Ever been out for a run and had to make a frantic dash to find a loo? Or perhaps you’ve been struck with mid-race stomach cramps which have derailed a PB attempt?
While runner’s diarrhea is anything but fun, it can be prevented if you’re able to get to the root of the cause. We spoke with leading sports dietitian Renee McGregor to understand what triggers runner’s runs, how to prevent mid-run stomach issues in the first place and what to do if you find yourself with a dodgy tummy during a race...
What causes runner’s belly?
According to McGregor, a combination of various things, detailed below, can cause runner’s diarrhea. However, the most obvious and general explanation, she says, is the actual mechanical movement of running causing the gut to loosen. ‘We know that, generally speaking, we are at rest most of the time, sitting and being fairly sedentary. But then, as soon as you start the actual motion of running, you’re jiggling around the food and everything else in your gut,’ she explains. ‘So, that can be one cause – particularly if you have quite a high-fibre diet, as it then takes a bit longer for food to move through the gut and the pull of gravity can cause the gut to loosen.’
Not leaving enough time between eating and running
McGregor says that this is exacerbated if you have too much food in your stomach to begin with. ‘Remember that, when we run, all the blood flow is directed away from the stomach and into our muscles to help us move,’ she says. ‘That means that there’s no blood flow in the stomach to allow for digestion to occur. So if you’ve got a full tummy, that could be another reason why you end up with the trots.’
Under-fuelling and running on an empty stomach
That said, and perhaps surprisingly, under eating – and running on an empty stomach – can also be to blame. ‘If you’re not consuming enough energy before you run, and maybe you’re just slightly chronically under-fuelled, that can actually lead to what we call guy permeability – something that is known as ‘leaky gut’ – where you tend to quite regularly get an upset stomach. So a lot of people will not eat before a run because they’ve associated runner’s trots with eating when, actually, it’s probably one of the worst things you can do.’
Dehydration
Dehydration is also a key cause of runner’s belly – be it simply from not drinking enough before heading out for a run, or due to chronic under-hydration during a long-distance training run or race. ‘When it comes to racing, people tend to blame energy gels. But, actually, that’s very unlikely – especially during endurance runs. So when you’re doing marathons or ultramarathons and if it’s also hot, people don’t tend to hydrate properly.’
This is due to the fact that, during a marathon, runners – particularly those who are going fairly quickly – don’t necessarily stop to drink, so can’t get enough fluid on board. ‘They just grab a drink and have a couple of sips and it’s quite difficult to swallow enough,’ says McGregor. ‘So over the course of marathon, they’ll have only had, say, 400-500ml, but if you’ve been out there for three or four hours, it’s not really enough.’
What happens then, explains McGregor, is that any energy taken on board (like energy gels, for example) tends to concentrate in the stomach. ‘And if it’s too concentrated in our stomach, it’s going to want to come out,’ she says.
Best winter running gear electrolytes which have been lost in your sweat can also cause dehydration. ‘People tend to make the mistake of drinking water but not realising they’re losing salt at the same time – you need the salt to draw the water back in.’
Pre-race nerves
It’s not just mid-race when runners can find themselves caught out by a bout of the trots. A look at the queue for the loos at the start line of races attests to the issues runners experience pre-race. ‘Basically, you’re in fight or flight, your adrenaline is high and a lot goes on when you’re in that phase of vigilance,’ notes McGregor. ‘Your heart rate goes up, your adrenaline is up and your bowels go – and that’s completely normal. So, anxiety and stress is absolutely more likely to cause you to go the toilet more often than you normally would.’
Time of the month
Health & Injuries menstrual cycle. ‘Females who have natural menstrual cycles – so those not on the pill – might experience more stomach issues during their luteal phase, which is the pre-menstrual phase. At this stage in your cycle, it’s worth being mindful of the amount of fibre you’re eating because it’s likely to cause more problems than it would in the middle of your cycle.’
Pushing too hard during a race
The risk of dehydration is heightened during a race – such as a marathon – where runners can end up overexerting themselves. ‘The thing with marathons is that people end up going out way too hard with excitement – because for the first half of a marathon, you feel great. But if you push a bit extra – even if it’s just 10 seconds or 15 seconds more per mile – you’re using up your energy stores very quickly, so you’re going to How long does it take to walk a mile quicker. But also, if you’re working hard, both your heart rate and your core temperature goes up, so you’ll be sweating more and losing a bit more salt – and then not replacing that fluid. And again, if you try to put something concentrated into your gut, like an energy gel, that can result in stomach issues – including vomiting – because your salt balance is off.’
