What happens when the condition of an athlete's gut gets in the way of the nutrition designed to power performance?
I have learned another new term. Gut training.
Evidently, athletes don't just train their muscles, cardiovascular systems, and nervous systems. They can train their guts too. Who knew? Well, sports dietitians, for starters. I certainly didn't. I didn’t even know teams had dieticians.
But the concept sent me down exactly the kind of rabbit hole you might expect from someone who spends an unreasonable amount of time thinking about gut health.
In the first two articles in this series, I looked at recovery and then travel for athletes. Both brought me back to the same place. We ask an extraordinary amount of an elite athlete's body, and the gut is involved in far more of that process than I had appreciated. This time, I started thinking about food, nutrition, fuel.
Professional athletes have access to exceptional nutritional expertise. Teams employ professionals who calculate energy needs, protein, carbohydrates, fluids, electrolytes, meal timing, and recovery nutrition. The science of putting fuel into the athlete has become remarkably sophisticated. But what happens when the athlete can't comfortably use it?
Eating the Fuel Is Only the Beginning
Food has to make it through the gastrointestinal system before much of it can become useful to the rest of the body. Carbohydrates have to be digested and absorbed and protein has to be broken down. Fluids and electrolytes have to be absorbed and vitamins and minerals have to become available to the body. Put that digestive system inside an athlete training or competing intensely and things get more complicated.
In 2025, Sports Dietitians Australia and the Ultra Sports Science Foundation published a joint position statement on something called exercise-induced gastrointestinal syndrome (Ex-GIS). Yes, there is an actual name for this. I was quite literally stunned, and had never heard of such a thing.
Apparently, exercise itself can disturb gastrointestinal function and integrity. Symptoms can include nausea, bloating, abdominal discomfort, reflux, diarrhea, and the sudden need to find a bathroom. Not exactly the performance metrics I expected to be reading about when I started this series, but they become performance issues when an athlete cannot comfortably consume or tolerate the fuel and fluid required for training or competition.
The world's most carefully designed nutrition plan has a problem if the athlete's gut can't handle it.
This Isn't Just a Marathon Problem
My first assumption was that this was primarily an endurance athlete issue for athletes in the runners, cyclist, triathletes realm. People who voluntarily exercise for lengths of time I personally find unreasonable. Wrong.
GI problems are certainly well documented in endurance sports. But a 2026 review looked specifically at highly trained and elite athletes outside traditional endurance sports. The studies varied considerably, so there isn't one reliable prevalence number that applies to every athlete or sport. Still, the researchers estimated that around competition, approximately one in two female athletes, and one in three male athletes may experience gastrointestinal symptoms. Suddenly, this isn't simply a digestive inconvenience. It can become a fueling problem. And a fueling problem can become a performance problem.
What Is Exercise Doing to the Gut?
During intense exercise, the body has priorities. Blood flow is redirected toward working muscles, the heart, lungs, and skin, which can mean less blood flow to the gastrointestinal tract. Exercise can also affect intestinal integrity and motility, while heat, dehydration, mechanical stress, anxiety, unfamiliar foods, and aggressive fueling strategies can add to the burden. That doesn't make exercise a nemesis for the gut. Obviously, regular physical activity has long been know to have enormous health benefits. What it does mean is that the gut has to perform too. And apparently, it can be trained.
Then I Discovered Gut Training
This may be my favorite thing I've learned so far in this series. Instead of suddenly asking the gut to process large amounts of carbohydrate or fluid on competition day, athletes can practice fueling strategies during training. Repeated exposure can help improve tolerance to the nutrition required during exercise. In other words, you don't just train the athlete. You train the digestive system that supports the athlete.
I love this. Not because it proves some grand theory about the microbiome. It doesn’t. I love it because professional sports already recognizes something I have become increasingly interested in from an entirely different direction: The gut is adaptable.
And Here Is Where I Have to Disappoint Myself
If you’ve been following me you can guess where my mind immediately went. Probiotics. Fermentation. Original Biome. Surely this is where I get to tell you that supporting the microbiome solves the whole problem. Unfortunately, the science isn't going to let me do that.
The 2025 position statement found stronger support for strategies, including appropriate nutrition, hydration, certain dietary adjustments, and gut training. Probiotics have been studied, but the evidence for preventing these specific exercise-related GI problems isn't consistent enough to call them the answer. So I won't.
What it does tell me is that gut health is considerably more complicated than any single intervention. Food, microbes, hydration, stress, sleep, exercise, medications, travel, and hormones can all influence the gastrointestinal environment. The microbiome is part of that environment. But once again, it isn't the whole story.
Where Fermentation Still Gets Interesting
Research in high level athletes is exploring relationships among the microbiome, metabolism, inflammatory response, recovery, and performance. The findings are intriguing, but there are still substantial gaps in the evidence. That's where my interest in fermentation remains.
Fermentation can bring together beneficial microorganisms, the substrates they use, and the postbiotics and other metabolites they create. Researchers continue to investigate how those components may interact with gastrointestinal health, immune function, metabolism, and the gut barrier. Could supporting that environment help an athlete better tolerate the nutritional and physiological demands placed on it? Maybe.
But if I'm going to argue that professional sports should look more seriously at gut health, then "maybe" has to remain maybe until someone measures it.
The Plan Still Has to Meet the Athlete's Biology
Professional teams can hire extraordinary dietitians. They can calculate what an athlete should eat and carefully manage ingredients, timing, hydration, supplementation, and recovery nutrition. But the plan still has to meet the athlete's biology, and sometimes that biology says “no”. Maybe that's why the concept of gut training struck me so strongly.
Sports performance already accepts that the gastrointestinal system can be conditioned, supported, monitored, and included in performance planning. So I'm left with another question: If teams are already training the gut to tolerate the fuel required for performance, how much more should we be learning about the biological environment inside that gut? I don't know.
Three articles into Performance Biology, I seem to be accumulating questions considerably faster than answers. I'm surprisingly comfortable with that. Because the more I learn, the more I think the gut deserves to be measured alongside the other systems professional sports already watches so carefully.
And maybe, eventually, supported just as deliberately.
References
Costa RJS, Gaskell SK, Henningsen K, et al. Sports Dietitians Australia and Ultra Sports Science Foundation Joint Position Statement: A Practitioner Guide to the Prevention and Management of Exercise Associated Gastrointestinal Perturbations and Symptoms. Sports Medicine. 2025;55:1097–1134. doi:10.1007/s40279-025-02186-6.
Haworth JJ, Bhatt N, Johnson KO, et al. Gastrointestinal Symptoms in Athletes Beyond Endurance Sports: A Scoping Review. Performance Nutrition. 2026;2:4. doi:10.1186/s44410-025-00019-1.
Carlone J, Parisi A, Fasano A. The Performance Gut: A Key to Optimizing Performance in High Level Athletes: A Systematic Scoping Review. Frontiers in Sports and Active Living. 2025;7:1641923. doi:10.3389/fspor.2025.1641923.
