Why Endurance Athletes Often Experience Diarrhea or Vomiting During Races

One of the common challenges endurance athletes face during races is gastrointestinal (GI) distress, often manifesting as nausea, vomiting, or diarrhea. This phenomenon can affect even the most experienced and well-prepared athletes, with research indicating that 30−90% of endurance athletes experience some form of GI discomfort during races​. For ultra-endurance athletes, such as long-distance triathletes or marathon runners, this figure can be as high as 93%​. Even professional athletes aren’t immune to these problems. Des Linden, the 2018 Boston Marathon champion, famously mentioned that she was trying hard "not to vomit" at the finish line.

Several factors contribute to GI issues during endurance events, including reduced blood flow to the digestive system, dehydration, and poor gut permeability. Additionally, poorly trained carbohydrate absorption plays a significant role.

1. Blood Flow Redistribution: During exercise, the body prioritizes sending blood to muscles, skin, and the cardiovascular system to sustain performance. As a result, the blood flow to the gut is reduced by up to 80% in extreme conditions​. This reduced blood supply can impair nutrient absorption and digestion, leading to nausea, vomiting, or diarrhea.
2. Dehydration: Dehydration worsens GI issues by further reducing blood volume and exacerbating heat stress. In hot environments or during high-intensity efforts, the risk of intestinal ischemia increases, making athletes more susceptible to GI symptoms like cramping and diarrhea​.
3. Gut Permeability: Prolonged or intense exercise can also increase intestinal permeability, leading to a condition known as "leaky gut." This causes undigested food particles and bacteria to pass into the bloodstream, triggering inflammation and worsening GI symptoms​.
4. Carbohydrate Malabsorption: A frequent cause of GI issues is the lack of training the gut to handle large amounts of carbohydrates. During endurance events, athletes often consume energy gels, bars, or drinks rich in simple sugars, which can overwhelm the digestive system if not properly trained. Without adequate training to absorb these carbs efficiently, the risk of diarrhea, bloating, or vomiting increases. To avoid this, athletes should practice consuming race-day fuels during training sessions to help the body adapt to higher carbohydrate loads​.

To mitigate GI distress, athletes can:

• Train the gut by gradually increasing carbohydrate intake during training to improve absorption and tolerance​.
• Avoid high-fiber foods and certain intolerances (like lactose or gluten) leading up to race day​.
• Hydrate properly by drinking water and electrolyte solutions in measured quantities to maintain fluid balance without overwhelming the gut.

By practicing and testing various nutrition and hydration strategies in training, athletes can better prepare their bodies for the demands of race day, minimizing the risk of gastrointestinal distress and optimizing their performance.

Don’t leave your performance to chance. With NeverLow, you can develop a personalized nutrition plan tailored to your needs, training, and race conditions. Let us help you optimize your fueling strategy and prevent GI issues so you can focus on reaching the finish line strong. Start now with NeverLow!