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Running Gels: Upset Stomach?

Updated: Jan 31

An upset stomach during sport, and running especially, is very common. In a piece of research 89% of marathon runners reported a stomach complaint. This is likely affecting performance due to discomfort of the issue, but also not absorbing water and nutrition vital to performance. In another piece of research it was found marathon runners were at least 11 minutes faster when following a nutrition strategy made by a professional. Likely because professional advice minimises stomach complaints and maximises absorption of nutrition.

So what is causing an upset stomach during running? And what can you do to manage it? Read on to find out.


Runners on a cobbled street

Your blood is in your muscles!

To start with lets learn a little about the intestines and how they work.

Diagram of the digestive system

Here is a diagram of the digestive tract. When you eat food enters your stomach. Depending on how much digestion it needs it may stay here or go into the small intestine. Once in the small intestine the bulk of nutrients are absorbed as the small intestine zig-zigs through your abdomen. The small intestine flows into the large intestine which does one big loop of your abdomen. The large intestine mainly absorbs water.

Diagram of absorption sites in the intestines used to explain why running and sports gels give people upset stomachs

Above is another diagram of the digestive tract. Relevant to absorbing nutrition in sport monosaccharides (sugar) are absorbed in the jejunum in the small intestine and electrolytes (sodium, potassium and chloride) are absorbed in the large intestine.

These nutrients are absorbed from the intestine into out blood. Here starts the problem. During sport blood is needed in high amounts in the muscle to deliver oxygen. This means less blood goes to your digestive tract and this means absorption is harder. This is why during sport very refined, highly processed foods are better. They need less digestion and are easier to absorb. This is also why you don't want to eat or drink big volumes. Your digestive tract won't be able to absorb it. And if it's not absorbed it will come out the other end, cause cramps or cause bloating.

Adrenaline is mostly responsible for redirecting blood away from your stomach to your muscles. This means the more nervous or the higher amount of adrenaline you have, the less blood available for absorbing foods.

This explains one reason running and gels give you an upset stomach, but there is another.

You're not used to nutrition you're eating!

Below is a diagram explaining how nutrients are absorbed. There are 'transport proteins' for each nutrient which move nutrients from your intestines into you blood. In this picture it is glucose and sodium. Each transported is specific to a nutrient and won't let other nutrients through.

diagram of transport proteins in the intestines absorbing glucose. This is used to explain why running and gels can upset your stomach

Maintaining the transport proteins requires energy and resources from your body. Your body won't maintain them if they are not being used. Same as using muscles, you lose them if you don't use them. If you eat foods your body is not used to your intestines cannot absorb them fast enough. This means the food sits in your intestines.

Food in your intestines can pull water towards it. If you've ever been cruel enough you may have put salt on a slug. You can see all the water be sucked out of it. Similar happens in your intestines. Water is pulled toward the food. This can give you very watery, urgent stools.

Also unabsorbed food can be fermented by the bacteria living in you colon (the end part of your large intestine). These produce gas as a byproduct when breaking down the food. This can cause cramps and bloating in your abdomen as the gas expands in your intestines.

This is another reason running and gels give you an upset stomach.

What can be done to prevent this and perform better?

The transport proteins in you intestines will increase in number if consistently exposed to a nutrient. The most relevant for sports performance are glucose, fructose and water. This demonstrates the importance of practicing race nutrition before an event.

Secondly, use highly processed, easily digestible foods during sport. This means less blood is needed to absorb it and it is more likely to be absorbed. Sports nutrition specific product are not compulsory, but you can be more certain your body will absorb it once you are adapted to handle the sugar content.

Finally, nerves will cause some symptoms due to adrenaline's effect on the intestines. This can be managed to an extent with techniques.

The adaptations discussed take time and planning. I have a 5 week nutrition strategy to adapt your intestines to handle sports nutrition available for you to buy. I also have event specific nutrition strategies available to buy which have the right amounts and right sugars planned.

Comment below with your preferred nutrition strategies and any problems you have encountered.

If you have found the information in the blog useful please like and share with people who need to read it.

Information Sources

Jeukendrup, A. (2018). Training the Gut for Athletes. Gatorade Sports Science Institute.

Hansen, E. A., Emanuelsen, A., Gertsen, R. M., & Raadahl Sørensen, S. S. (2014). Improved Marathon Performance by In-Race Nutritional Strategy Intervention. International Journal of Sport Nutrition and Exercise Metabolism, 24(6), 645–655.

Hoogervorst, D., van der Burg, N., Versteegen, J. J., Lambrechtse, K. J., Redegeld, M. I., Cornelissen, L. A. J., & Wardenaar, F. C. (2019). Gastrointestinal complaints and correlations with self-reported macronutrient intake in independent groups of (Ultra)marathon runners competing at different distances. Sports, 7(6), 1–18.

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