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Home / Training / Swim / Can cola prevent stomach upsets after swimming in open water?

Can cola prevent stomach upsets after swimming in open water?

We look at what steps you can take after a swim in the great outdoors to reduce your chances of picking up a stomach bug

Open-water swimmers

It’s easy to understand the appeal of swimming in open water. However, one drawback is that there’s a small risk of picking up a stomach bug from swallowing contaminated water, particularly when training in water with less than exemplary quality standards.

And even though the quality might be good most of the time, it can vary dramatically – for example after very heavy rainfall, which can suddenly wash effluence from the land into the water, dramatically increasing the risk of picking something up.

A good example of this comes from the 2010 and 2011 Copenhagen triathlons, which had a 3.8km open-water swim. The 2010 race took place after extreme rainfall, which resulted in flooding and run-off from the land into the sea, increasing the concentration of E. coli bacteria at the time of the race up to around 20,000 bacteria per 100mls of water. By contrast, the 2011 event had no such pre-race rainfall and the water quality remained ‘good’, being well within the EU Bathing Water Directive criteria of less than 500 E. coli per 100mls of water.

The result of this was that in 2010 42% of the participants fell ill (mainly diarrhoea and vomiting), while in 2011 just 8.2% of participants reported similar symptoms. The researchers also discovered that, in the 2010 event, the risk of athletes falling ill increased with the number of mouthfuls swallowed.

Bugs and cola drinks

One popular strategy employed by open-water swimmers is to consume fizzy drinks such as Coca Cola immediately after a swim in order to help prevent the risk of stomach bugs. The theory is that the ingredients in these drinks (such as phosphoric acid) can help kill off harmful bacteria in the digestive system. But is it just an urban myth?

No study has ever tested this directly (in other words, by giving swimmers cola after a swim) but one study looked at the effects of exposing harmful bacteria to virgin olive oil, vinegar, fruit juices, Coca Cola, coffee, beer, and red and white wine. The vinegar and virgin olive oil showed the strongest bactericidal activity against all the bacteria tested. Red and white wines also killed most strains after 5mins of contact but all the remaining drinks (including Coca Cola) had absolutely zero effect. The bug-killing capacity was put down to the high acidity of the vinegar, antioxidant compounds called phenolics in the virgin olive oil and the alcohol in the wine.

While it’s true that pure phosphoric is a strong acid with a pH of around one (the lower the number, the stronger the acid), the acidity of cola drinks that contain it is far weaker – typically having a pH of around 2.5, which is about 30 times weaker. Plus, you should keep in mind that your stomach already contains hydrochloric acid, whose job is to help break down protein you eat and kill any bugs as part of the bargain. Stomach acid has a typical pH of around 1-2 – in other words, some 10 times more acidic than any cola drink.

The reality is that when you survey all the literature, there’s simply no scientific evidence whatsoever that drinking Coca Cola or any similar drink after an open-water swim will reduce the likelihood of getting an upset stomach. Drink cola after a swim if you enjoy it but don’t expect it to protect you.

Saying that, the science also tells us that some basic and sensible precautions can help keep the bugs at bay, as you can see below.

Takeaway tips

Minimise your risk of catching a stomach bug by following these three simple suggestions:

– Choose your training locations with care; look for blue-flag awarded beaches and check the water-quality data on the Environment Agency’s website

– Modify your swimming technique and/or stroke (especially in rough conditions) to minimise the amount of water swallowed

– Refrain from swimming in open water after heavy rain/flooding when water quality is likely to be significantly poorer

(Main image: akunamatata)

For lots more advice on how to get the most from your swim, head to our Training section

Profile image of Jamie Beach Jamie Beach Former digital editor


Jamie was 220 Triathlon's digital editor between 2013 and 2015.