Why GI Distress Is The Leading Cause of DNFs in Ultrarunning

Written by:

Cliff Pittman

CTS Pro Ultrarunning Coach

Updated On

April 8, 2026

CHOOSE YOUR FORMAT

►WATCH THE VIDEO

►LISTEN TO THE PODCAST

►READ THE ARTICLE

Why GI Distress Is The Leading Cause of DNFs in Ultrarunning

Most ultramarathon DNFs aren’t caused by injury, or lack of fitness. They’re caused by something far more common, and far more preventable: GI distress. The symptoms are familiar: nausea, bloating, loss of appetite, even vomiting and diarrhea. Not only are the symptoms unpleasant, once they begin fueling becomes difficult. Energy and fluid intakes drop, starting a cascade of problems that often leads to a DNF. The good news is that GI distress in ultramarathons is almost entirely preventable.  

The Scope of the GI Distress Problem

Research summarized in the International Society of Sports Nutrition position stand on gastrointestinal distress shows that 30% to 90% of endurance athletes experience GI symptoms during competition. (PMID: 31699159) That’s a big range, but incidence of GI distress increases as events get longer, and ultramarathons sit firmly at the high end the spectrum.

Two Types of GI Distress

GI symptoms in endurance sports typically fall into two categories:

Upper GI Symptoms

• Nausea
• Reflux
• Bloating
• Vomiting

Lower GI Symptoms

• Cramping
• Diarrhea
• Urgent bowel movements

Athletes may experience one, a combination of a few, or all of these symptoms during training sessions and races. Upper GI symptoms are more likely to disrupt fueling and hydration by making it uncomfortable to consume more food or fluids or impossible to keep food or fluids down. Lower GI symptoms disrupt movement, making continued forward progress difficult or ill advised. Either or both types of symptoms can make an already challenging race or training session impossible to finish.

What Causes GI Distress During Ultramarathons?

According to the ISSN position stand, there are three primary drivers of GI distress during exercise:

1. Reduced Blood Flow to the Gut
   During exercise, your body prioritizes blood flow to deliver oxygen to working muscles and to the skin for cooling. Blood flow to the digestive system can drop significantly, a condition called splanchnic hypoperfusion. This reduces oxygen delivery to the stomach and intestines, which slows gastric emptying and digestion. Food and fluid sit in the stomach and small intestine longer than normal, leading to compromised gastrointestinal function, nausea, bloating, and vomiting.
2. Mechanical Stress
   The mechanical stresses from repeated foot strikes reverberate throughout your body. The relentless jostling impairs gastric emptying and disrupts digestion. This is one reason cyclists (who don’t experience the same jostling) are often able to consume more carbohydrate calories per hour than runners, even at similar exercise intensities.
3. Nutritional Load
   Modern endurance fueling strategies often recommend 60–90+ grams of carbohydrate per hour, with some athletes pushing the envelope to consume upwards of 120-140 grams of carbohydrate per hour! High-carbohydrate fueling strategies can be highly effective, but only if your gut can handle it.If intake exceeds your capacity to absorb food and fluids, carbohydrates remain in the intestine. In response, additional water is drawn into the gut. This can result in cramping, bloating, and eventually diarrhea. Even if you have the capacity to absorb a massive amount of carbohydrate, you can also overload your gut by consuming way more carbohydrate than you need, based on your exercise intensity and energy expenditure.

The Solution to GI Distress: Train Your Gut

Just like your muscles and cardiovascular system, your digestive system adapts to repeated stress. With consistent practice, you can improve:

• Gastric emptying
• Intestinal absorption
• Carbohydrate transport efficiency

The key is to proactively train your gut to process more fuel and fluid per hour in the weeks and months before your goal event.

Coaching Example: Hydration Problem Disguised As A Nutritional Problem

I worked with an ultrarunner preparing for a mountainous 100K. They had strong fitness and a history of consistent training, but every long race was plagued by the same destructive pattern:

• Around hour 4–5 → nausea started
• Shortly after → they stopped eating
• Then → performance collapsed

 At first, it looked like a fueling issue. When we dug deeper, we found the real problem:

They were significantly under-hydrating in hot conditions. Dehydration impacts digestion more than most athletes realize. When fluid intake is too low and sweat rate is elevated, plasma volume decreases. This reduces blood flow to the gut, leading to the splanchnic hypoperfusion mentioned above. In my athlete’s case, this led directly to nausea, loss of appetite, and an inability to continue fueling.

The Fix: Improve Hydration to Keep the Gut Moving

The solution for my athlete’s GI distress, and many athletes I’ve worked with, wasn’t to reduce carbohydrate intake. It was to improve their hydration strategy to match their sweat rate, including education on how to change hydration strategies in hotter weather and during higher intensity efforts when sweat rate increased. Then we practiced those hydration strategies on long runs. Once the hydration strategies were working well, we gradually increase carbohydrate intake.

Within a couple of months, the athlete was completing 5- to 6-hour runs with consistent high-carbohydrate fueling and no nausea.

Practical Takeaways to Avoid GI Distress

If you want to avoid GI distress, focus on these three areas:

1. Practice Race-Day Nutrition in Training
   Don’t save fueling strategies for race day. Learn to overcome nutritional challenges in training. This includes using the foods and sports drinks that will be available on course, if they are different than what you normally use.
2. Increase Carbohydrate Intake Gradually
   Jumping straight to high intake (e.g., 90g/hour) without adaptation is a common mistake. First, go for a long run and carefully record what you actually eat and drink now. You may be surprised to find you’re only consuming 40-60 grams of carbohydrate per hour despite thinking you’re consuming way more. It’s important to know your starting point so you know how to gradually increase your intake.
3. A Good Nutrition Plan Starts with a Good Hydration Plan
   Adequate fluid intake maintains blood volume, supports gastric emptying, and improves efficient nutrient absorption. Fueling and hydration are not separate, they’re interdependent.

The Bottom Line

Most athletes focus on miles, hours, pace, and intervals, but overlook the most common point of failure in ultramarathon running: the gut. Training your digestive system is as important as training your muscles and cardiovascular system, because if you can’t consistently take in and deliver fuel and fluids during prolonged exercise, your whole body shuts down. And at the other end of the performance spectrum, training your gut to process high amounts of fluid and fuel can unlock a whole new level of performance.

References:

Tiller, N. B., Roberts, J. D., Beasley, L., Chapman, S., Pinto, J. M., Smith, L., Wiffin, M., Russell, M., Sparks, S. A., Duckworth, L., O’Hara, J., Sutton, L., Antonio, J., Willoughby, D. S., Tarpey, M. D., Smith-Ryan, A. E., Ormsbee, M. J., Astorino, T. A., Kreider, R. B., McGinnis, G. R., … Bannock, L. (2019). International Society of Sports Nutrition Position Stand: nutritional considerations for single-stage ultra-marathon training and racing. Journal of the International Society of Sports Nutrition, 16(1), 50. https://doi.org/10.1186/s12970-019-0312-9