are individual. Foods should not be removed unnecessarily, and any significant dietary restriction is best discussed with a qualified professional.
Key takeaways
- Gastrointestinal symptoms are common in endurance sport and usually have multiple contributing factors.
- The digestive system can adapt to a repeated fuelling routine.
- Carbohydrate and fluid intake should be progressed according to the event and individual tolerance.
- Sodium contributes to fluid balance and intestinal transport, but replacement needs vary.
- Race nutrition should be practised under realistic training conditions.
- Persistent or severe symptoms should be assessed by a qualified healthcare professional.
References
- American College of Sports Medicine. Position stand: Exercise and fluid replacement.
- International Olympic Committee. Consensus statement on sports nutrition.
- Jeukendrup AE. Training the gut for athletes. Sports Medicine.
- Costa RJS, et al. Research concerning exercise-induced gastrointestinal syndrome.
- Shirreffs SM, Sawka MN. Fluid and electrolyte needs for training, competition and recovery. Journal of Sports Sciences.
- National Center for Biotechnology Information. Research concerning sodium-glucose transport and intestinal absorption.
