The #1 Reason Ultrarunners DNF...And How to Avoid It (#4)

Gastrointestinal (GI) distress is a primary cause of ultramarathon DNFs, often outweighing injury or lack of fitness. Between 30% and 90% of endurance athletes experience symptoms like nausea, vomiting, or diarrhea, which are driven by reduced blood flow to the gut, mechanical impact from running, and excessive nutritional loads. The digestive system is a trainable organ; repeated exposure to carbohydrates during exercise improves gastric emptying and intestinal absorption. A coaching case study reveals that nausea often stems from dehydration, which compromises plasma volume and causes fuel to "back up" in the stomach. Effective prevention requires a gradual increase in carbohydrate intake during training and maintaining fluid balance to support digestion. Treating the gut as a trainable system—equivalent to the aerobic system—is essential for sustaining the energy intake required for high-level performance and successful race finishes.