The Science of Gut Training for Endurance Athletes

On September 18, 2026, The Guardian published a detailed feature examining exercise-related gastrointestinal distress following Australian athlete Joanna Wietrzyk's bowel accident during the Hyrox Games in Beijing. She remarkably won the fitness race despite the incident and later apologised. The subsequent coverage highlighted that gastrointestinal problems are incredibly common across the entire sport. The article reports that approximately 60 percent of endurance athletes experience some form of gastrointestinal symptom during training or competition.

Furthermore, the American Medical Society for Sports Medicine estimates that prevalence may actually reach as high as 90 percent. These varying figures reflect different definitions, populations, and symptom thresholds. Gastrointestinal distress covers a wide severity range, from mild bloating and cramping to nausea, diarrhoea, and rare cases of bowel control loss. The fundamental takeaway from the recent coverage is that race day digestive issues are multifactorial.

The Guardian describes exercise-related symptoms as a complex interplay rather than the result of one universally applicable food or supplement. Professor John Hawley of Australian Catholic University explains that prolonged exercise drastically redirects your blood flow. During hard efforts, blood is driven toward working muscles to sustain power output. In hot conditions, blood also moves toward the skin to facilitate cooling.

Blood Flow and Mechanical Stress

This dual demand significantly reduces the digestive tract's ability to process and absorb food and fluid. Consequently, consuming carbohydrate drinks, gels, and bars while your absorptive capacity is compromised can lead to severe urgency or diarrhoea. The Guardian notes that these symptoms affect athletes across various endurance formats. Incidents involving marathoners, triathletes, and race walkers show that bowel urgency is widespread.

However, prolonged, weight-bearing exercise is particularly likely to produce severe symptoms. Running lasting 90 minutes or longer sits at the top of this risk profile. Hawley attributes this heightened risk to the repeated vertical impact of running. He notes that cyclists may be able to tolerate more food and fluid simply because they experience far less intestinal jostling.

The underlying research area is now often described clinically as exercise-induced gastrointestinal syndrome. Kayla Henningsen is a Monash University PhD candidate researching exercise gastroenterology and extremes physiology. She argues that digestive symptoms are not necessarily evidence of poor preparation or poor fitness. Instead, her practical recommendation is to treat your gut like any other part of your performance system.

The Gut as an Athletic Organ

Hawley similarly describes the gut as an athletic organ that can be trained to tolerate nutrient loading. Evidence reviewed by Ricardo Da Costa and colleagues suggests that carbohydrate ingestion during prolonged exercise may actually support the gut. Possible benefits from proteins, particularly glutamine, have also been investigated by researchers. However, The Guardian cautions that evidence for glutamine, low fibre diets, and gluten free diets remains completely unclear.

These restrictive approaches should never be presented as universally effective treatments for all endurance athletes. The clinical perspective on these issues has matured significantly over recent years. A 2025 joint position statement addressed the prevention and management of exercise-associated gastrointestinal perturbations and symptoms. Published in Sports Medicine, the statement was developed by Sports Dietitians Australia and the Ultra Sports Science Foundation.

This document serves as a vital practitioner guide for endurance coaches and sports dietitians. It confirms that digestive problems are a serious sports nutrition issue rather than merely an embarrassing mishap. The 2025 joint position statement specifically emphasises this shift toward proactive management. Its existence underlines that digestive problems require strategic intervention from coaches, dietitians, and athletes alike.

Translating Science for the Veteran Athlete

The field is clearly moving beyond merely observing these problems to providing actionable, practical guidance for everyday training. The broader research direction is moving toward integrated exercise gastroenterology. Hydration and heat are now viewed as interacting factors alongside mechanical impact, carbohydrate delivery, and physical stress. The Guardian reports that the gut-brain axis may also connect performance stress with gastrointestinal function through the microbiome.

Our team at Reendure knows that hitting your forties brings a harsh reality check. The track workouts might not get slower, but recovering from them feels significantly heavier. In our experience, forcing an old Tuesday and Thursday intensity schedule often leads to diminishing returns. We learned to push our second hard session to Friday to allow an extra forty eight hours of recovery.

While total weekly volume stayed the same, the quality of our intervals skyrocketed. This exact principle of calculated, structured adaptation applies directly to gut training. As masters athletes, our bodies change. Our tolerance for untested race day strategies diminishes over time.

This targeted approach is crucial because balancing high ambition with the realities of an aging body requires immense strategic patience. Many ambitious endurance athletes aged 35 to 65 continue to enter demanding, long distance races globally. Simultaneously, they must balance intensive training with heavy work schedules, frequent travel, and cumulative daily stress. These lifestyle factors actively influence the gut-brain axis and can exacerbate physiological fatigue.

