How To Test Your Sweat For Better Trail Racing Performance

We tried three at-home sweat tests so you don’t have to. Here’s what sweat tests are best, and how to use that information for a trail racing breakthrough.

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Developing a fueling and hydration strategy is integral to running performance.   Hydration is especially important because we lose fluids and electrolytes through sweat when our bodies thermoregulate. .  For endurance sports that are particularly long or in extreme conditions like heat, humidity or altitude, total sweat and electrolyte losses can cause fluid and electrolyte imbalances that can hinder performance, and cause health concerns.  

RELATED: The Science of Hydration

Understanding Sweat and Electrolyte Losses

Sweat production is an essential physiological process that helps maintain body temperature during exercise. The body’s core temperature rises during exercise, particularly during higher intensities and when exposed to different environmental conditions (hot temperatures and humidity). Sweat is produced through specialized glands that are dispersed throughout the body and when it evaporates on the skin it releases heat

Sweat fluid loss rates can change depending on a number of factors including: genetic predisposition and number of sweat glands, body weight, metabolism, and fitness level, while sodium sweat concentration is not nearly as variable. It is well known that excessive fluid loss amounts of more than two percent of body weight of water can impact performance, so, minimization of losses is key. 

Sweat composition is highly individualized and contains predominantly: sodium, chloride, magnesium, calcium, and phosphorus. Sodium is the electrolyte lost in the highest amounts in sweat, found in the largest amounts in our intracellular space, and has the biggest impact on total body hydration status. When trying to determine fluid and electrolyte requirements during exercise, we really only want to focus on sodium sweat concentration results from testing.

Unlike fluid losses (which can change often), sodium losses do not need to be tested repeatedly. Jennifer Gibson, MS, RD CSSD, reminds us that our sodium sweat concentration does not actually change all that much—it is very much genetically predetermined. This can actually be beneficial to athletes because once they have an idea of what their sodium sweat concentration looks like, they don’t have to keep frequently testing. Once you have the fluid and sodium sweat concentration, you can apply this concentration to the fluid loss rates to help you determine hourly sodium losses during exercise. 

In an ideal world, we would collect whole-body sweat losses without interruptions in the natural sweating process to figure out sweat composition, but, without running in a climate-controlled bubble, this isn’t possible.   While all fluid loss tests have their limitations, here is what experts recommend. 

Method of Fluid Loss Testing

  • Whole Body Mass Pre and Post-Exercise: This is the easiest and most practical method for assessing fluid losses during exercise.  To perform, weigh yourself naked pre and post-easy effort training sessions between 45 and 75 minutes in duration.

Methods of Sodium Sweat Concentration Testing

  • Whole Body Wash-Down: A wash down of the body is done post-exercise, sweat is analyzed, and results can give a picture of full body sweat losses without interruption of the normal sweat process.  While it is the most accurate method of sodium sweat testing, it is very impractical due to it having to be done in a lab and often doesn’t reflect real life sport conditions during collection.
  • Sweat Patches: Sweat patches apply an absorbable patch to the skin, which picks up sweat.  The patches are then placed in a sealed tube and sent off to a lab for analysis. A downside of the patches is that they can block normal sweat glands from functioning because of the covering.

Companies: Levelen, Gatorade

  • Biometric Sweat-Testing Devices (Wearables): These devices use technology to sense fluid and electrolyte losses, and they analyze and transmit the data results to an app for real-time results. Many of these methods have not been validated with scientific research for accuracy.

Companies: hDrop, Nix 

Until recently, figuring out sodium sweat concentration was inaccessible to most athletes due to the sheer cost of the tests, which can range from tktk to tktkt. A few companies, Levelen, Nix, and hDrop, are starting to develop more affordable at-home testing options. We tested three of these at-home testing options to figure out if they are worthy of purchase.

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Sweat Tests We Used

Cost: $129 for the rechargeable sensor and four patches. Additional patches must be purchased separately

Pros: Comfortable when wearing, sleek design, did sync well with the app

Cons: Patches are nonreusable, but can be recycled through TerraCycle, pods are reusable and rechargeable, doesn’t separate out electrolytes in sweat in the analysis (which leaves the user guessing what their individual sodium losses are), huge inconsistencies with fluid loss rates

Cost:  $259.99, on pre-sale for $159.99 for the Gen 2.0 version (reuseable) 

Pros: When connectivity worked, the app layout was sleek with user-friendly electrolyte and fluid losses, rechargeable (can use in different conditions), no scale needed, our fluid losses were congruent between scale fluid losses and sensor fluid losses

Cons: Armband was a little clunky and uncomfortable, and connectivity issues (however, per the hDrop CEO, this is to be fixed in their hDrop 2.0 version being released this Fall 2023.)

Cost: $119 for the single-sport test (one-time use)

Pros: Straightforward directions, quick result turnaround time (even though we had to mail it in), easy to interpret results 

Cons: Need to use a scale to weigh self pre and post exercise for fluid loss rates, one time use only (however, can do at home fluid loss testing and match sodium losses), must be careful to mess up the test 

While it was valuable being able to trial three different sweat-testing methods, Levelen is the only company out of the three that works the closest off of the wash-down method, while the other two do not have validated research that compares their product to the wash-down method.  

RELATED: Running in Humidity and Heat Is Tough. Here’s How to Make it Easier.

Create a Winning Fluid and Electrolyte Plan

For best results, athletes should do their sweat testing in conjunction with a professional like an RDN.  Getting fluid and sodium sweat loss rates dialed in can be a huge step in the right direction.  

Once you have your sodium sweat concentration figured out in mg/oz, remember that fluid loss rates can change depending on a number of factors. This is why if you don’t use a wearable sweat-testing device, you may need to take regular scale weights to determine where your fluid loss rates fall in different weather conditions and during different parts of your training cycle. 

The Bottom Line

It is important to remember that for electrolyte and hydration protocols, while the tools for testing are promising, they are still just that at the moment—tools.  In sifting through all of the sweat science out there, it appears that there is still a lot of room for improvement and research in this subject matter.  

While at-home sodium sweat tests may have some downsides, Colette Vartanian, MS, RDN, a sports dietitian and athlete manager for Skratch Labs,likens the sweat-testing methodologies to finding a T-shirt size that fits you; sometimes the T-shirt might be a little tighter or looser, but it fits. For sodium sweat concentration: You might not be able to figure out with exact precision what your sodium sweat concentration is, but they can get into the right category to help you dial in what intake would be best fit—light, moderate, or heavy salty sweater.

And regardless of what sodium sweat concentration and fluid loss rates you come up with, you still need to figure out how to interpret the data and create an optimal fueling strategy for yourself. Remember to work with a sports dietitian who understands the testing methodologies and fueling protocols to ensure you are setting yourself up for success in training and racing.  


Kylee has a B.A. in Biology/Pre-Veterinary Medicine from the University of Richmond, B.S. in Human Nutrition from Metropolitan State University in Denver, and completed her dietetic internship through the University of Northern Colorado. She offers virtual sports nutrition coaching for endurance athletes and is a running coach for Microcosm Coaching.



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