You’ve heard about runner’s high – the incredible endorphin rush some people are lucky enough to get while pounding the pavement. But for every up there has to be a down, and that can come in the form of ‘runner’s gut’. It’s a less well-known side effect of exercising (maybe due to the fact that no one wants to discuss their digestive issues), but it can be common among a certain subset of people who enjoy high-intensity exercise.

Explaining the symptom in a recent Instagram post, Dr Hazel Wallace (aka The Food Medic), said: “Runner’s gut refers to digestive issues that can occur during exercise, particularly endurance sports such as running, cycling and triathlons. Common symptoms include: nausea, reflux, bloating, excess wind, diarrhoea, urgency to poo, stomach pains and cramps [and] blood in stools.”

“We don’t know for certain why this happens but there seems to be a few factors that may influence it,” writes Dr Wallace. These include:

1. Changes in the gut:

“During high-intensity exercise, blood flow is redirected from your gut to your working muscles, head, lungs and brain. This causes your gut to be slightly low in oxygen, which impairs the absorption of nutrients or fluid from the gut.”

Studies – such as 2017 review on exercise-induced gastrointestinal syndrome published in Alimentary Pharmacology & Therapeutics – have found that blood flow to the stomach can decrease by 20% within 10 minutes of running at 70% of your maximum oxygen intake, and by 80% after an hour.

Plus, “changes in the movement of the gut and relaxation of the lower esophageal sphincter (the gateway from the food pipe into the stomach) during exercise can also lead to symptoms of reflux, belching, nausea and vomiting,” says Dr Wallace. 

2. Mechanical issues

“The repetitive up-down movement of running can trigger symptoms like diarrhoea, urgency and wind. It’s thought that this could also damage intestinal lining and contribute to bloody diarrhoea,” writes Dr Wallace.

Interestingly, posture could play a role. “Cyclists are more likely to experience symptoms related to the upper part of the digestive system, like reflux,” she says, due to their bent over positioning. Take that as a reminder to work on your upright running posture.

3. Fluid 

Both too much and not enough water can be problematic for runner’s gut sufferers. Dehydration already puts the body at risk of nausea, but having water move around your stomach can also make you feel unwell. How you pair your drinks with food is also crucial.

4. Nutrition 

“Foods that can produce extra gas and draw extra water into the gut, like fructose, fibre and highly concentrated carbohydrate drinks may trigger symptoms. Fructose may be better tolerated when it’s found in foods that also contain glucose,” Dr Wallace says. They can include foods like certain tinned fruits, some breads and chocolate milk or fruit yoghurts.

“Nutrients that reduce the speed at which contents leave the stomach, like fats, fibre and protein may also impact gut symptoms,” says Dr Wallace. Remember, that’s because the body directs oxygen away from the gut and towards the muscles and organs that are needed to power your workout – essentially halting digestion. “Other common gut irritants like caffeine other stimulants and sweeteners are sometimes found in pre-workout food, drinks and supplements.” 

Anyone can experience runner’s gut – even those who aren’t runners. It’s thought to occur in those who perform high-intensity exercise, so you shouldn’t be at risk if you’re taking on a walk or a yoga class, but may be if running, HIIT or cycling is your jam.

It’s mainly thought to impact those who are performing the exercise for long periods of time – hence why runner’s gut is associated with marathon or ultra marathon runners. “It seems that as exercise intensity and duration increases, so does the likelihood of having gut issues,” says Dr Wallace.

However, this may depend on the types of exercise, too. In the same 2017 review, cyclists were found to have the most pronounced increase in oxygen restriction to the gut within the first 10 minutes of exercise.

Stress – both prior to and during your workout – is another factor that could put you more at risk. “Stress hormones such as adrenaline and cortisol [which are released during intense exercise] may contribute to this. While there isn’t much research exploring this in athletes, perceived life stress and anxiety has been linked with GI symptoms in runners.”

And those who already have a sensitive gut could be more at risk, she says: “Unfortunately, those with a history of gut issues (such as IBS, IBD or reflux) are more likely to suffer from GI symptoms during exercise.”

Dr Wallace shared her nine tips for ensuring your training session isn’t ruined by runner’s gut.

• Reduce your intake of high fibre and gas-producing foods the day before as well as the day of a race or longer period of endurance exercise. Examples of these foods include bran cereals, cabbage, cauliflower, broccoli, beans and lentils
• For some people, energy gels and bars can cause problems, as can high-fructose food and drink. Examples of these include honey, dried fruit and fruit juice. Jelly sweets may be better tolerated
• Try diluting highly concentrated (those with more than 10% carbohydrate content like sports drinks) carbohydrate drinks
• Try to avoid consuming large volumes of food and/or fluid immediately before or during exercise aim to have your pre-run meal 2-3 hours beforehand, or opt for a snack if less than 1-2 hours
• Make sure you’re well-hydrated when you start your run
• Try going to the toilet and emptying your bowels prior to the race, so make sure you get up early enough to do this
• Be careful with gut stimulants like caffeine
• Don’t make any big last-minute changes or experiments with a new nutrition regime or supplement too close to race day, or on race day itself
• Give yourself time to find out what works for you and give your gut time to adjust. This can take some trial and error but it is so important 

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Images: Getty