Thirty to fifty percent of marathon runners experience significant gastrointestinal distress during races. In Ironman-distance triathletes, that figure exceeds 60%. GI issues, nausea, cramping, bloating, diarrhoea, vomiting, are the leading cause of athletes failing to execute their race-day nutrition plan, and one of the most common reasons for DNFs in ultra-distance events.

The frustrating part is that most athletes treat GI distress as an inevitable feature of hard racing rather than a problem to be solved. It is, in most cases, a solvable problem. Understanding the mechanisms that cause exercise-induced GI dysfunction opens up practical solutions that most athletes never attempt.

Why runners and cyclists have GI problems

Exercise creates a fundamentally hostile environment for digestion. Several mechanisms combine to impair gut function during intense effort:

Reduced splanchnic blood flow: During intense exercise, cardiac output is redirected from the gut to working muscles. Blood flow to the gastrointestinal tract can decrease by 60–70% during maximal effort. Reduced blood flow means reduced absorptive capacity, increased intestinal permeability, and compromised gut motility.

Mechanical trauma: Running specifically creates repetitive vertical impact forces that literally jostle intestinal contents. This is why runners experience significantly more GI distress than cyclists at comparable intensities, the mechanical component is absent in cycling.

Hyperthermia: As core temperature rises during exercise, intestinal barrier integrity decreases. Increased gut permeability ("leaky gut") allows bacterial endotoxins to enter the bloodstream, triggering inflammatory responses that compound GI symptoms.

Stress hormones: Cortisol and adrenaline released during intense exercise affect gut motility, often accelerating transit time and triggering the urgency many runners are familiar with.

The exercise-gut microbiome connection

Research over the past decade has established a clear relationship between exercise and the gut microbiome, the community of trillions of microorganisms living in the gastrointestinal tract.

Elite endurance athletes have significantly more diverse gut microbiomes than sedentary populations. They show enrichment of specific bacterial species, particularly Akkermansia muciniphila, Faecalibacterium prausnitzii, and various Prevotella species that are associated with improved metabolic efficiency, reduced inflammation, and better gut barrier function.

The causality runs both ways: exercise improves the microbiome, and a healthy microbiome improves exercise capacity. Athletes with richer, more diverse microbiomes show better substrate utilisation during endurance exercise and faster recovery from hard efforts.

The practical implication: your dietary choices, not just your training, shape the gut ecosystem that determines how well you absorb nutrients during exercise.

Gut training: teaching your intestines to absorb more

One of the most underutilised strategies in endurance sport is gut training, systematically practising consuming carbohydrates during exercise to train the gastrointestinal system to absorb and utilise them more efficiently.

The intestinal glucose and fructose transporters (SGLT1 and GLUT5) upregulate in response to demand. Athletes who consistently practice consuming 60–90g of carbohydrate per hour during training improve their absorptive capacity over 4–8 weeks, reducing GI distress at those intake rates.

Athletes who never consume carbohydrates during training, then attempt to execute a 60g/hour race-day fueling plan, are asking a gut that is not adapted to high rates of carbohydrate absorption to perform at levels it has never experienced. Distress is predictable.

Gut training protocol:

  • Start with 30g/hour during long sessions and progressively increase over 6–8 weeks
  • Use the same products you plan to race with, gut training is product-specific
  • Practice on runs of 90+ minutes and rides of 2+ hours
  • Train in heat conditions similar to your race to adapt to the temperature-gut interaction

Foods that cause problems during exercise

Certain foods dramatically increase GI distress risk during exercise, particularly in the 2–6 hours before training or racing:

  • High-fibre foods: Bran, beans, lentils, cruciferous vegetables, whole grains, these slow gastric emptying, increase bowel content, and often cause bloating and urgency during running
  • High-fat foods: Fat slows gastric emptying significantly. A high-fat pre-race meal means partially digested food sitting in the stomach during the event, causing nausea and GI distress
  • Spicy foods: Capsaicin directly increases gut motility and can trigger urgency and cramping during exercise
  • Fructose without glucose: Fructose in isolation (as in many fruit juices, agave) overwhelms the GLUT5 transporter, causing osmotic diarrhoea. Fructose consumed alongside glucose (2:1 ratio) uses a different transporter and is much better tolerated
  • Artificial sweeteners: Sorbitol, xylitol, and other polyols are poorly absorbed and cause osmotic water drawing into the gut, a recipe for mid-race distress
  • Dairy (in some athletes): Lactose intolerance or high-fat dairy can cause significant GI issues during intense exercise

Foods that improve gut health and performance

Long-term dietary patterns that support a healthy, exercise-adapted gut microbiome:

  • Diverse plant foods: A variety of vegetables, fruits, and legumes (consumed away from training) feeds diverse beneficial bacteria. Aim for 30+ different plant foods per week.
  • Fermented foods: Yoghurt, kefir, sauerkraut, kimchi, miso, these deliver live bacteria and support microbiome diversity. A 2021 Stanford study showed fermented foods outperformed high-fibre diets for increasing microbiome diversity.
  • Prebiotic fibre: Inulin, FOS (fructooligosaccharides), and resistant starch feed beneficial bacteria. Found in garlic, onion, asparagus, bananas, oats, and cooked-and-cooled potatoes.
  • Polyphenol-rich foods: Blueberries, dark chocolate, green tea, olive oil, polyphenols are selectively metabolised by beneficial bacteria and inhibit pathogens.

Probiotics: what the evidence actually shows

Probiotic supplements are widely used by athletes, and the evidence is more nuanced than either enthusiastic proponents or sceptical dismissers suggest.

Meta-analyses show that specific probiotic strains reduce upper respiratory tract infection duration and severity in athletes, a meaningful benefit given training-induced immunosuppression. The strains with the strongest evidence for immune support include Lactobacillus rhamnosus GG and Lactobacillus acidophilus NCFM.

Evidence for direct GI performance benefits during exercise is weaker and more strain-specific. Probiotics are not a substitute for gut training, dietary management, or adequate hydration. They're a potentially useful adjunct for athletes with persistent GI issues who have addressed the fundamentals.

If using probiotics, choose a product with documented, named strains at a dose of at least 10 billion CFU, and take consistently for a minimum of 4 weeks before drawing any conclusions about efficacy.

Pre-race gut preparation protocol

In the 48–72 hours before an important race:

  • Reduce dietary fibre (shift from whole grains to white grains, reduce raw vegetables)
  • Eliminate high-risk foods (spicy, high-fat, artificial sweeteners, new foods)
  • Increase hydration to ensure you start the race well-hydrated
  • Avoid alcohol, it disrupts gut motility and sleep quality
  • Consume familiar, low-fibre carbohydrate sources (white rice, pasta, bread, bananas)
  • On race morning: eat at least 2.5–3 hours before start time. Familiar food. Low fat, low fibre.
  • Give your body time for a bowel movement before the start, this requires waking early enough

When to see a sports dietitian

If you experience severe GI distress during training despite implementing the above strategies, or if symptoms persist outside of training (indicating a possible underlying condition such as IBS, SIBO, or coeliac disease), consultation with a sports dietitian or gastroenterologist is warranted.

Conditions like exercise-induced ischaemic colitis, though rare, require clinical investigation. Severe chronic GI symptoms during exercise are not something to simply manage through nutrition adjustments alone.

For most athletes, however, GI distress during exercise is a solvable training and nutrition problem. Gut training, strategic pre-race dietary management, and long-term investment in microbiome health will, for the majority, eliminate or significantly reduce GI issues that currently limit performance.