A lot of the time I can’t get through a pool swim without needing at least one toilet break, which is a little bit weird and irritating.

And then there’s open water swimming when it’s even more frequent.

I’m proud to report that I’ve taught myself to pee in my wetsuit although on my list of skills to learn is the art of peeing without interrupting my stroke which will be helpful while I’m swimming the channel.

On a side note it’s an interesting conversation to have with a safety kayaker when I’ve paused to take a comfort break and they want to know if I’m Ok…..”sorry I’m just having a wee”. 

But apart from that – what’s it all about? How come I need to go to the toilet so frequently during or after a swim? But not during or after a bike or run? And how is that going to affect my hydration status during a marathon swim?

The science name for it is Immersion Diuresis and is caused by the Gauer-Henry Reflex.

Here are a few things that contribute to the need to pee.

Temperature effect: The swimming pool water is cooler than air and our normal environment (even though a pool isn’t cold like a lake or the sea).  So our body attempts to conserve the heat by narrowing the blood vessels (vasoconstriction) at the extremities such as the skin, arms and legs.  The blood moves from these extremities back to the vital organs the heart, lungs and large internal blood vessels to keep these areas warm and functioning properly. Our bodies sense this extra volume of blood and liquid and send a signal to kidneys to get rid of the excess water. Thus the need to pee.

Pressure effect: The external hydrostaticpressure due to being immersed in water is different and also causes a shift in blood from our skin, arms and legs to the central circulation. Again the body detects the shift and assumes that blood volume is too great and sends a signal to the kidney to get rid of the excess and thus the need to pee.

Gravity: When we swim we’re lying down supported by the water which takes away the effects of gravity.  While on land, gravity causes blood to stay in our legs and arms. As soon as we enter the water and lie down to swim the blood is no longer held in our legs and arms by gravity so blood moves back towards our chest area and major organs. Again the body senses this increase in blood volume and the need to pee more.

Coffee Effect! I love a morning cup of coffee…..and we all know coffee is a diuretic and makes you pee more!  

Another useful titbit of information I learned during this research is that  “free water clearance by the kidneys actually increases slightly during very low intensity exercise, due to the rise in mean arterial blood pressure”  So when you go on an easy recovery ride you might need to pee  more often than normal.  

If you’ve got any tips on how you cope with this during a long swim let me know! Or how about any other weird and unusual things that happen to you during your training.