Managing GI Distress in Endurance Athletes

Gastrointestinal distress is one of the most common and frustrating challenges endurance athletes face, particularly during high-intensity racing and double training days. It can turn a well-planned session into a write-off, and left unmanaged, it starts to eat into fuelling, performance and confidence. Understanding the interplay between gut motility, fibre intake, carbohydrate load and exercise intensity can help athletes train and race more comfortably.

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What this looks like

The most common presentation is GI distress during high-intensity training and second sessions of the day, ranging from food reflux through to vomiting. This is frequently linked to slow gut motility combined with a high fibre intake in the lead-up.

A few things tend to show up together as risk factors: slow gut motility, high fibre meals eaten too close to exercise, carbonated sodium supplementation, large meal boluses, and high-intensity or threshold exercise, where blood flow is diverted away from the gut and towards working muscle.

The symptoms usually follow a pattern. Bloating and a feeling of being overwhelmed by food intake are early signs. Some athletes experience food reflux during low-intensity, zone 1 exercise, sometimes several hours after eating. At the more severe end, vomiting can occur during threshold-intensity racing.

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Why it matters

GI symptoms don’t just cause discomfort in the moment. They can compromise second training sessions and race-day performance directly, and if the underlying issue isn’t addressed, they often lead to inadequate energy intake.

Left untreated, ongoing GI distress creates a difficult cycle. Athletes underfuel to avoid symptoms, which increases the risk of muscle tissue breakdown and reduces performance, particularly at threshold intensity where the demand for fuel is highest.

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Prevention and management

The good news is that most of this is manageable with a few targeted changes, trialled one at a time.

• Reduce the fibre content of meals before afternoon running sessions. As an example, halving the rice and broccolini in a pre-session meal and adding a drink mix and flat cola can maintain carbohydrate load while lowering fibre load.
• Trial lower-fibre carbohydrate sources where possible. White bread is a useful swap for sourdough — three slices of white bread carries around 3g of fibre compared to roughly 4.3g in sourdough.
• Remove carbonation from sodium supplementation. Switching from dissolvable salt tablets to capsules avoids the added gas load carbonation brings.
• Consider flat cola as a carbohydrate source before a second session — it’s easy to tolerate and settles well.
• Trial dairy-free options to assess tolerance. Dairy is a common but under-recognised contributor for some athletes.
• During threshold racing, consider reducing carbohydrate intake to around 120g per hour rather than the commonly recommended 135–150g per hour.
• Delay carbohydrate intake for the first 15–20 minutes after a swim leg, giving heart rate time to settle to steady state before adding fuel on top.
• Lean on energy-dense fats to top up calories without adding bulk. Avocado, macadamia or cashew nuts, rice pudding and jam are useful options that don’t sit as heavily as high-fibre or high-volume foods.
• Where possible, reduce salt water ingestion during the swim leg — this adds an unnecessary load on top of everything else the gut is managing.

Professional support has a role here too. A gastroenterology review — including colonoscopy or gastroscopy where indicated, and acid-reducing medication pre-race — can help rule out or manage underlying issues. Dietitian-guided food logging, using a tool like Cronometer, gives a clear picture of what’s actually going in and when. Input from a hydration specialist can also help fine-tune the sodium and fluid side of the equation.

Yours in Health,
Sean Cornish

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