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Why Test Sweat?

Dana Eshelman, MS, RDN, CSSD, METS I

Sweating leads to relatively rapid fluid and electrolyte losses as your body offloads excess heat to the environment. As you sweat there are physiological and perceptual alterations that occur such as reduced blood volume, increased rate of perceived exertion (RPE), increased cardiovascular strain, decreased muscle and brain blood flow, increased core body temperature and an increased use of glycogen. These effects cause a dip in performance at fluid losses as low as 1-2%.

As an endurance athlete, you need to replace both water and electrolyte losses in order to minimize the effects of dehydration so you can maintain maximal performance. This is especially true in more extreme conditions such as heat, humidity, and altitude, at higher intensity, and during events lasting over 1 hour.

Why test sweat?

Sweat rate varies among individuals as well as from situation to situation in a single person. This is where understanding your needs comes into play. Developing a personalized hydration protocol can maximize your performance potential.

A study from 2015 looked at 2 groups of half ironman athletes. The experimental group aimed to replete 71% of sodium lost through sweat, while the control group replaced 20% of their sodium. The triathletes replacing 71% of their sodium losses finished the race 26 minutes faster than the control group. This is not necessarily the right equation of sodium intake, but paints a picture of the importance of replacing your losses and athletic performance.

Sweat testing is usually performed with athletes to look at:

  1. How much total sweat volume you are losing

  2. What are you losing in that sweat (ie electrolytes - sodium, magnesium, potassium, chloride, calcium)

Most portable sweat tests/ wearables will measure the total body sweat rate (how much fluid you are using) and the key electrolyte lost in sweat, sodium. But, there are important considerations to keep in mind when relying solely on a wearable:

  • Testing in similar conditions (environment, effort, etc) to the race/event you are targeting to understand the sweat rate and sodium concentration. It is important to remember it will take about 30 minutes for your body temperature to increase from resting, so placing the wearable on after 30 minutes is ideal.

  • Day to day sweat rates can vary around 15% due to what you ate the day before, temperature, humidity, airflow, clothing, and/or variation in the hormone that regulates sodium losses, aldosterone.

Why is sodium important?

Sodium has multiple functions in the body:

  • Maintain fluid balance

  • Cognitive function

  • Muscle contraction

  • Absorption of nutrients in the gut

  • Nerve impulse transmission

As mentioned earlier, when you sweat, you are not only losing fluid, you are losing sodium. That means this sodium needs to be replaced during activity with drinks, chews, gus, gels, and/or food. Drinking plain water can work up to a certain point, but when intensity and duration of sessions increase, you are training in hot, humid conditions, or if you are a salty sweater, you need to begin replacing those electrolyte losses to prevent your blood from becoming diluted. This is called hyponatremia; this is just as dangerous as becoming dehydrated and can be fatal. I recommend opting for an electrolyte with activities lasting longer than 60 minutes to support electrolyte losses.

Where do I start?

If you do not want to use all the fancy gadgets, a great place to start is understanding your sweat rate. You can do this by taking a nude weight after you have used the restroom pre-session and a nude weight of you post-session with your sweat dried off. I recommend doing this for an hour long session and trying not to use the restroom (#1 or #2) to avoid having to do extra calculations. This will give you the amount of fluid you are losing during your session. You will want to aim to replace losses to prevent >2% dehydration.

For example, Susie weight 140 pounds.

140 x 0.02 = 2.8 pounds

140 -2.8 = 137.2 pounds

Susie's optimal hydration zone = 137.2 to 140 pounds

** this is the amount you will not want to exceed in sweat losses

From there, you want to consider sodium. A great starting place is 300 to 500 mg sodium/liter (190 to 315 mg sodium/ 20 ounces). If you find you have more salt residue on your skin or apparel, are getting headaches after training sessions, or are craving salty foods after or during training, then you are not consuming enough sodium in your training beverage (and potentially your daily intake). You may try to increase this based on personal tolerance and palate.


A key reminder: more is not better. Consuming too much sodium an also hinder your performance; and, consuming too much fluid volume with too low of sodium (ie. plain water) can be dangerous and fatal. It is important to understand your unique fluid needs, so you can support your training and racing efforts! Things to keep in mind:

  • Personal sweat rate or how much fluid volume are you losing per hour. Sweat rates range from 0.5 L/hour to over 2.5L/hour. Understanding your fluid needs in different scenarios is key! You will want to aim for fluids losses less than 2% to prevent decrements in performance.

  • Environment - heat, humidity, altitude

  • Intensity of your training session. Higher intensity will cause increased sweat and sodium losses.

  • Sodium sweat losses can vary from 200 mg to 2400 mg/ hour. Start at 300 to 500 mg/L and increase based on individual feedback such as headache post session or craving salty foods.

  • Test and measure your fluid and sodium losses. Practice your hydration strategy in training. Periodically re-check your sweat losses and monitor variables of hydration status after training.

If you are looking for 1:1 coaching or interested in having a few strategy sessions with a sports dietitian to help you fine tune your race day strategy, the A Dash of Dana team is here for you! Fuel fiercely!


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