by Mark Wolff

Over the last few years I have engaged with a fair amount of athletes who constantly suffer from gut issues during exercise especially on race day. It is a very important issue to touch on and through this blog I hope to shed some light on the pitfalls of GI (gastrointestinal) distress and how to try to manage it. I myself have a very sensitive stomach and over the years have developed and followed some very key methods to keep the stomach issues a bay.

It is a fact that more runners will suffer from GI distress than cyclists. It is a much higher impact sport and the digestive system takes a bit more of a beating with the foot pounding. However that is not to say cyclists do not have their issues, they certainly do and this can be related to posture on the bike which can also add to the discomfort. Let me start by saying there is no full and sure way to completely understand exactly what your unique trigger is but there definitely are plenty of reasons as to why GI distress can occur and if properly managed they can be mitigated.

There are conditions that occur during exercise which place the digestive system under stress.
The first of course is the fact that when embarking on a training or racing session blood is required to transport oxygen to the muscles that are actively working. This pulls blood away from the digestive tracts and diverts that flow to the active muscles as well as to the surface of the skin as those vessels are required for cooling the body. This is an effect called splanchnic hypoperfusion which can lead to intestinal distress and compromise the intestinal barrier function. In short this increases intestinal permeability which means that a type of leaky gut syndrome occurs which can cause major discomfort in the form of pain, bloatedness due to excess gas as well as nausea and diarrhoea. The higher the intensity you perform at the more blood is moved to the muscles and the more sensitive the digestive system becomes.

Another aspect of GI distress is the brain gastrointestinal tract communication pathway. It is a fact that when we have digestive discomfort in any form the brain is signalled but at the same time when we have certain emotions such as nervousness, anxiety, sadness, anger or happiness our brain speaks to our gut. Without a doubt stresses can play a big role in determining how the gut bacteria behave. It is a two way communication channel and our health is very much determined by our mindset. Pre-training or racing nerves can have a major influence on how your stomach will behave on the day. I am sure many of you remember training days where you get up relaxed and off with a group of friends for a nice training session. However on race day you are up and down to the loo hoping you will make it to the start line.

Sports nutrition also plays a crucial role in that highly concentrated carbohydrates such as gels or certain drink mixes can cause digestive discomfort. Believe it or not taking a high amount of carbohydrates can upset the fluid energy balance pulling more water into the digestive system and actually increasing the risk of dehydration which will certainly upset the stomach. Feelings of nausea, irritability, stomach pain can very much be associated with a build up of glucose in the gut.

A simple example is if you consume a gel during exercise you have to take in a certain amount of water with it to ensure the dilution process is correct. Some gels are pre mixed with water but those that are not need to be diluted to reduce the risk of GI distress and onset of dehydration. So for example a gel with 20-25grams of carbohydrates in it would require approximately a fluid consumption of around 100ml per 6-10grams of carbohydrate ingestion to be on the safe side. So this means around 200 – 400ml of fluid. Quite a fair amount and this is definitely one of the major pitfalls of most athletes. The failure to understand the energy / fluid requirements to ensure a balance in the body which allows for both fuelling and hydration.

Another crucial aspect of sports nutrition is the ability to oxidize carbohydrates. Many athletes train fasted or train low and race high to improve their fat burn efficiency at higher intensities and try to gain the extra benefit of carbohydrate intake on race day. However if you do not train your gut to cope with the food intake on the day of your race you will not have the ability to properly break it down and absorb it efficiently. This places you at risk.

Finally the last trigger of course can be issues with your daily diet. I am of the firm belief that if you feel discomfort with any food then you need to eliminate it. There are however some foods that can certainly cause more discomfort than others. The top of my list is dairy for those that are lactose intolerant it is certainly one of the triggers of GI distress during exercise and best to avoid if you are not sure. Many people are fibre intolerant I personally limit fibre intake from 72hrs before any long or racing session. Fibre can cause gut havoc. High fat and protein intake before a session can also cause GI distress. They are not easily broken down and as opposed to carbohydrates and in many cases this can also be a trigger. Additionally on the nutrition side of things is the method of high FODMAP food reduction.

What are FODMAP’s?
FODMAP stands for Fermentable, Oligosacchiride, Disacchiride, Monosacchiride, and Polyols.
Developed in Australia, the FODMAP diet was developed to deal with IBS (Irritable Bowel Syndrome) fairly new to other parts of the world the diet has seen some good success in reducing the symptoms associated with irritable bowel.

