Marathon Mastery Series: How to avoid an upset tummy

As we approach peak training load for those running Spring marathons, we often hear of runners complaining of an upset stomach and/or feeling sick during and after a run. Read on for advice about how to avoid it….

Run strong without feeling ill
Run strong without feeling ill


For many runners, stomach pain, feeling sick and needing the toilet urgently, are all unfortunate consequences of an increase in marathon training and long training runs. There are a number of factors at play that cause this, but one could be taking on too much carbohydrate in the form of gels.

‘If you take on too many gels or sports energy products, that’s when you get into trouble,’ explains nutritionist Fran Taylor, ‘Your body can only absorb so much sugar and any excess accumulates in the gut and draws in extra water which all contributes to making you feel pretty uncomfortable.’

Research shows that if you want to run faster and keep going, 30-60g of carbohydrates per hour when running for up to two hours (although remember the muscles hold enough fuel for 90 minutes so you won’t need to start taking them until you’ve run for an hour), which equates to about two gels or 8-10 Jelly Babies.  For runs longer than 2 1/2 hours, aim for 90 grammes, about three gels or 16 Jelly Babies.

The above paragraph should also illustrate the benefits of getting used to gels for racing – it takes 20 seconds to swiftly swallow a gel (and we’d recommend doing it fast, they’re not something you want to savour!) compared to trying to chew your way through lots of Jelly Babies.

But there can also be other factors at play when it comes to runners’ gut.


‘When we run for a long time, blood flow to the stomach is reduced in favour of the blood that is powering the muscles to run, and this is  exacerbated by dehydration along with an increase in stress hormones which are released when we exercise, all which can contribute to making you feel sick,’ explains Fran Taylor.

Try to replenish lost fluids when you finish your run and do some yoga stretches (check our YouTube channel for examples) which will help the body to recover and off-set the harmful effects of the stress hormones.


Here’s our guide to what you can eat to fuel your runs…

The Food


Disgusting flavours (Salted Watermelon or Toasted Marshmallow anyone?) aside, gels are easy to swallow and digest, and become available for the body to use as energy almost immediately. But lots of runners avoid them because they taste horrible and this can get confused with people assuming that this is what causes an upset  stomach.

But the reason they cause an upset stomach is usually because someone is taking on too much. Your body can’t use any more than 3 gels in an hour, or 60 grams or less. Then they don’t overload the system and while not tasting great, they’ll do the job.

I can personally vouch for this, I hate gels, hate the taste and the texture, as does almost every runner of every level that I know, BUT shoot them down in one and don’t overdo them and they DO work.  I’m a fan of High 5 Isogels, which are a little bit more liquid than most.

Electrolyte Drinks

If you’re constantly battling an upset stomach and it’s not nerves, switching to liquids may be an alternative. Nowadays, electrolyte mixes from the likes of Tailwind  and Scratch Labs have been specifically designed to fuel the body while moving in the easiest, most digestible form. This can help to a certain degree, but be aware that getting enough carbohydrate in from liquid alone requires drinking a lot. So, it provides an alternative but not without supplementing with another carb source.


Performance blocks like Clif Shot Bloks and Honey Stinger Chews (which are also vegan and use honey as their carb source) are similar to gels but come in a denser form. They provide around seven grammes of carbohydrates at a time, and so this can enable you to test at what point, your body reaches overload and you begin to feel sick.

Longhaul Endurance

These food sachets are increasingly becoming popular with ultra runners, who can’t fuel their runs with gels and sweets because it causes bonking not due to hitting the wall but crashing into a sugar wa (keep feeding the body too much sugar and your sugar levels will spike and then fall, which over a long distance race is not sustainable). They come in flavours such as Turmeric and Chicken and Sweet Potato and Sesame, and provide between 32.6g and 23.7g of carbohydrate respectively. Not easy to take on you’re racing at full pelt but worth investigating for training runs.

Sweet Potatoes

Boil or roast sweet potatoes and you’re left with a creamy, fast-digesting carbohydrate which would work well at the beginning of a race, as long as you take the skin off first which contains fibre and which could cause a tummy upset or require a sudden need to go to the loo.


Pretty much the first thing I can even think of stomaching after a hard run marathon is crisps. They won’t fuel you as well as gels, but they break up the monotony of just eating sweet stuff and contain salt to stabilise your electrolytes and they’re great for afterwards . If you’ve ever watched or taken part in an Ironman event, you’ll see crisps and flat cola on all the refreshment stations on the run.

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