The science behind perspiration

How analysing your sweat can make you a better runner

PH Sweat Test - Inducing a small patch of Lucy Bartholomew's forearm to sweat maximally at rest.png

Understanding the science behind your own sweat is vital when it comes to trail running, whether you’re aiming to win the UTMB, with its massive temperature swings, or just avoiding hydration issues like cramp on your weekend outings.

The thing about sweat is that it’s all very personal. Sitting on the Tube on a particularly hot and humid August afternoon, you can easily deduce who among us sweats copiously. And equally, we can also see who doesn’t. “We’re all different,” says Andy Blow, founder of Precision Hydration. Andy, an Xterrra World Age group champion, founded the company to create bespoke hydration plans for its clients. Because what works for one runner won’t necessarily do the job for another. 

We all, however, sweat to some degree. We have to. Sweating is the body’s cooling system. We heat up causing the sweat glands to excrete a water-rich secretion. Evaporation of this sweat from the skin has a cooling effect, without which we’d soon overheat and stumble to a halt. 

How we manage our fluid/electrolyte replenishment can dramatically affect our performance. It’s all a question of hydrating. No, check that, of hydrating effectively. Runners who sweat more often drink slightly more, but runners who don’t sweat a lot may still be losing a lot of sodium in their sweat, and so need to adequately replace it if their performance is not to suffer.

The blood flood

“Boosting your blood plasma volume before exercise is a way to enhance your performance,” explains Andy. “Having more blood makes it easier for your cardiovascular system to meet the competing demands of cooling you while delivering oxygen to muscles.

“Drinking a strong electrolyte drink to optimise your hydration status before key runs can significantly improve your performance. We call this ‘preloading’ and it’s about getting you to the start line ready to perform at your best. It’s especially useful before races as starting fully hydrated reduces or removes the need to disrupt your rhythm by drinking on the move. But it’s also worth doing before longer training sessions or training runs in a hot climate.”

What’s really interesting in all of this is that today’s runner will undoubtedly head out of the door, bottle in hand, ready to hydrate at the earliest opportunity. It’s regarded as very old fashioned not to be obsessed by hydration, in fact it’s damn-near foolhardy, right? Wrong. For many, this approach is overkill.

Andy explains: “As a rule of thumb, very few runners can comfortably drink much more than 750ml (24oz) per hour – especially when running hard – so unless experience tells you otherwise, it’s unlikely you’ll need to drink more than that, especially if you got to the start line well hydrated.” In other words, it’s important to have fluids available, but everyone’s needs are individual and you should largely drink to thirst. Experimenting within the guidelines (above) will help you fine tune what works for you.

In search of sodium

A few of you may already have a good idea of your sweat rate. But the crucial factor is how much sodium you’re losing (it’s the sodium that makes your sweat taste salty and leave white marks on your kit). Sodium helps you absorb and retain fluid and it’s this that boosts your blood plasma volume. If you don’t replace the sodium you lose, over time it’ll start to impact your performance and may even cause cramp as a reduction in your sodium stores triggers a contraction of the interstitial fluid compartment around your muscles and a misfiring of nerve impulses.

A Precision Hydration Advanced Sweat Test tells you how much sodium you lose in your sweat, which is largely genetically determined and which can vary hugely from athlete to athlete. Some people lose as little as 200mg of sodium per litre of sweat, while others lose upwards of 1800mg/l. Anywhere between 900-1000mg/l is average. 

Once you know how much salt you lose you can manage its replacement. This will help you reduce the risk of cramp and improve your performances in hot weather. An Advanced Sweat Test with Precision Hydration is a simple but very accurate procedure. The rest is non-invasive and done at rest, so no jumping up and down in a bid to produce buckets of sweat! A couple of electrodes are just attached to your forearm to induce you to sweat. The sweat is then collected using a watch-like device. The PH team then create a personalised hydration plan to suit your particular needs. They’ll tell you when, what and how much you might need to drink before, during and after training and races. And they have a range of electrolyte supplements to match your plan, the strongest of which has three times more sodium than typical supplements. 

In the weeks following the test you’ll receive a series of emails outlining a strategy to make it easy to implement, we found these really useful. Easy peasy, no sweat…

The above piece was published in the April-May 2018 edition of Trail Running