Let’s break down what you would actually do with a sweat test and the reality of sweat and sodium replacement.

First: What is the purpose of your testing and how will you use it? If you’re not using a sweat test to inform fluid and electrolyte replacement strategy, then there is no purpose at all. In top-level NCAA DI sports sweat testing is increasingly commonplace, so that teams’ Registered Dietitians can formulate beverages specific to each athlete to ensure optimal hydration, allowing for better performance during training and more rapid recovery after training. Better blood volume maintenance during training simply means better cardiac output and more sustained performance ability over long training sessions.  

Similarly, among triathletes, there exists an entire market of sweat testing products and companies designed for the dedicated athlete to learn what they ought to be intaking during training and racing. Without replacing what you lose each hour while you train, you will without question be worsening your performance by reducing blood volume and the oxygen carrying capacity of your blood. Loss of hydration is the most surefire way to have higher heart rates at the same paces and efforts. Not good!

What Do Sweat Tests Really Measure?

So just what do these sweat tests test? Primarily, the content of your sweat—specifically, the sodium content. Testing the sodium content is quite sensible, given that it’s sodium that is primarily lost in sweat and contributes most strongly, by a wide margin, to blood volume, tissue hydration, and performance, over any other constituent of sweat—outside of water, of course.

Other than the specific sodium concentration of your sweat, though, you’ll also need to know just how much you are sweating, so you can match that with your fluid replacement and so that you can figure out just how much total sodium is being lost. With just a sodium concentration and a total body sweat volume rate, you can figure out how many milligrams of sodium are being lost per hour and then seek to match that with you consume.

Starting to think this is going to be a bit of a “process” yet? Good. It is. And it will be. There is no shortcut that provides a meaningful test result with which to guide your fueling strategy. You need to know how much sweat you are losing, and you need to know how much sodium is in your sweat. Or do you?

Some Facts About Sweat, Sodium Losses, and Athletes Interested in Sweat Testing

Let’s look at some facts of sweat testing and then examine the tradeoffs at play. Here are some realities of the people who typically pursue sweat testing:

1. Performance-interested athletes or coaches. These are folks who care a lot about performing to the best of their ability. They train near their recoverable limits and this places limitations on their time and energy for other things.
2. They might be heavier sweaters. That is athletes who are interested in sweat testing probably sweat at least an average amount or more, or have had hydration or cramping issues.
3. They may train in hot or humid environments where fluid and sodium loss are the most rapid and likely to hinder performance.
4. They may just be “Type A” or data-driven folks. Yes, I am looking at you—you are reading an article that promised to be math-heavy to test the electrolyte content of your sweat to make you go faster in training and racing. You are Type A.

Here are some realities about sweat.

1. Adults sweat usually between 0.3-3.0 L/hr in warm conditions during exercise. For frame of reference:
   1. A 120-pound person running 7:30min/mile loses about 0.9L/hr in 85-degree temps with mild humidity and no direct sunlight.
   2. A 150-pound person running 8:30min/mile loses about 1L/hr in 75-degree temps with mild humidity and no direct sun.
   3. A 200-pound person running 10min/mile loses about 1L/hr in shaded 65-degree temps in mild humidity.
   4. Warmer temperatures would bump up those rates of loss by about 10% per 10 degrees increase.
   5. Direct sunlight would increase rates of sweat loss by conservatively estimates about 10-30% more per hour.
   6. Elevated humidity increases sweat rate even further.
   7. Lower fitness levels mean greater sweat rates for given paces and body sizes.
   8. Increase any one of the variables mentioned in the examples above and sweat rate goes up.
2. Average salt content of sweat is very roughly 750-950mg sodium/L of sweat.
   1. Pulsatility—ie. a calculation of blood flow—of effort (undulation, like intervals or climb/descending) may increase sweat sodium content.
   2. Lower fitness likely increases sweat sodium content.
   3. Higher sodium consumption, especially during training, may slightly increase sweat sodium content.
3. Potassium loss via sweat is probably of negligible concern for blood volume maintenance or for cramping issues among exercising athletes.

It’s easy to see how sweat sodium losses quite routinely exceed 1000mg per hour during routine training in relatively fit individuals, even in mildly warm conditions.

The Limits of Replacing Sodium Losses

The kicker to all of this is you cannot just haphazardly attempt to match sodium losses without considering if your gut can effectively absorb that amount of sodium. It may be possible that your sweat rate (volume loss per hour) and your sweat sodium content (mg of sodium per liter of sweat) completely preclude you from matching what you’re losing, no matter how optimally and how well-planned your hydration and electrolyte replacement strategy is. Sip every four minutes, lick the salt, add sodium citrate to your beverages or purchase expensive electrolyte mixes and down them during exercise with the precise timing of a metronome to your heart’s content. You still may (and often will) fall short of replacing what is being lost.

Let us investigate this issue, the limits of sodium and fluid consumption during exercise.

1. Maximum fluid consumption and absorption per hour for most folks starts near 1.0-1.2 L/hr during exercise. Dehydration of any degree will quickly, and unfortunately, reduce that.
2. Maximum sodium tolerance without GI issues is probably somewhere between 1000-1800 mg/L for most folks, with the upper end of that requiring very delicate timing, and the use of something like sodium citrate instead of sodium chloride (table salt). If using table salt, you are wise to limit it to 1000mg/L or you’ll risk upsetting gastrointestinal cramping and later (or sooner) diarrhea.

If you exceed any of the values, you are likely to find performance acutely or remarkably hindered due to gut cramping and the urgent need for a restroom. Too many ions in the gut tend to do that!

Importantly, short of causing any gut issues, you can quite safely consume much more sodium than you’re losing via sweat and the body will simply respond by concentrating the sodium content of your urine a bit, and by sweating out a bit more concentrated sodium in your sweat itself—simply in response to your intake outmatching your sweat output. Replacing more than you lose during exercise, within the limits of what your gut can handle, poses no risk, assuming you don’t have high blood pressure and happen to be quite responsive to increases in dietary sodium intake. If that is you, chat with an MD first.

The Tradeoffs of Sweat Testing

If you are reading this, you are likely to frequently be losing upwards of 1000mg of sodium per hour via sweat, which is approaching the limits of replacement already, and there is little harm in over-consuming sodium within your personal gut constraints.

You are also probably competitive and driven in more ways than just in your sport. You have limited time, if for no other reason than your sport itself. It is a safe bet that you occasionally cut sleep short or cut other restful things out of your life already just to fit everything in.

Sweat testing is costly financially, though not prohibitively so for very interested folks—but the time-cost is huge; the reliability, repeatability, and permanence of the tests is low; and for virtually all folks for whom it matters, they should all be maxing out their gut-tolerated sodium consumption rates during exercise anyway, or at least it won’t hurt them to do so.  (And they may benefit tremendously from it.)

That means you might as well start by testing out your gut-tolerated sodium consumption rates before you spend the effort and headache on sweat testing. Then spend the time you might otherwise be sweat testing instead with your family, catching up on work, napping, or getting to bed earlier. Seriously, it’s a better tradeoff for performance—you’ll go faster.

Dr. Alex Harrison, a certified USA Triathlon coach, holds a PhD in Sport Physiology and Performance. He is the author of The RP Diet for Endurance, creator of the RP Endurance Macro Calculator, and has authored and contributed to dozens of articles. When he isn’t pumping out training and nutrition plans in his RV-garage-turned-mobile-office, he can be found on his bike, clinging for dear life to his wife’s wheel.