How to stay hydrated during a swimrun

Jul 27, 2017

Jakob Edholm

By Andy Blow

Hydration is routinely cited as a crucial factor for performance in endurance sports like swimrun and to a large extent this is true.

Significant fluid loss is comprehensively proven to negatively impact endurance performance, so it’s extremely important, if you want to have your best possible race, to avoid getting excessively dehydrated when racing an ÖTILLÖ swimrun.

When does dehydration start to impact your performance?

Nearly all athletes finish endurance events dehydrated to some extent. This is normal and to be expected as your fluid losses tend to outpace your fluid intake when you’re racing. However for performance limiting dehydration to occur most people need to lose at least ~2% of their bodyweight in fluid, assuming they started exercising well-hydrated.

There’s also evidence that some athletes can tolerate much more than 2% dehydration and still perform at a high level, so that number is not set in stone, just the likely starting point for when problems could occur.

To put this figure into a more meaningful context, 2% of the bodyweight of a 70kg person is 1.4kg. As sweat weighs 1kg per litre, that equates to ~1.4l of un-replaced sweat loss as the likely starting point for performance issues.

Human sweat rates vary pretty wildly from person to person, from less than 500ml/hr to over 3.5l/hr in more extreme cases. But if you’re looking for an average, something around 1-1.5l per hour is considered pretty normal in adults.

This all points to the fact that getting fluid balance right in a swimrun race is a very individual process.

What are the hydration challenges specific to swimrun?

There are three main issues that can make hydration for swimrun a bit more challenging than it can be for other endurance events.

1. Running in a wetsuit

Over long distances and in hot conditions this can really increase your sweat rate quite dramatically, leading to higher fluid losses than you’d normally experience running in regular clothing in the same conditions.
Anyone (like me) who did the recent ÖTILLÖ Isles of Scilly race will know about this first hand! The very warm air temperatures and blazing sunshine all day out on the course made for some heavy sweating.

2. Carrying fluids requires a big performance compromise

While it’s theoretically possible to carry fluids during a swimrun, doing so adds additional weight and drag in the water, which means that a majority of teams (especially those wanting to be competitive) choose to rely solely on fluids from the Energy Stations on course to get their fluids in.

3. Immersion in cold water can increase fluid loss

This is because it causes diuresis (i.e. it makes you pee more than normal). For a great explanation of this see this recent blog by one of our athletes.

Competing in a swimrun can lead to greater overall fluid losses through sweating and urine output and make fluid intake more difficult than in more “regular” endurance events.

So, what can you do to stay hydrated during a swimrun?

1. Start well hydrated

To avoid becoming significantly dehydrated when racing a swimrun event, the first and probably most important point to get right is getting to the start line optimally hydrated. This gives you the biggest possible reservoir of fluid to draw on when you begin, so that any losses are less impactful as the race unfolds.

The big thing to note about prerace hydration though is that human beings are not camels! We cannot simply drink loads and loads of water in the final few days (as many athletes try to do) and store it away for future use. All that drinking lots of extra water does is to increase the amount you need to pee and it can actually be very counter productive if this leads to dilution of the electrolyte levels in your blood (see this Training Peaks article for a more in-depth explanation of this)

Instead of dramatically increasing water intake in the build up to the race it’s a good idea to increase sodium (salt) and fluid intake a bit, as the additional sodium helps your body to retain the little bit of extra fluid you consume in your bloodstream, where it can be used to help you maintain your performance.

For this reason we recommend drinking a 500ml bottle of PH 1500 the night before the race and another one in the last 2 hours before the gun goes off. You can also add a little extra salt to your food in the final 24 hours before the start.

PH 1500 has 3 times more sodium than a regular sports drink and this little “top up” is enough to make sure your body’s fluid levels are optimised, while at the same time reducing the likelihood that you will just spend most of the prerace build up in a queue for the toilet (or peeing in your wetsuit on the start line if you really get desperate!). That is pretty much all of the “extra” hydrating you need to do prerace, assuming you’re otherwise drinking a sensible amount of fluids on a daily basis.

Jakob Edholm

2. Understand your individual fluid/electrolyte needs for during the race.

During the race itself the amount you need to drink will depend a lot on your individual circumstances – notably your sweat rate and the amount of time it’s taking you to get from Energy Station to Energy Station (if you’re not carrying drinks yourself).

Water and HIGH or LOW strength Precision Hydration electrolyte drinks are available at all of the ÖTILLÖ Energy Stations and we’ve developed a Swimrun Sweat Test to give you guidance on which strength to go for and how much you’re likely to need to take in. It’s free to take and gives you a detailed output on your own individual requirements, so is worth spending 2 minutes having a look at that.

Fluid/electrolyte requirements during a race are highly individual (and require a bit of trial and error to perfect). As an example, in the recent ÖTILLÖ Scilly Isles event where the weather was very hot, I took in 2-3 full cups of the HIGH strength PH electrolyte drink at each of the 6 Energy Stations.

This was inline with my past experiences of how much I can comfortably consume without getting too bloated and uncomfortable and was also enough to adequately quench my thirst each time we stopped to refuel and rehydrate. This was very important in the warm weather – there’s nothing worse that getting really thirsty halfway into one of the longer run sections as you can literally feel your power draining away when that happens.

I aimed for at least 2-3 cups per station knowing that I have a pretty high sweat rate and a very high amount of sodium in my sweat (1,842mg/l in fact). And this approach – along with about 2 SweatSalt capsules per hour washed down at the Stations – got me around the race in just under 5 and a half hours without any hydration or cramping issues (which I used to be prone to). There will have been athletes who drank more than I did for sure, as well as many who have got away with drinking a bit less, but for me the above approach felt like it worked pretty well and your aim should be to find a level that works well for you.

The sodium in Precision Hydration is a crucial component when sweat losses are high as it helps you replace what you’re losing in your sweat and maintain better fluid balance than you would get by just drinking plain water all of the time. It can also help you avoid cramping up.

I found myself pretty thirsty at the finish after a relatively long final run in the heat, but didn’t feel it was detracting from my performance (certainly not as much as a general lack of running training was anyway!) and that was a sign that I got my intake about right.

For those of you in training for the World Championship race in Sweden in September, one thing worth highlighting is that paying attention to replacing more fluid early on in the race is more important than it is in the shorter qualifying events. With the extra distance and time out on the course, dehydration in the later stages can be more likely the big race if you skimp early on and rush through the Energy Stations too quickly.

I hope this helps you perform at your best out there.

Andy Blow founded Precision Hydration to help athletes solve their hydration issues. He has a few top 10 Ironman/70.3 finishes and an Xterra World Age Group title to his name. He previously finished in the top 10 at the ÖTILLÖ World Championships in 2014 and in the top 15 at the 2016 and 2017 ÖTILLÖ Isles of Scilly races. He has a degree in Sport and Exercise Science and was once the Team Sports Scientist for Benetton and Renault F1 teams.

Swimrun Sweat Test

Take the free Swimrun Sweat Test to get a personalised hydration strategy for your next race to know whether to pick up the HIGH or LOW strength Precision Hydration drink at the Energy Stations.

Want to pick up some PH 1500 to start the race optimally hydrated or some blister-packed SweatSalt capsules to carry with you on the course? Use the code OTILLO15 to get 15% off your first order at

Article first published in Swimrun Life Magazine Issue #3 (July 2017)

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