Compared to land-based sports like cycling and running, hydration needs for swimming are less widely studied, talked about and frankly a lot less understood.
There have been a few studies looking at sweat rates in competitive pool swimmers and water polo players (two decent examples can be seen here and here) and these, along with a lot of anecdotal data, seem to point to the fact that normal swim training in a pool environment leads to some sweating - in the region of about 250-750ml per hour. This is likely to be lower than when working at an equivalent intensity on land doing other aerobic activities, where average sweat rates are closer to 1-1.5l per hour.
This makes sense because pool water is generally pretty cool. Or at least it should be in the kind of pools proper swimmers train in regularly; according to FINA rules, a competition pool must be kept between 25 and 28C / 77-82F. This means the pool does a good job of taking some body heat away without the need to sweat much.
It also seems likely (according to some more recent research) that swimmers’ sweat responses and heat acclimation statuses are generally less developed than runners who stimulate more adaptation (and therefore higher sweat rates) through their typical land-based training regimes.
During pool training sessions
The main thing pool swimmers have to do is try to show up to training well hydrated in the first place, top up as they feel they need to throughout a standard 45-90 min session and all will generally be right with the world from a hydration point of view.
Though it does probably make sense to use a light carb/electrolyte based drink like Precision Hydration 500 or 1000 during long, hard swim sets, or when swimming is not your only training session of the day, to help optimise your recovery.
When swimming in open water
Open water swimming is a whole different kettle of fish to pool swimming as it invites a new set of variables into the hydration equation.
Water temperature and the wearing of a wetsuit (or not) are probably the biggest determining factors as to how hydration can potentially affect performance for open water swimmers and triathletes, if not in the swim itself then in the bike and run sections that come after it.
As water temperature rises it becomes increasingly difficult for the body to offload heat to it and your sweat rate increases considerably in response. Add a wetsuit (something that’s specifically designed to help you retain body heat) and a bout of hard swimming (that drives up heat production from your muscles) into the mix and you can go from being comfortable in the water to overheating - and therefore sweating a lot - pretty quickly.
This is acknowledged in the ITU competition rules for triathlon, where for age group athletes wetsuits are not allowed when the water temperature exceeds 22 degrees C for swims up to and including 1500m swims and 24.6 degrees C for longer distances (i.e. half and full Ironman).
When it comes to ‘short’ duration open water swims (i.e. up to about 5,000m), it’s generally the case that all hydration (and nutrition) intake can (and arguably should be) taken care of before and after the swim.
A degree of dehydration and energy depletion is inevitable, especially once you’re swimming for over an hour, but stopping to interrupt your rhythm to eat or drink in a race of that length is unlikely to offer benefits that significantly outweigh that of just cracking on and getting the job done without stopping.
You then generally have to rehydrate (and refuel) as soon as you’re out of the water. Because of this, your main aim needs to be to start ‘shorter’ open water swim races and the swim sections of triathlons optimally hydrated to give yourself the biggest possible reservoir of fluids and electrolytes to draw on once the race gets under way.
Drinking a high strength electrolyte drink before you race will help you retain more fluid than just drinking water alone. The extra sodium in stronger electrolyte drinks allow boost your blood plasma volume without triggering lots of additional unwanted trips to the bathroom before you start. That higher blood volume reduces cardiovascular strain, enabling you to maintain your performance for longer. Aim to drink 500ml of an electrolyte drink with more than 1000mg of sodium per litre (like Precision Hydration 1500) about 90 minutes before the gun goes off, finishing it no less than about 45 minutes before you get in the water so you have plenty of chance to absorb the fluids.
Similarly using some Precision Hydration 1500 - or other strong electrolyte drink - after the event (or on the early stages of the bike, if you’re doing a triathlon) can help to rapidly replace any fluid you’ve lost through sweating more effectively than drinking water alone.
This is extremely important in Ironman races when the water temperature is warm but still “wetsuit legal”, as top age group athlete from Team Freespeed Ali Rowatt found not so long ago when she did Ironman Switzerland on a really hot day…
“One of the biggest mistakes I made in an Ironman was not recognising the potential for getting dehydrated in the swim and not rehydrating enough after the swim to compensate for this. In this particular race the water temperature was warm (24.5 deg) and after swimming hard for an hour in the wetsuit I came out very dehydrated. I didn't take on enough fluid in the early stages of the bike ride and by the second half of the bike I was struggling to eat or drink at all.”
If you start racing in really long swim events (i.e. significantly over 5km) then hydrating. along with taking carbohydrate energy on board, starts to become increasingly necessary to maintain your performance over the full duration of the event.
As with shorter races, drinking a stronger than average electrolyte drink before you set off will help you absorb and retain more fluid. The sodium will increase your blood plasma volume, reducing the strain on your cardiovascular system when you’re pushing yourself hard, enabling you to maintain your performance for longer. Drinks with more sodium in are also effective at alleviating (or helping you avoid) cramp.
Drink about 500ml of an electrolyte drink with more than 1000mg of sodium per litre (like Precision Hydration 1500) the night before your event and another serving 90 minutes before the gun goes off, finishing it with at least 45 minutes to spare so you have time to absorb the fluids.
When it comes to during the swim, any generic recommendations for fluid and energy intake need to be treated with a serious amount of caution as individual requirements vary significantly for both of these. However, it seems quite common that many experienced long distance swimmers find that they can tolerate less fluid and carbs per hour than they could during an equivalent kind of exercise on land.
Paul Newsome (founder of SwimSmooth and winner of the Round Manhattan Island Swim amongst other accolades) describes this well…
“Nausea is a big problem for many marathon swimmers. The old adage of 1g carbohydrate per 1kg body weight per 1hr of exercise for fuelling often seems to be too much for the body to process on long marathon swims. Equally trying to match your hydration rate with your perceived sweat rate often sees people trying to over-consume, and this, I believe, adds to that risk of nausea.”
He adds “I’ve cut back by about 30-40% on the “magazine recommendations” over previous years and personally seem to have benefitted from it.”
Of course, it’s likely that water temperature and individual work rate will have a big impact on what you need (and are able) to take in during a very long swim. At times when the water is warm and you’re working hard, your sweat rate is likely to rise and so more fluids and electrolytes are likely to be beneficial. If the water is cold and you’re not moving so fast, then you can probably get away with a little less.
The bottom line in longer marathon swims is that it’s important to work out a plan for replacing both fluids and electrolytes. Precision Hydration developed a free online Sweat Test to help you get started with personalising your hydration strategy and you can refine from there using a bit of trail and error. It’s likely that you’ll be taking in a bit less than you would do for the equivalent duration of workout on land, but your intake is also going to be heavily influenced by water temperature and work rate.
If you want to try some of Precision Hydration’s stronger, all-natural electrolyte drinks as part of your personalised hydration strategy, use the code HUUB15 at their checkout to get 15% off!
Andy Blow has a few top 10 Ironman and 70.3 finishes and an Xterra World Age Group title to his name. He founded Precision Hydration to help athletes solve their hydration issues, they work with a long list of elite sports teams and athletes including HUUB athlete Henri Schoeman. He has a degree in Sport and Exercise Science and was once the Team Sports Scientist for Benetton and Renault F1 teams.