Why Your Race Fueling Plan Looks Good on Paper, But Falls Apart in Your Gut

So you have put in the hard work. You have trained consistently, organised your crew, planned your leg times, worked out when to change shoes, when to get the poles out, and mapped every bite, nibble and sip from carb loading to the finish line.

Well done. Planning is a huge part of setting yourself up for a great day out in an endurance event.

But despite all the careful carbohydrate calculations, drop bag logistics, and backup nutrition options your crew is carrying, your fueling plan still falls apart in your gut on race day.

You have probably spent hours pouring over past race records, trying to work out where things went wrong. Maybe you have trialled new gels, changed sports drink mixes, or gone down the path of trying to race on “real foods" only. Yet here you are again, with a gut that does not want to play the game.

Gut issues are one of the most common reasons for poor performance, reduced enjoyment, and DNFs in endurance events. The key point is this: a fueling plan can look excellent on paper, but still fail if it does not address the underlying mechanism(s) that contribute to your gut symptoms.

Read on to learn why this happens, and what you can do about it.

What pathways contribute to gut symptoms?

When working out how to reduce gut symptoms, it is critical to understand the main pathway, or combination of pathways, likely contributing to the problem. Four main pathways have been identified in exercise related gut symptoms.

1. Circulatory pathway: blood flow or “the plumbing”
During exercise, blood flow is redirected towards the heart, lungs and working muscles, at the expense of the gut. This leaves the gut with less oxygen and fewer nutrients than it needs, which can impair digestion and absorption. It can also affect the integrity of the gut lining, increasing the risk of endotoxaemia, inflammation and symptoms such as nausea, abdominal pain and ongoing gastrointestinal upset after exercise.

2. Neuroendocrine pathway: stress response or “the electrics”
Exercise reduces parasympathetic activity, including the activity of the vagus nerve, which plays a major role in gut function. This can slow the movement of food and fluid through the gut, reduce digestive secretions, and impair digestion and absorption. Food and fluids that are usually well tolerated at rest may suddenly feel difficult to manage during exercise. Recovery nutrition can also be less well tolerated when overall stress levels are high.

3. Mechanical pathway: jostling or “the mechanics”
Running places repeated mechanical stress on the gut. The jostling of the abdomen, along with tissue irritation and swelling, can contribute to urgency, cramping and bowel motions. This pathway is particularly relevant in running compared with lower impact sports.

4. Metabolic pathway: when metabolism is under strain
If blood glucose drops too low during exercise, nausea can occur, along with fatigue and reduced performance. High exercise intensity can also increase lactate production, which may further contribute to nausea in some athletes.

Why a plan that looks good on paper can still fail

A good plan is not just about hitting theoretical carbohydrate or fluid targets. It also needs to match the pathway driving your symptoms.

For example, an athlete affected mostly by the circulatory pathway may benefit from early, frequent carbohydrate intake in smaller amounts, helping to support gut blood flow. But that same strategy may do very little for an athlete whose symptoms are mainly driven by the mechanical pathway.

This is one of the reasons why “just trial it in training” can be such a frustrating approach. Without understanding the main drivers of your symptoms, you can spend a long time testing products and strategies without really solving the problem.

Working out your main causal mechanism is where input from an Accredited Sports Dietitian can be incredibly valuable. There are, however, some key strategies that are worth considering across the board.

Three big questions to ask if your gut keeps letting you down

1. Am I fueling with the right amount, type and timing of carbohydrate?
Carbohydrate intake before and during exercise is critical for performance in endurance events lasting more than 45 to 60 minutes, but it is also highly relevant for gut tolerance. The right carbohydrate strategy can help reduce stress on the body, support gut blood flow, and reduce the risk of hypoglycaemia and nausea.

2. Am I maintaining appropriate hydration?
Adequate hydration before and during exercise supports circulation, helps reduce overall physiological stress, and may reduce strain on the gut. Understanding your fluid needs is important, and this is best based on multiple sweat rate assessments collected across training in relevant conditions. It is just as important to practise your drinking strategy in training so you know what your gut can comfortably tolerate.

3. Am I well trained and physically adapted for the demands of the event?
Being fit and appropriately prepared for the task can reduce overall stress, mechanical strain and extreme metabolic responses. Long term progression matters. Athletes often jump from race to race without building the specific physical and physiological adaptations needed for their target event. Better preparation can make race day feel less stressful to the body, improve tolerance of fueling, and reduce the chance of gut symptoms derailing your performance.

The answers to these questions will often give you excellent clues about why your gut is not cooperating on race day.

If you are keen to reduce gut symptoms during exercise so you can enjoy your event and perform to your potential, working with an Accredited Sports Dietitian can help fast track the process. I help runners work through gut issues every day, and it is incredibly rewarding when an athlete finally has an event where their gut does not hold them back, and they know exactly what helped.

If that sounds like what you need, reach out for a complimentary 30 minute Performance Strategy Call and let’s make a game plan.

 References

Costa, R.J.S., Gaskell, S.K., Henningsen, K., Jeacocke, N.A., Martinez, I.G., Mika, A., Scheer, V., Scrivin, R., Snipe, R.M.J., Wallett, A.M. and Young, P. (2025) ‘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, 55, pp. 1097–1134. doi: 10.1007/s40279-025-02186-6.  

Costa, R.J.S., Young, P., Gill, S.K., Snipe, R.M.J., Gaskell, S., Russo, I. and Burke, L.M. (2022) ‘Assessment of exercise-associated gastrointestinal perturbations in research and practical settings: methodological concerns and recommendations for best practice’, International Journal of Sport Nutrition and Exercise Metabolism, 32(5), pp. 387–418. doi: 10.1123/ijsnem.2022-0048.  

Gaskell, S.K., Rauch, C.E. and Costa, R.J.S. (2021) ‘Gastrointestinal assessment and management procedures for exercise-associated gastrointestinal symptoms’, Aspetar Sports Medicine Journal, 10, pp. 36–44.