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

Most ultrarunners spend months building fitness, logging miles, and dialing in pacing, but never train the one system that causes more DNFs than anything else. In this episode, Coach Cliff Pittman breaks down the science behind GI distress in ultramarathons, explains why so many athletes accidentally cause their own stomach problems, and walks through exactly how to train your gut so it holds up when it matters most.

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HOST

Cliff Pittman is the Coaching Development Director at CTS, leading the Ultrarunning and Cycling Coaching staff with a specialty in guiding athletes from first-time ultrarunners to elite competitors at races like Western States 100, Leadville 100, and the Triple Crown of 200s. A competitive trail and ultra athlete himself, Cliff brings firsthand experience and a rare ability to turn complex training science into simple, actionable coaching.

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Transcript

Why Most Ultra DNFs Happen

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Most ultra marathon DNFs are not caused by injury, and they're also not caused by a lack of fitness. They're caused by stomach problems. GI distress is one of the most common reasons ultra runners fail to finish races. This includes nausea, vomiting, diarrhea, loss of appetite, and once those symptoms start, fueling becomes difficult, energy intake drops, and performance falls apart. The good news is that most of this is preventable. And in this video, I'm going to explain why GI distress happens in ultramarathons, why many athletes accidentally cause it, and how gut training can dramatically reduce the risk. I'm Cliff Pittman, Pro Ultrarunning Coach, and Director of Coaching at CTS. I coach athletes ranging from first-time ultrarunners to professionals competing at the highest level of the sport. And one of the most common problems we solve in ultramarathon training is nutrition tolerance during long runs. Let's start with the big picture. Research summarized in the International Society of Sports Nutrition position stand on gastrointestinal distress shows that between 30 and 90% of endurance athletes experience GI symptoms during competition. And the longer the race, the more common those symptoms become. Ultramarathons sit at the extreme end of that spectrum. Symptoms usually fall into two categories upper GI symptoms, things like nausea, reflux, bloating, or vomiting, and lower GI symptoms, cramping, diarrhea, urgent bowel movements, any of these can interfere with fueling. And once fueling breaks down, performance usually follows. So why does this happen? The ISSN position stand describes three major factors that contribute to GI distress during exercise. First is reduced blood flow to the gut. During exercise, your body prioritizes blood flow to working muscles and to the skin for cooling. As a result, blood flow to the digestive system can drop significantly. When the gut receives less oxygen and less blood flow, digestion becomes slower and more difficult. The second factor is mechanical stress. Running creates constant impact forces. Every foot strike creates small mechanical jolts through the abdomen. Over time, this repeated movement can disrupt digestion and gastric emptying. This is one reason runners tend to experience more GI distress than cyclists. The third fact is nutritional load. Modern endurance nutrition often targets high carbohydrate intake, sometimes 90 grams per hour or even more. That strategy can support performance, but it only works if the gut can actually process that fuel. If carbohydrate intake exceeds absorption capacity, these carbohydrates remain in the intestine. And when that happens, they draw water into the gut, and that can lead to cramping, bloating, and diarrhea. This is why gut training matters. Just like your muscles, your digestive system can adapt to training. Repeated exposure to carbohydrate intake during exercise can improve several things gastric emptying, intestinal absorption, and the activity of transport proteins that move carbohydrates across the intestinal wall. In simple terms, your gut learns how to process fuel when you're running. This concept has been emphasized for years in ultrarunning coaches. Your gut is a trainable system, but it only adapts if you practice fueling during training. Now let me give you a coaching example. I was working with an ultrarunner preparing for a mountainous 100 race. They had good fitness, their training was consistent, but in longer races they repeatedly developed nausea around four or five hours. Once the nausea started, they stopped eating, and the race would unravel from there. At first glance, it looked like a fueling problem, but when we reviewed their data and race reports, another issue appeared. They were significantly underhydrating in hot conditions. Fluid intake was far below their sweat rate, and that matters, because dehydration can reduce gastric emptying and slow intestinal absorption. When plasma volume drops during dehydration, blood flow to the gut can be further compromised. And that makes it harder for the stomach to empty its contents into the intestine. So even though they were trying to consume adequate carbohydrate, the fuel was essentially backing up in the stomach, which created nausea and loss of appetite. The solution was not simply more carbohydrates, so we focused first on restoring fluid balance. During long runs in similar conditions, we gradually increased fluid intake to better match their sweat losses. We also practiced their fueling strategy during those runs. And once hydration improved, gastric emptying improved, and their ability to consume carbohydrates during long efforts became far more sustainable. Within a couple months, they were completing five to six hour training runs without the nausea that previously stopped them. There are a few practical takeaways from this. First, practice your nutrition during long runs. The gut adapts through repeated exposure. Second, increase carbohydrate intake gradually. Jumping straight to very high intake without adaptation often causes problems. And third, make sure your hydration supports digestion. Adequate fluid intake helps maintain blood volume and supports gastric emptying during exercise. Let's zoom out for the bigger picture. One of the most common reasons ultrarunners' DNF is GI distress. But in many cases, it's not random. It's the result of physiological stress combined with an untrained gut or poorly practiced fueling. When athletes train their digestive system the same way they train their aerobic system, they dramatically improve their ability to fuel during long races. And consistent fueling is one of the biggest factors in ultramarathon performance. If this video helped clarify how to avoid GI distress in long races, subscribe to the CTS channel for more evidence based endurance training advice. And if you want to learn more about fueling strategies for ultramarathon athletes, I'll link additional resources in the description below.