There are very few downsides to exercise. But a recent viral story highlighted one of the more embarrassing – and potentially dangerous – issues that can occur when your body reaches it limit with exercise.Joanna Wietrzyk, the winner of the recent women’s pro Hyrox race in Beijing (a challenging fitness race that blends running and strength), found herself in the spotlight not for her performance – but because she soiled herself during the event.While there has been plenty of commentary about what happened – and discussions over whether she should have been allowed to continue competing – one thing that hasn’t been said is that such occurrences are not uncommon in athletes.Those who watched the 2005 London Marathon will remember Paula Radcliffe stopping at the 22-mile mark to “do her business” on the side of the road. Triathlete Taylor Knibb also suffered a similar incident during 2024’s Dubai T100.Gastrointestinal problems, including diarrhoea, are up to three times more common in professional athletes than in recreational athletes. The impact it has on runners is so common it has even has its own term: runner’s diarrhoea. It’s estimated that up to 90% of runners experience this issue at some point.But diarrhoea isn’t the only symptom people undertaking strenuous exercise for a prolonged period will suffer from. Abdominal pain, heartburn, nausea, vomiting and even gastrointestinal bleeding may all occur.There are a few key reasons why intense exercise can have such a detrimental effect on our gastrointestinal system.The first has to do with the digestive tract itself. The human digestive tract is approximately three times our height. This means it contains a lot of material. It also means our digestive tracts are rarely ever empty. This is particularly true when people are doing long or intense forms of exercise, where they may need to consume pre-workout snacks or drinks to give them energy, alongside fluid or gels during exercise to keep going. If any problems occur with food when it’s in the stomach, it usually goes back out the way it came in. But if any issues occur with food once it leaves the stomach, it’s a one-way ticket “downwards”. The small and large intestines hold approximately 300ml and 600ml of liquid content at a time – although this varies from person to person. So you can perhaps see why it can be difficult to stop problems when they arise during exercise. Our digestive tracts can hold onto a lot of material. Andrii Iemelianenko/ Shutterstock The second factor relates to blood flow. Our body has a limited amount of blood – around five litres on average, which is circulated around the body. Whatever activity we’re doing changes the distribution of blood around the body. When we exercise, nervous and hormonal signals tell the blood vessels in the muscles and lungs to dilate, increasing the amount of blood flowing to them. The same signals that cause those vessels to dilate also tell vessels to the gastrointestinal system to constrict. This temporarily reduces blood flow to these organs.A reduction in blood flow to the intestines during endurance exercise can cause bowel ischaemia. This is where the blood supply to the organ is reduced so much that it can no longer function properly due to insufficient oxygen and nutrients. Standard exercise doesn’t usually cause this, but prolonged and intense exercise increases the risk.This reduction in blood flow has been shown to cause symptoms such as diarrhoea in athletes. In rare cases, it can also result in bowel blockages.A prolonged reduction in blood flow, oxygen and nutrients to the intestines can also damage the cells that line the gut. These cells are responsible for absorbing nutrients and water into the blood stream, meaning performance is impaired and dehydration may occur.The physical effects of intense exercise also cause problems. Our gastrointestinal contents are repeatedly sloshing around with the impact of every step, jump or lunge. This not only places additional force on our gastrointestinal system, it also causes physical microtrauma to the tract’s lining, which can sometimes result in intestinal bleeding.Thankfully the lining of the intestines has a good capacity for regeneration, so longer term issues are rare. But if microtrauma and intestinal bleeding do occur frequently, it can lead to a “leaky gut”, where the contents of the gastrointestinal tract (including partially digested food, pathogens and bacterial toxins) can leak into the bloodstream. This can lead to a systemic immune response, resulting in pain, cramping and diarrhoea – a condition referred to as exercise induced gastrointestinal syndrome (EIGS).Once the body decides that it doesn’t want the contents in the intestines any more, it has limited power to stop what comes next. This is despite the fact that the body has two anal sphincters – an internal one and an external one – which work in tandem to stop the gastrointestinal tract from emptying when we’re not ready.Once enough pressure has built in the gastrointestinal tract, you will experience an urge to “go”. And in the context of Hyrox, the running, pushing, lunging and lifting competitors perform exacerbates the pressure that may already naturally be happening in the gastrointestinal tract.Wietrzyk’s “accident” isn’t as uncommon as what people may think. As four-time Boston and New York marathon winner Bill Rodgers put it: “More marathons are won or lost in the porta-toilets than at the dinner table.” Wietrzyk’s story is just the latest example of the toll competing at an elite level can have on your body.Adam Taylor does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.