How to prevent runner’s belly
Stay hydrated
When it comes to preventing diarrhea during a race, McGregor says that the most important thing you can do is to keep on top of your hydration, as this will prevent the contents of your stomach from concentrating. Recommendations on how much additional water we should consume per hour when running vary from 300-800ml, but it’s also important to consider the outside temperature and how much you sweat, as well as your thirst.
Take on electrolytes
In addition to drinking enough water, it’s important to replace the sodium lost in your sweat, which will help to draw water back into your body. ‘While sports companies promote gels, they often don’t contain huge amounts of electrolytes, so people think they’re getting what they need with a gel when they’re not,’ says McGregor.
For distances over a half marathon, McGregor recommends taking on electrolytes mid-run. ‘Once you get into the realm of the marathon, people will usually be running for over three hours, which means you’ll be sweating for over three hours. So, you’re going to need to replace those electrolytes.’ McGregor suggests using salt tablets, electrolytes, energy drinks and even food if you can stomach it (like salted peanuts and cured meat).
Have something small to eat and drink before a run
To ensure you’re adequately hydrated – and fuelled – before early morning runs, McGregor suggests having a little something to eat and drink before heading out. ‘If you’re constantly getting an upset stomach, try having a glass of water and a banana before you head out,’ she suggests. ‘A lot of people think they need to have their breakfast before they go, but you don’t need to have your whole breakfast. For example, when I go for a run in the morning, I might have a piece of toast half an hour to 45 minutes before I go, so I’ve had something. It’s not heavy – but I’ve had something and that tends to help.’
Take gels over 4-5 minutes
To prevent a direct glucose hit to the stomach, which can, fundamentally, cause the body to want to expel, McGregor advises runners to take their time when consuming energy gels. ‘Taking a gel over the course of, say, four or five minutes is much better than trying to take a gel all in one go, which the body can’t cope while it’s trying to run at the same time.’
Practise your nutrition in training
Most runners know that they should practise their mid-race nutrition in training, but does this mean that most runners heed this advice? Apparently not. ‘The biggest mistake I see is with people who won’t train with the products they’re going to use on race day. Then, on race day, they start using them and think they’re going to be fine when they don’t know if the body is going to tolerate it or not. There will be some products that your body doesn’t like very much and you do have to work that out before you go out.’
That’s why it’s so important to not take unfamiliar products off the course. ‘Sometimes, people don’t want to carry the gels they’ve practised with, so they take the gels the marathon is giving out because this think a gel is a gel, which is a mistake.’
Keep a diary
McGregor recommends that runners – particularly female runners – keep a diary of when they’re noticing stomach issues. ‘Female runners might find a pattern in their cycle, which is something you can work around,’ she says. ‘But it’s thinking about other things like when you had your last meal, or whether you had a lot of more fibre on a particular day that led to issues one or two days later. It’s not always the last meal immediately before a run that causes problems – it can be one you had 12, 24 or even 36 hours before.’
Although we do need fibre in our diet, excess fibre can cause us to be a little more mobile in the gut region. ‘It’s just about being aware that things like lentils, beans and pulses can take a bit longer to digest, hence why they are good for you – so it’s just being mindful and seeing if you can see patterns.’
She also recommends looking at your dairy intake. ‘While it’s very rare, some people do find that having dairy, particularly in the run up to a race, can mean that they’re more likely to end up with stomach problems. But, it is quite rare.’
It’s about finding out what might be the cause for you because everyone is different. If you’ve tried everything and are still struggling, McGregor suggests taking a probiotic supplement. ‘That tends to help a lot of people if they’re really stuck. Our gut biomes are so affected by stress, lifestyle, exercise and so on, that sometimes, if you have a little bit of disequilibrium in the gut – if you’ve got a bit of dysbiosis going on – taking a probiotic can help to sort that out and encourage good bacteria to colonise and allow equilibrium back in the gut again.’
What should you do if you get runner’s belly mid-race?
If you find yourself with runner’s trots during a race, try not to panic. Slow down your pace and take on some fluids and electrolytes, if you know you’ve not taken on enough by that point. ‘Slow things down, have a few sips of water, take a salt capsule and see if that makes a difference,’ advises McGregor.
What to eat before and after a run Runner’s World, overseeing RW’s new membership programme, set to launch later this year. She has been with the brand for close to three years now – previously working as deputy digital editor, where she covered all manner of running topics, spanning training, health, injury, nutrition and gear. Over the years, she’s interviewed an abundance of awe-inspiring athletes, from top-tier ultrarunners and Olympic champions to everyday runners who have accomplished extraordinary things. Jen has been a sports journalist for 10 years; she is the former editor of Women’s Running magazine and has also worked as Sports Editor at Red Bull. She started running a decade ago and likes to dabble in triathlon a little, too. You’ll find she’s happiest plodding down the Thames path or chowing down on a post-run pastry.