Realistic Rehearsal Conditions

The sources support a deeply personalised approach to nutrition precisely because these external variables fluctuate constantly. Relying on sheer grit or copying a younger athlete's fueling plan is a recipe for disaster. Instead, ambitious athletes aged 35 to 65 must treat their nutrition strategy with immense precision. The primary directive is to practice your complete fueling protocol during training.

Henningsen recommends gradually exposing your gut to the specific foods and drinks you plan to use. This means using the exact carbohydrate gels and bars during selected long sessions. You cannot simply introduce unfamiliar products or large quantities on the morning of a competition. Hawley advises rehearsing exactly what you will consume in competition rather than guessing on race day.

Progressive overload applies to your digestive tract just as it does to your working muscles. Introduce race carbohydrates and fluids in manageable amounts early in your training block. Build your tolerance systematically over time to build and test a personalized plan safely. When starting this process, athletes often wonder whether to prioritize practicing with fluids or solid foods.

The sources do not mandate a universal starting point, but gradual exposure is the universal principle. You might begin by introducing small amounts of carbohydrate liquid during a manageable steady state run. Once your stomach accepts that volume without complaint, you can incrementally introduce a single gel. Over successive weeks, you can layer these components until you simulate your full race nutrition plan.

Testing your nutrition under realistic conditions is another nonnegotiable step for long events. You should include long duration sessions at race pace to simulate actual race day stress. When it is safe to do so, practice in environmental conditions that resemble your expected competition. Heat can severely worsen symptoms by increasing competition for blood flow between the gut and the skin.

Tracking and Objective Refinement

If your event will be hot, your gut must learn to process fuel while actively cooling your body. When the core temperature rises during an event, the body prioritizes survival over digestion. The heart pumps aggressively to circulate blood to the skin, where sweat evaporation dissipates accumulated heat. If an athlete consumes a heavy concentration of carbohydrate in this precise moment, the gut simply lacks the resources to process it.

The sugar sits unprocessed in the stomach, drawing in additional water through osmosis. This fluid shift rapidly leads to the bloating, cramping, and sudden urgency that derails so many races. Many endurance runners look for evidence based marathon hydration strategies to maintain their output. The Guardian identifies dehydration as a clear possible contributor to digestive distress.

Staying well hydrated is crucial, but athletes must determine their specific fluid needs through rigorous practice. A generic hydration formula will never account for your individual sweat rate or specific environmental challenges. The psychological component of racing also plays a significant role in digestive stability. Performance stress can trigger a hormonal response that actively alters how your stomach functions.

The gut-brain axis ensures that anxiety or intense focus directly impacts your physical digestion. Recognizing this connection helps athletes understand why a nutrition plan might work perfectly in relaxed training but fail on a stressful race day. This is exactly why rehearsing under race like conditions is so critical for true preparation. Henningsen specifically recommends avoiding unfamiliar foods right before a competition.

She also strongly recommends keeping a detailed training log to monitor your ongoing physical progress. You should record your food intake, fluid consumption, physical symptoms, and environmental conditions meticulously. Your log should also note the intensity of the session when symptoms occurred. Digestion operates differently during a relaxed recovery run compared to a high threshold interval session.

By cross referencing intensity with your nutritional intake, you can pinpoint exactly when your digestive capacity drops. This level of granular tracking separates a hopeful finisher from a consistently prepared competitor. Over time, this detailed data will reveal vital recurring associations. You might notice that a particular product, a specific temperature, or a certain dehydration level triggers your distress.

Tracking these variables ensures that your training log becomes an invaluable diagnostic tool. Because individual triggers vary widely, the ultimate goal is not to copy another competitor's nutrition plan. Instead, you are looking to develop a highly tolerable personal protocol through repeated practice. Identifying these patterns allows you to make informed, objective adjustments to your race day strategy.

It is also incredibly helpful to evaluate your performance objectively after every single event. Implementing a structured evaluation system helps endurance athletes refine their pacing and fueling plans. Treat your physical symptoms as useful physiological information rather than a test of your character. While most symptoms are mild and transient, severe or worsening issues warrant a proper medical assessment.

Individual testing and professional medical advice are always more defensible than sweeping dietary bans. Endurance athletes must actively train their digestive systems through gradual, event specific rehearsal to secure reliable race day performance.

Sources

1. There's a long history of athletes pooing themselves mid-race – why, and how can you avoid it?
2. The Most Underused Component of Endurance Race Preparation Is Not a Training Session. It Is Practising Your Nutrition Protocol in Training — and Most Athletes Never Do It.