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How to Test Your Sweat Rate

As some of you may know from personal experience, hydration mishaps are all too common among endurance athletes - whether it's a fluid issue, electrolyte issue, or both. Both fluid and salt needs vary greatly among athletes, and thus these two things must be individualized. Thankfully, both of these can be tested, and today we'll be focusing on measuring sweat rate, or the amount of fluid you lose per hour during exercise, using a home scale.


How to test your sweat rate using a home scale

But First, Some Background on Sweat


About 60-70% of our body is made up of water, and it's essential both in everyday functioning as well as during exercise. We need adequate water to transport nutrients to your working muscles, remove waste products, support the immune system, hydrate your tissues and organs, maintain blood pressure, and maintain an appropriate body temperature.


As for the electrolytes, sodium, chloride, and potassium all help maintain fluid balance within the body, while magnesium and calcium promote optimal muscle function and aid in energy metabolism. We lose electrolytes through sweat and urine, and lose sodium and chloride in the highest concentration. If we're not replacing our losses adequately, we develop electrolyte imbalances that disrupt normal bodily functions, leading to symptoms like dizziness, headache, fatigue, irregular heartbeat, muscle cramps, dark-colored urine, mental confusion, or nausea and vomiting.


It's easy to get dehydrated, especially if you're exercising for many hours - in fact, our performance suffers when we lose as little as 2% of our body fluid! What makes it even more challenging is that sweat rate changes based on environmental factors. For instance, if you measure fluid losses in moderate temperatures and then go into the heat, you would expect your sweat rate to increase, thus increasing your fluid needs per hour. Equally, a more intense workout will lead to greater fluid losses compared to a lower intensity one.


This is why it's important to measure sweat rate in different conditions, and more than once in a training cycle (e.g. if you're training during the winter for a spring event). Thankfully, we can measure our sweat rate easily using a scale, and then use the results to individualize fluid goals during training and competition!


How to Test Your Sweat Rate: Instructions


Pick which exercise session you plan to test. Ideally, we are targeting a session lasting 60 minutes, but anywhere between 45 and 90min works. Make note of the weather and your exercise intensity, for reference (e.g. it's helpful to know if you're testing in similar or very different conditions from the event you're training for). If you are a multi-sport athlete, you may also want to do more than one test (e.g. one on the bike and another on the run).


Once you've fueled/hydrated, gone to the bathroom, and you're ready to exercise, here's what you do:

  • Weigh yourself (naked) before your workout

  • Make note of how much fluid you drink (in ounces) during exercise, if any

  • Avoid using the bathroom during exercise, if possible

  • Once you're back, remove sweaty clothes, towel off any excess sweat, and weigh yourself again (naked)

  • For every 1 lb that you lose, that equates to 16oz of sweat lost

  • Then, do the math using the equation below!


Sweat rate = (pre-exercise weight - post exercise weight in lbs) x 16 oz of sweat loss + fluid consumed (oz) / exercise time (hr)


Example: Let's say I do a 60min run and I lose 1 lb and I also drink 16oz. That means my sweat rate is 1 x 16 + 16 / 1 = 32oz/hr. Made that one super easy math-wise!


What to do with your sweat test results


First, off, you are considered a heavy sweater if you lose over 1 L or 32oz/hr of sweat. But we see athletes with sweat rates all over the map - I've had athletes lose well above 2L/hr of sweat, and as little as 8oz/hr. This is why we personalize!


Note that the goal here isn't to replace 100% of our losses. We may try to replace closer to 50-75% of losses during exercise, with adjustments made based on how heavy of a sweater you are, what conditions you're in, and what intensity you're aiming for. Very heavy sweaters may not be able to tolerate much above 1L/hr, especially over extended periods of time. This is a great example of why we want to start all exercise well-hydrated!


How to estimate hydration targets if you can't weigh yourself


If you don't have access to a scale or you find the scale to be triggering, you do NOT have to do this test. We can determine estimated fluid needs in other ways. First, ask yourself if you think you are a heavy sweater, and then use a more general fluid goal range of 12-28oz/hr, making adjustments based on estimated sweat amounts, weather, and intensity.


If you're struggling with any of the above, you may be a great candidate for 1:1 nutrition coaching! Please reach out or book a FREE 10min call to see if we're a good fit.


My 7-part self-paced course "Peak Performance for Endurance Athletes" is another great option to help you learn how to fuel yourself better to crush your health and athletic goals. You can listen for FREE to module 1 in this podcast episode, and you can visit the course page to watch a free video preview and read detailed course FAQs.


Questions? Email me at claire@eatforendurance.com.

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