‘‘More marathons are won or lost in the porta-toilets than at the dinner table.’’ This quote from Bill Rodgers, a marathon legend from the 1970’s, sums up the plight of many a distance runner. Depending on the event and conditions, studies have found that between 30% and 90% of endurance athletes experience intestinal problems while competing.
Symptoms vary in intensity and can include nausea, vomiting, stomach cramps, diarrhoea and even bloody stools. The research seems to suggest that some people may have a genetic predisposition to developing these problems, but that does not mean that you can’t improve your situation.
In this article:
- How exercise can cause an upset stomach
- Mechanical causes
- Nutritional causes of stomach problems during exercise
- Medication can aggravate your gut
- 10 steps to fewer stomach problems while running
How exercise can cause an upset stomach
One of the main causes for stomach problems during exercise is a decrease in blood supply to the gastrointestinal system. During high intensity exercise, up to 80% of the gut’s blood supply is shunted away from it to the exercising muscles and organs (e.g. the lungs). This leads to a lack of oxygen and damages the cells that line the gut wall causing nausea, vomiting, abdominal pain and/or diarrhoea.(2)
You may feel that there’s not much that can be done about this, but research suggests that the reduction in the intestinal blood flow is less severe in athletes who train regularly.(4) Low intensity exercise does also not seem to affect the gut as much as high intensity exercise. You may thus improve your symptoms by training regularly and by decreasing the intensity to a level where you do not experience symptoms.
There are also some nutritional strategies that you can follow. For instance, ingesting nitrate (found in beetroot and green leafy veg) during exercise has been found to increase the gut’s blood flow and improve performance. I’ll explain this in more detail in the advice section below.(4)
High intensity exercise can also contribute to reflux and nausea by decreasing the contractions of the oesophagus, making its valve less effective and causing food to remain longer in the stomach. You can read more about how to manage acid reflux during running here.
These are either related to impact or posture. Symptoms of the lower intestines are for instance more prevalent in runners than in cyclists. The repetitive high impact jostling of running is thought to damage the intestinal lining which contributes to symptoms such as diarrhoea.
Cyclist, on the other hand, complain more of symptoms like reflux which may be exacerbated by the position that they are in on the bike.
The good news is that these mechanical causes can be relieved by training! Researchers have found that regular training makes people’s guts less susceptible to mechanical strain.(2)
Nutritional causes of stomach problems during exercise
It has been well documented that what we eat or drink during exercise can have a strong influence on gastrointestinal complaints. Studies done on Ironman athletes have found that ingestion of fibre, fat, protein and concentrated carbohydrate solutions all made it more likely for athletes to develop symptoms.
All of these foods delay gastric emptying (stay for longer in the stomach) or causes more fluid to move from the body into the gut which can lead to nausea and diarrhoea.
The problem is that we need carbohydrates during exercise and high concentrations of carbohydrates have been linked to better performance. Luckily researchers have found that endurance athletes seem to tolerate concentrated carbohydrates much better if they are made up of different types, such as glucose and fructose, rather than just glucose. This is because glucose and fructose are absorbed through different pathways and the processes can run in parallel, thus allowing the carbs to be absorbed more quickly.(1)
Dehydration has also been found to contribute to symptoms. The reason for this is most likely because it causes an even bigger decrease in blood flow to the digestive system.
Medication can aggravate your digestive system
I am often asked by people if it would help if they take pain tablets before sport. My answer is always NO!
Aspirin and nonsteroidal anti-inflammatory drugs (NSAIDs), of which the most well-known example must be ibuprofen, have been shown to lead to a five-fold increased risk of gastrointestinal complications in athletes. These can range from nausea to intestinal bleeds.(2,4)
You also risk kidney failure if you use them in a dehydrated state.
10 Steps to combat stomach problems while running
1. Do not use aspirin or NSAIDs (especially ibuprofen) before/during training or racing.(2,4)
2. While a high fibre diet is needed to keep your bowel regular, stay away from high fibre foods the night before or on the day of competition. The same goes for spicy or fatty foods.(2)
3. Avoid high fructose food and drinks. Rather use drinks that contain a combination of fructose and other carbohydrates.(2)
4. Ingest carbohydrates with plenty of water or use drinks with lower concentrations of carbohydrates so that it can be absorbed quickly. When using a single carb solution (e.g. glucose only) it is recommended that you ingest 30–60 g/h (6% concentration) to avoid discomfort. For a combined fructose-glucose or fructose–maltodextrin solution, studies have found that athletes could tolerate 90-105g/h (8-10% concentration). Avoid solid forms of carbohydrates as this will be harder to digest and absorb.(1)
5. Carbohydrates enhances performance, but there is evidence that it is not necessary to ingest large amounts during exercise that lasts less than 60 minutes. In this case, rinsing your mouth with a carbohydrate solution is enough to enhance performance. The carbohydrates stimulate receptors in your mouth that activate the reward system in the brain and helps you to perform better. You must, however, make sure that you start your exercise fully carbo-loaded.(1)
6. Avoid dehydration by starting the race well hydrated and drinking according to thirst. You can learn more about how to tell if you are dehydrated here.(2)
7. Practice your nutritional strategies during training. This will “train” your gut to absorb nutrients better as well as tolerate the mechanical strain of food and drink during exercise. It may further highlight food that should be avoided.(1,2)
8. There is also research emerging that suggests that you can train your body to rely more on fat during endurance events which will decrease your need for carbohydrates, but more research is needed before one can make specific recommendations for this.
9. Taking in nitrate during exercise can decrease the damage in the gastrointestinal system since it helps to increase the blood supply to the gut. Researchers suggest that you use a natural source e.g. beetroot juice, since it also contains vitamin C which is needed in order for the nitrate to work its magic. The only downside is that the natural food sources can also cause stomach discomfort of their own.(4)
10. Lastly, reduce your training intensity to a level that does not cause you stomach trouble while you test out different strategies. Keep it at this level for long enough to allow your digestive system to adapt. It is thought that regular training at a comfortable level increases the digestive system’s ability to cope with the strains caused by exercise. Once your stomach has settled, slowly increase the intensity.(2)
- de Oliveira EP, Burini RC. Carbohydrate-Dependent, Exercise-Induced Gastrointestinal Distress. Nutrients 2014;6(10):4191-99. doi: 10.3390/nu6104191
- de Oliveira EP, Burini RC, Jeukendrup A. Gastrointestinal Complaints During Exercise: Prevalence, Etiology, and Nutritional Recommendations. Sports Med 2014;44(1):79-85. doi: 10.1007/s40279-014-0153-2
- Rowlands DS, Swift M, Ros M, et al. Composite versus single transportable carbohydrate solution enhances race and laboratory cycling performance. Applied Physiology, Nutrition, and Metabolism 2012;37(3):425-36.
- van Wijck K, Lenaerts K, Grootjans J, et al. Physiology and pathophysiology of splanchnic hypoperfusion and intestinal injury during exercise: strategies for evaluation and prevention. American Journal of Physiology-Gastrointestinal and Liver Physiology 2012;303(2):G155-G68.
Maryke is an expert sports physiotherapist who provides online physio consultations using Skype. She is registered with the Chartered Society of Physiotherapy as well as the Health and Care Professions Council.