So many athletes will up their carb intake during a long run or ride and get hit with bloating, nausea or stomach cramps, and then assume their body just can't handle that much, or discount that particular supplement as not working for them. 

This doesn't have to be the case if you strategically train your gut to adapt to what you're asking it to process. Think about how you progressively train your muscles and cardiovascular system for the demands of an event. It's much the same with gut training:  practicing consuming different amounts of carbohydrates during exercise allows your gastrointestinal system to get more accustomed to your race-day fuelling strategy.

Why does your gut need training?

As we covered in our guide to how many carbs per hour you actually need, higher carbohydrate intakes can become useful during longer endurance events. Once you start looking at consuming 60-90g per hour, however, you need to know that your body is able to digest and absorb that amount of carbohydrates comfortably while exercising.

Exercise itself can make this harder. During prolonged or strenuous exercise, blood flow is redirected towards the working muscles and skin, and gastrointestinal function can be reduced. This, alongside heat, dehydration, race intensity, nerves and a large intake of carbs can all contribute to gut problems that can affect your performance.

Trying a high-carb strategy for the first time on race day is a gamble we do not recommend.

Research suggests you can you actually train your gut.

Endurance runners completed two weeks of gut training while consuming up to 90g of carbohydrate per hour during exercise as a part of a study. Results showed that the athletes experienced reductions in overall gastrointestinal discomfort, including upper and lower GI symptoms and nausea. Measures of leftover carbs unabsorbed in the gut also reduced. 

Another study using a similar two-week protocol found gastrointestinal symptoms reduced by around 60% in the carbohydrate gel group and 63% in the carbohydrate food group following gut training. 

Based on this, it looks like the gut is indeed trainable. However, this doesn't mean everyone needs to train themselves to tolerate 90g per hour. We'll reiterate: the target should still match what your event actually requires.

What is actually changing within the gut?

Repeatedly consuming carbs at incrementally higher doses during exercise can encourage adaptations throughout the gastrointestinal system that may help the body process carbs as energy more effectively.

Research shows gut adaptations can include not only improved carbohydrate absorption but also better tolerance to larger volumes of food or fluid. There is also evidence that a diet consistently higher in carbohydrate can increase the intestine's capacity to absorb glucose. 

Simply put: a carbohydrate intake that initially feels uncomfortable may become considerably easier to manage after repeated exposure.

Don't jump straight to 90g.

If you're currently consuming one 30g carb gel every hour without any issues, you shouldn't suddenly quadruple that intake in your next session but instead, build towards it.

For example, an athlete aiming to comfortably consume around 90g per hour might progress something like:

Current tolerance	Next progression
~30g/hr	Practise 40-45g/hr
~45g/hr	Progress towards 50-60g/hr
~60g/hr	Gradually introduce 70-75g/hr
~75g/hr	Practice towards race target

This is just a guide - some athletes will progress faster and others will need considerably longer at each intake. You just need to give yourself enough training sessions to learn how you respond rather than treating 90g per hour as a number you need to hit immediately.

Practise the actual race-day strategy.

If you plan to use gels or carbohydrate drinks during the event, practise with those exact products at the exact intervals, at the same speed or intensity as race day.

This allows you to test not only whether your gut can currently handle a certain number of grams, but also how easy the packaging is to open while moving, how much fluid you need with a gel, whether you get flavour fatigue after three hours, and so on. This will help you feel more confident in how you'll perform on race day, because you've rehearsed it.

Another piece of important advice: don't just use another athlete's race fuelling strategy unless you've tested it first and know it works for you. Research shows substantial individual variation in feeding tolerance and gastrointestinal symptoms, so it's important you know your own training response.

Several studies demonstrating improvements in carbohydrate absorption and tolerability have used relatively short periods of repeated carbohydrate exposure, including protocols of ~1-2 weeks. For most athletes, though, it makes sense to incorporate fuelling practice progressively throughout the training block rather than waiting until two weeks before your event. Your long sessions are an obvious opportunity for testing. As the duration of those sessions increases, your carbohydrate intake can gradually become more representative of what you intend to use on race day.

What to do if you're still getting stomach problems.

It's not just about the number of carbs you consume. Gastrointestinal symptoms during endurance exercise can be influenced by how quickly you consume them, the type and concentration of carbohydrate, fluid intake, dehydration, heat, exercise intensity and your individual tolerance.

Carbohydrate source matters too. As discussed in our article on glucose-to-fructose ratios, combining different transportable carbohydrates can help increase carbohydrate absorption when intake becomes high.

So if 70g per hour causes problems, don't immediately conclude that 70g is beyond your limit. Look at how you're delivering those carbohydrates as well.

Consider your gut in your race plan.

You would never get to race day without having completed the physical training required for it. Your fuelling strategy deserves the same preparation.

Work out approximately how much carbohydrate your event requires, gradually build towards that intake during training sessions and pay attention to how your gut responds. By race day, you want the fuelling to feel as familiar as the exercise.

References.

1. Jeukendrup AE. Training the Gut for Athletes. Sports Medicine. 2017;47(Suppl 1):101–110.
2. Jeukendrup AE, McLaughlin J. Carbohydrate ingestion during exercise: effects on performance, training adaptations and trainability of the gut. Nestlé Nutrition Institute Workshop Series. 2011;69:1–12.
3. Costa RJS, Miall A, Khoo A, et al. Gut-training: the impact of two weeks repetitive gut-challenge during exercise on gastrointestinal status, glucose availability, fuel kinetics, and running performance. Applied Physiology, Nutrition, and Metabolism. 2017;42(5):547–557.
4. Miall A, Khoo A, Rauch C, et al. Two weeks of repetitive gut-challenge reduce exercise-associated gastrointestinal symptoms and malabsorption. Scandinavian Journal of Medicine & Science in Sports. 2018;28(2):630–640.
5. Gaskell SK, Rauch CE, Parr A, et al. The Effect of Gut-Training and Feeding-Challenge on Markers of Gastrointestinal Status in Response to Endurance Exercise: A Systematic Literature Review. Sports Medicine. 2023.
6. Nutritional strategies for minimizing gastrointestinal symptoms during endurance exercise: systematic review of the literature. 2025.
7. Burke LM, Hawley JA, Wong SHS, Jeukendrup AE. Carbohydrates for training and competition. Journal of Sports Sciences. 2011;29(S1):S17–S27.