Marathons: Training Your Gut

When training for a marathon, most runners focus on the total mileage, speed sessions and what type of shoes to wear. One area that is often overlooked is their nutrition plan and how their digestive system will tolerate their fuelling on race day. But what you eat, when you eat and how your body tolerates supplements, can be the difference between finishing strong and struggling through the last stretch of your race. 

During endurance exercise, your body relies heavily on glycogen, the stored form of carbohydrate in your muscle and liver. A well-trained athlete can store roughly 300 to 700 grams of glycogen in muscle mass and another 80 to 120 grams in the liver, giving a total of around 400 to 800 grams of stored glycogen. That equates to about 1,600 to 2,300 calories of fuel. While this may seem like a lot, at marathon pace, this supply only lasts about 90 to 120 minutes. Once reserves runs low, runners experience glycogen depletion, known among runners as “hitting the wall” or “bonking”.

"During running and other endurance events, the sympathetic nervous system is activated..."

To prevent this, you need to keep topping up your energy stores with easily digestible carbohydrates. A common challenge, is that the digestive system isn’t naturally built to handle large amounts of fuel while the body is under stress. During running and other endurance events, the sympathetic nervous system is activated, directing blood flow toward the working muscles and away from the intestinal tract. This shift can slow gastric emptying and reduce carbohydrate absorption, making fuelling more difficult when you need it most. As a result, runners may experience stomach discomfort such as nausea, bloating, vomiting and diarrhoea and can contribute to underperformance in long races.

Research has shown that through training your gut and taking a few simple strategies into consideration, this issue can be alleviated. 

The process begins with gastric emptying, the step where carbohydrates and fluids leave the stomach and enter the small intestine for absorption. If gastric emptying is delayed, the stomach remains fuller for longer, which can lead to bloating, nausea, and even vomiting. Factors such as high-intensity exercise, prolonged efforts in the heat, and dehydration are all known to slow this process down. Hypertonic carbohydrate solutions, which are highly concentrated with sugar (more than 8g of carbohydrate per 100grams), also hinder gastric emptying. 

Fortunately, there are strategies to speed up gastric emptying and improve tolerance to larger volumes of sports supplements. Training your gut during long runs by practicing the same nutrition plan you intend to use on race day- helps the stomach adapt to larger volumes and a higher carbohydrate intake. Consuming carbohydrate solutions that are isotonic or hypotonic, rather than hypertonic, also encourages faster emptying. 

Drinks are isotonic if they contain between 4 and 8 grams of carbohydrate per 100g and hypotonic if they contain less than 4g per 100g. Additionally, if you start your fuelling early on and spread your carbohydrates evenly throughout your run it reduces the load on the stomach at any one time. Over time, these practices teach your digestive system to handle the demands of race-day fuelling, allowing energy to reach your muscles more efficiently and reducing the risk of discomfort.

Once carbohydrates pass into the intestine, they are broken down into simple sugars and absorbed into the bloodstream. If absorption here is delayed, it can cause diarrhoea, often referred to as runner’s tummy.

"This means that if an athlete consumes only glucose, absorption is capped at around 60 grams per hour"

Most sports gels and drinks contain a mix of sugars designed for quick absorption. Monosaccharides like glucose, fructose and galactose can be absorbed directly in the small intestine. Disaccharides such as maltodextrin (made of glucose units) and isomaltulose (glucose + fructose) must first be broken down into their monosaccharide components before absorption.

Absorption occurs via specific transporters. Glucose and galactose use the SGLT-1 transporter, which is coupled with sodium absorption. This transporter can absorb around 60 grams of carbohydrate per hour. Fructose, however, is absorbed via a different transporter called GLUT-5, which can only absorb around 30 grams per hour. This means that if an athlete consumes only glucose, absorption is capped at around 60 grams per hour. But by combining glucose with fructose in about a 2:1 ratio, both transport systems can be utilised, maximising absorption to around 90 grams per hour.

Because absorption is limited by the capacity of these transporters and can further be reduced by exercise stress, dehydration, or high-intensity effort, training the gut becomes crucial. By practising fuelling and adapting the digestive system, athletes can increase transporter efficiency and upregulate the expression of these transporters (meaning even more carbohydrates can get absorbed and be utilized for energy).

Research has shown that these adaptations can occur, not only from regularly practicing fuelling during training runs, but also from consuming a higher carbohydrate diet in the days leading up to a race. Interestingly, even three days of dietary manipulation with higher carbohydrate intake have been shown to improve tolerance of larger carbohydrate volumes. In practice, this means that carbo-loading before a marathon not only fills glycogen stores but also primes the digestive system to handle the extra carbohydrates you will need to consume during the race itself.

Training the gut is not something that can be left until race day. It should be treated with the same consistency and planning as long runs or interval sessions. This involves practicing with the same gels, chews, and sports drinks you intend to use in competition, gradually increasing the amounts until you reach the recommended intake of 60 to 90 grams of carbohydrate per hour for events lasting longer than 2 hours and 30 minutes. Through repeated practice, the stomach learns to tolerate larger volumes, and the intestines become more efficient at absorbing and oxidising the fuel.

In the end, gut training is the bridge that connects your fuelling strategy to your performance. Without it, even the best carbohydrate plan can fall apart under the strain of race conditions. With it, your body can efficiently move fuel from your mouth to your muscles, keeping you energised and in control. Training your gut means fewer surprises on race day, fewer moments of discomfort, and a greater chance of hitting your performance goals.

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