How does it work?
FODMAPS are basically a group of short chain carbohydrates which occur in many foods such as wheat, dates honey and milk to name a few. These short-chain carbohydrates have been shown to increase osmotic load in the small intestine which means it will pull water into it and upset the digestive system. These food types are broken down with a fermentation process by gut bacteria which creates gas and discomfort especially in an athlete that is exercising. Best is to avoid high FODMAP foods in the week leading to a race or important training sessions in an attempt to reduce the risks of GI distress. If you suffer severely from IBS or Proctitis this could be a good direction to take.

Another important item to consider which I have not touched on yet is alcohol. As much as you want to argue with me, it causes GI disruption and can be considered another major player in causing GI distress. It also causes dehydration and we know that this is another trigger of GI distress. If I cannot convince an you to reduce or quit  then you should take jump and read a previous blog I wrote Endurance vs Alcohol. My personal opinion is that you should never have any alcohol in a race week or anywhere close to long training sessions.

So with all this in mind how do we tackle the dreaded tummy troubles that impact so many athletes across the globe.

A number of steps need to be adhered to:

No eating foods that negatively impact you
If eating any food impacts you negatively just avoid it. Identify those foods that you are emotionally attached to however cause tummy trouble, list them and avoid them. Just find a replacement there are plenty of options available.

Avoid dairy, fibre high fat and protein meals close to your important training or racing sessions.
We know these food types put more strain on the digestive system especially when under stress. So best is to avoid them to be sure. Proper experimentation will guide you, but rather start off clean and introduce to test and monitor the impact.

Train the gut
Do not just train low (low carbohydrate or fasted) you also need to train high. There is a benefit to fasted or low carbohydrate training in order to became more fat burn efficient. However failing to train the gut to adapt to fuel intake on the day will certainly cause issues. It is a fact that increasing carbohydrate intake during exercise will also allow your digestive system to become more efficient at oxidizing those carbohydrates. Make sure you are using the correct product for you and that you know exactly how to use it. You want to get the best of both worlds. Check out my previous blog So You Really Think you have Your Sports Nutrition Right?

Avoid high FODMAP foods
As mentioned above this has shown to quite successful in many that suffer from irritable bowl syndrome. Here is a link to an application which can be installed to help with food selection when it comes to FODMAP eating
FODMAP Mobile App

Avoid Alcohol
Check out my blog for a more in-depth discussion on this Click Here

During Exercise
I am a firm advocate of drip feeding during an endurance event. Taking in small amounts of carbohydrates more frequently over a longer period of time reduces the risk of GI distress caused by a glucose overload. Separate out your hydration and energy requirements and ensure there is balance. I am an advocate of mineral loading to help with fluid absorption and reduce the risks associated with dehydration which is also a major factor in GI distress. Check out my blog on Simply Hydrate if you are wanting more information on proper hydration.

Relax Relax Relax
We know so well that endurance sport is very much a mental game. As mentioned above anxiety and nervousness can absolutely trigger an irritable bowel. Find ways to calm yourself and stay relaxed. Maybe its speaking to others, or putting on your favourite playlist to shut out the world. Whatever your coping mechanism find one that works to keep you happy, content, confident and relaxed.

I hope this helps, please feel free to email me if you have any questions

all the best


Yours in Health and Fitness

Mark Wolff is a certified exercise & sports nutritionist, endurance nutrition and physiology expert with over 20 years experience. An endurance multi-sport athlete with a running, triathlon, mountain biking and weight lifting background, he works extensively with professional and amateur athletes in a variety of sports disciplines as well as those just wanting to change their lifestyles. He firmly believes that a person can only reach their full potential when their health and nutrition is given the proper focus. Mark’s focus on nutrition and physiology is not just on training and racing, but he places major emphasis on recovery, immune system health, emotional stability, stress management and performance. Mark is co-founder of 32Gi, a sports nutrition company, focused mainly on health and endurance nutrition. He is also co-founder of Rapid Recover focussed on pneumatic compression equipment to improve circulation for recovery and health.



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OmniRunner February 18, 2019 - 3:59 pm

Great article. A friend of mine has been seeing doctors for the past few months to figure out her gut.
I avoid fiber and high fat foods before a race but still have problems at times. For marathons I’ve resorted to taking Imodium before the race. My doctor said I shouldn’t take it very often, and I only use it for marathons.

Xolani Fomana February 18, 2019 - 6:27 pm

Thanks Mark.

Very helpful.

Regards Xolani

Sent from my iPhone


Athini Ziyanda Ndobeni February 20, 2019 - 5:56 am

Great article and for someone whose sulphur intolerant and has SIBO I find myself struggling on comrades day especially after 5hrs on the road as I just consume whats on the stations .I need to work on a plan and put it into practise during training.


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