Understanding how much sweat and sodium you lose when you exercise—and knowing how much you need to replace for optimal performance—can have a significant impact on your training and racing. Today, more than ever, being able to race in the heat is key to any tri plan, and dialing in your fluid intake (and “outtake”) is crucial.
But before we dive into the details of today’s big leaps in sweat testing with our first-hand experience and its takeaways, we’ll take an important look at the science of sweating and the importance of sweat testing.
Sweating is our body’s way of cooling down and enables us to continue to exercise for long periods of time. But if you lose too much fluid and electrolytes (without replacing them) then you’ll soon struggle to keep going at the same pace, if at all. While sweat testing is not new, it’s not something that all athletes opt to analyze, which can have far-reaching effects, especially if you happen to be a high-sodium sweater. We’ll explain more about this later, but when it comes to racing in hot and humid conditions it can really be a critical part of achieving your true racing potential.
Every athlete’s sweat is different, which is why sweat testing is so important—there is no one-size-fits-all approach or generic rule of thumb to follow: what works for Athlete A could be entirely disastrous for Athlete B. In fact, sweat rates can vary fivefold between individuals while sweat sodium concentrations can vary by as much as 10 times. The results from a sweat test can help you to tailor two key factors: your sweat rate (how much fluid you lose per hour) and sweat sodium concentration (the amount of sodium you lose in your sweat). From there, you can draw up an approximate sodium and fluid intake plan that you can test in training and then use on race day, feeling confident that you’re taking care of one of the most fundamental aspects of your physiology.
The Test Itself
We went to Full Cycle Colorado Multisport in Boulder, Colorado, for this sweat composition test, which was conducted by performance director Ryan Ignatz and took approximately an hour. Surprisingly, you do not need to exercise for the sweat test to be effective. He explained that the test can be carried out at any time, but it’s particularly useful if you do it well ahead of any races you have planned, so you can implement the findings in training for optimal results on race day. And while it can be helpful for athletes racing any distance, those doing longer distance racing (four hours or more) will likely stand to benefit the most from their results, he said.
It begins with a small patch, known as a macroduct (that is a little bigger than a quarter), being strapped to the inside of your forearm. There is a coil of tubing inside of it. Your sweat glands are activated to sweat by using light electrical current (you feel a slight tingling sensation) that is pushed to the patch, which helps activate protein channels (that are the sweat glands themselves). Hey presto! You’re sweating and you’ve not moved an inch. (And it’s worth noting that this elicits the exact same sweat response you’d have if you were working out, making it far more convenient and comfortable).
Ignatz said: “This process is like opening the door for sweat to come out, and once you’re sweating, we’ll collect that, which takes about five minutes.”
You can sit and watch as the tubing fills with your sweat; it’s mixed with a blue-colored sodium-free dye that turns your sweat blue, so you can see what’s happening and how fast it’s filling up. “The faster this happens, the higher your sweat rate,” Ignatz explained.
Compared to most physiological testing that we’ve undertaken, this was entirely painless and simple—and after the five minutes had elapsed, Ignatz was removing the tubing from the macroduct and running it through the small machine for full sweat composition analysis.
This version of the test costs $175 and was performed locally at Full Cycle Colorado Multisport, in tandem with Precision Hydration, a UK-based sports science company co-founded by former elite triathlete Andy Blow. While tests like these are not widely available, the Precision Hydration website offers a free online sweat test to help get you started, and you can also search for test centers near you on their site. If you have an exercise physiology lab or sports medicine center near you, it’s also well worth reaching out to them to see if this type of testing is available.
Much to my surprise, the results showed I’m a low salt sweater, losing 512mg of sodium per liter of sweat. The spectrum ranges from 200mg of sodium per liter of sweat (very low) to 2200mg of sodium per liter of sweat (very high). For me, this is good news when training and/or racing in warmer climes, as I don’t need to take on a huge amount of sodium in order to stay replenished. The results were broken down further to show that in order to replenish 100% of sodium I’d need to take on 364mg of sodium per 24oz (750ml) of fluid, which is easy to do with just two scoops of something like Osmo’s Active Hydration (320mg sodium), for example, or one sachet of SOS (330mg sodium).
“Remember that you can’t replace 100% of sodium lost, but you should try to get as close to it as possible,” Ignatz said. “When racing Ironman, it’s better to use the bike as an opportunity to stay on top of hydration because that helps set you up for the run, where it’s much harder to stay hydrated.”
He also explained that it’s much harder to catch up on hydration if you fall behind, so it’s far better to sip fluids frequently and stay well hydrated, because once you fall behind it can be extremely hard to bring things back around.
Equally, it is important to remember that you can overdo the hydration and also take on too much sodium, which can have dire consequences. For example, if a low sodium sweater like me starts to take on high volumes of salt, it won’t end well. There is a balance to strike, as Ignatz warned: “Sodium is stored in the bloodstream, which makes it an extracellular electrolyte. When you ingest sodium, it goes into the gut, moves across the gut lining into the bloodstream where it is stored. If we ingest too much sodium, your bloodstream gets saturated with it, and the sodium in your gut has nowhere to go, which now creates a weird draw for water—instead of water moving across into the bloodstream it now backflows into the gut, creating a sloshy feeling and dehydrating the rest of your body.”
In short, while you might have had the best of intentions taking on all that sodium, it could, in actual fact, be setting you up for GI (gastrointestinal) distress, dehydration, and a host of other problems—none of which are going to help get you to the finish line faster.
Learning that I’m a low sodium sweater is something I implemented in my training immediately, taking on much lower levels of electrolytes and drinking more plain water. The results (so far) have been terrific: on a 50-mile training run last month (as I prepare for Leadville 100 Trail Run in August) I had zero GI issues, very stable energy levels, and felt mentally alert all day (nine hours of running covering 7,000+ft of elevation gain). My biggest takeaway from the testing has been that plain old H20 is just as much my friend—if not more so—than all of those fancy electrolyte tablets and powders.
If you don’t have access to an in-depth sweat composition test like the one outlined here, panic not, there’s still plenty you can learn and implement in your own training:
- First, think about whether you regularly get dehydrated when you train and/or if you suffer with cramps. If you do, then the chances are you’re a high sodium sweater who could benefit from paying closer attention to your hydration needs. Also pay attention to your kit after training on hotter days: Is it stained with salt? If so, you’re likely a medium/high sodium sweater.
- Ignatz recommends that the baseline amount of fluid to take in per hour (for the “average” person) is ~240z per 750ml when training, so if you’re taking in less than this (and not feeling good in training or taking longer to recover) then consider consuming more fluids.
- If you want to get some baseline figures on your own sweat rate, there’s a simple at-home test you can do pre-/post-training, which involves weighing yourself before your workout and then weighing yourself afterwards. You’ll also need to know how much fluid you consumed. Once you’re done with your workout, subtract your pre-exercise weight from your post-exercise weight, remembering to factor in how much fluid you drank too. You’ll then add together the weight lost plus fluid consumed and divide that by the total time spent working out. This will give you a sweat rate per hour. If that sounds complicated or confusing there are online calculators to help guide you, such as this one from Precision Hydration’s website and this one from the Gatorade Sports Science Institute.
- Once you have all of this data, start implementing it in your training, making notes on how you felt and performed (objective and subjective data, ideally) so that you truly have it dialed by the time your race arrives.