By Dan Baumgardt, Senior Lecturer, School of Physiology, Pharmacology and Neuroscience, University of Bristol. Originally published in The Conversation (opens in a new tab) in April 2025.
Running a marathon is no small feat. Athletes can expect to cover between 50-60,000 steps, burn over 3,000 calories (opens in a new tab) and expel multiple litres of sweat (opens in a new tab) to keep cool.
Marathons and other long distance events can be associated with several dangers (opens in a new tab) – including dehydration, electrolyte imbalances and heatstroke. All important reasons as to why it's crucial to train adequately for the big day, and come prepared.
But there's another condition that can affect long distance runners – one that can be more than a little embarrassing.
It comes under many guises (opens in a new tab): runner's trots, runner's gut, runner's stomach, runner's runs. What we're referring to is the overactive gastrointestinal tract (opens in a new tab) brought on by the whole-body effects of running. This results in urgent, sometimes explosive diarrhoea.
Runner's diarrhoea is actually a triad of symptoms: diarrhoea (opens in a new tab), cramping abdominal discomfort plus a heightened urge to open your bowels. It's actually more common than you think (opens in a new tab) – with up to 96% (opens in a new tab) of endurance runners reportedly experiencing some sort of gastrointestinal symptom during a race.
In most cases, runner's trots are not considered concerning, especially if the condition is adequately managed as you would any episode of acute diarrhoea (opens in a new tab) – with fluid and electrolyte replenishment. But in some extreme cases, there have been signs of blood in the runner's faeces (opens in a new tab). This suggests that in some people, the condition may be caused by mechanical damage to the bowel – perhaps as a result of this organ being sloshed around in the abdomen during a long run.
Why it happens
But what causes it in the first place? We aren't entirely sure (opens in a new tab), but most experts have established several different causes which might play a role in generating these symptoms.
The first clue may lie within the blood supply. When you start to exercise, your body shifts its attention away from resting and digesting, and diverts blood to tissues and organs that need it more – namely the heart, lungs and muscles. Prolonged reduced blood flow to the gut could irritate and inflame its lining. This might also affect the bacterial colonies (opens in a new tab) that reside within the gut. This may explain why a recent study suggests probiotics (opens in a new tab) may work as a treatment.
Other studies have considered the effect of nutrition (opens in a new tab) upon gut activity. Certain foods are associated with increased gut activity and fermentation, such as protein, fat and fibre. This is why most runners avoid foods high in these before a long run, often consuming a breakfast which is higher in simple, easily digestible carbohydrates.
In addition, some of the other nutrients and substances we commonly use as a welcome boost for heavy exercise might be culprits. Take caffeine, for instance. These stimulants might boost our energy, but they can also have a laxative effect (opens in a new tab) in some people.
And carbs are not entirely without blame (opens in a new tab). Evidence suggests that some carbs (opens in a new tab) can not only increase the speed at which foods move through the gastrointestinal tract, they can also cause fluid retention and fermentation within the gut, making diarrhoea and gas more likely. These include the lactose in cow's milk products and high fructose fruits, such as apples, pears and grapes.
Finally, it's possible that an attack of the nerves may be (in part) to blame for the runs. Not only do stress and mood have associations with irritable bowel syndrome (opens in a new tab), it has also been suggested that psychological factors (opens in a new tab), such as anxiety, may be associated with runner's diarrhoea.
What can you do?
Is there anything you can do to avoid needing the loo somewhere on your marathon route?
Nutrition does seem to be key (opens in a new tab). Eat an energy-rich and familiar breakfast (one you know won't bother your stomach) with the minimum of fibre, fat and protein (opens in a new tab). It's not good to choose the day of a big event to challenge your gut with unfamiliar foods. One example might be a toasted bagel (or indeed plain white toast), or a low-fibre breakfast cereal, that's based on rice or corn rather than bran. Fruits lower in fructose include strawberries, raspberries and bananas.
Ideally, give your chosen breakfast a test drive before a training session to see what the effect is. In addition, separating your meal and the starting line by 2-3 hours gives the gut some time to get going on digestion, and to mobilise fuel for action.
Hydration is important. Make sure not only to drink water, but replenish electrolytes as well since both water and sodium are easily lost in sweat. Consuming water or sports drinks in small but regular bursts can help alleviate the problem of needing the toilet.
Perhaps one of the best ways to recognise and prevent runner's diarrhoea is to sit back, observe and listen to your own body. Yet another reason why preparation is so important. Training for a marathon should take place over months, not days or weeks. This gives ample time to recognise gut symptoms, but also what might be triggering them.
Some athletes find it useful to keep a diary, detailing symptoms and activity for the day, in order to spot trends more easily. You can also trial simple tactics, such as specific nutrition, hydration and training plans to see what effect they have. Everyone is different, and will respond to exercise and diet in different ways.
In any event, if you get caught foul (sometimes quite literally) of runner's trots, try to take a break, slow down and rehydrate. Also remember that most marathons have toilets at frequent intervals, in case the moment should grab you.
This article is republished from The Conversation (opens in a new tab) under a Creative Commons licence. Read the original article (opens in a new tab).
