Triathlon/open-water swimming: do things really go better with Coke?

Does drinking cola after an open-water swim help to prevent tummy upsets? And if not, are there any other strategies you can use? Andrew Hamilton looks at what the science says

It’s easy to understand the appeal of swimming in open water as Nature intended. However, one drawback is that there’s a small risk of picking up a tummy bug from swallowing contaminated water, particularly when training in sea/lake/river water with less than exemplary water quality standards. And even though the water quality at any particular beach, lake or river might be good most of the time, it can vary dramatically – for example after very heavy rainfall, which can suddenly wash effluent from the land into the water, increasing the risk of picking up a tummy bug dramatically.

A good example of this comes from the 2010 and 2011 Copenhagen triathlons, which had a 3.8km open-water swim leg(1). The 2010 race took place after extreme rainfall, which resulted in flooding and run off from the land into the sea. The result of this was that the concentration of E. coli bacteria at the time of the triathlon shot 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 outcome for the triathletes was that in 2010, 42% of the participants developed illness (mainly diarrhoea and vomiting), while in 2011, just 8.2% of participants reported a tummy upset (see figure 1). The researchers also discovered that in the 2010 event, the risk of falling ill increased with the number of mouthfuls of water swallowed.

Figure 1: Number of cases of gastrointestinal illness onset following 2010 & 2011 Copenhagen triathlons

The 2010 competition took place on 15 August (blue arrow) and the 2011 competition on 14 August (red arrow).

Bugs and cola drinks

One popular strategy employed by open-water swimmers is to drink cola beverages such as Coca Cola and Pepsi Cola immediately after a swim in order to help prevent the risk of tummy bugs. The theory is that the ingredients in these drinks (eg phosphoric acid) can help kill off harmful bacteria in the digestive system. But how true is this, or is it just an urban myth?

No study has ever tested this directly (ie by giving swimmers Coke 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(2). 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 5 minutes 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!

And while it’s true that pure phosphoric is a strong acid, with a pH of around 1 (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(3). Bear in mind too that your stomach already contains strong acid (hydrochloric acid), whose job is to help break down protein you eat and kill any bugs into the bargain. This hydrochloric acid has a typical pH of around 1.0-2.0 – ie some 10 times more acidic than any cola drink!

The reality is then that when you survey the scientific literature, there’s no solid scientific evidence whatsoever that drinking Coke or any other cola drink after an open-water swim will reduce the likelihood of getting a tummy upset. Drink Coke after a swim if you enjoy it but don’t expect it to protect you from tummy upsets. However, the science also tells us that some basic and sensible precautions can help keep the bugs at bay; you can see these in the practical advice panel below.

Practical advice for open water swimmers and triathletes

To minimise your risk of catching a tummy bug when swimming in open water, follow these guidelines:

  • Choose your training locations with care. In Europe for example, look for blue-flag awarded beaches, which meet minimum water quality standards for coastal waters. For lakes and rivers, you can check the water-quality data on the Environment Agency’s website (
  • Consider modifying your swimming technique and/or stroke (especially in rough conditions) to minimise the frequency of water swallowing;
  • Refrain from swimming in open water venues after heavy rain/flooding; this is when water quality is likely to be significantly poorer due to run off from drains and agricultural land.

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