As some of you may know from personal experience, hydration mishaps are all too common among endurance athletes. Sometimes it's a fluid issue, other times an electrolyte issue, or it may be both.
Fluid and salt needs vary greatly among athletes, which is why these two things must be individualized. Thankfully, you can test both your sweat rate as well as your sodium composition!
In this post, I'll teach you how to test your sweat rate (the amount of fluid you lose per hour during exercise) using a home scale.
But First, Some Background on Sweat
Water is essential both in everyday functioning as well as during exercise. In fact, water makes up about 60-70% of our body!
You 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. You lose electrolytes through sweat and urine, and lose sodium followed by chloride in the highest concentration through sweat.
If you're not replacing your losses adequately, you will develop electrolyte imbalances that disrupt normal bodily functions. This leads 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, your performance suffers when we lose as little as 2% of our body fluid!
What makes this even more challenging is that your sweat rate changes based on external 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.
It's important that you learn how to test your sweat rate so that you can measure it 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, all you need is a scale, and then you can use the results to individualize your fluid goals during training and competition!
How to Test Your Sweat Rate: Instructions
The first step to testing your sweat rate (aside from finding a scale) is to pick which exercise session you plan to test. Try to target a session around 60 minutes, but anywhere between 45 and 90min works.
Once you've fueled, gone to the bathroom, and you're ready to go, here's how to test your sweat rate:
Weigh yourself (naked) before your workout
Start your session and make note of the following:
How much fluid you drink (in ounces) during exercise, if any
Duration of your session
Exercise intensity / session type (e.g. easy run vs speed session)
Weather/temperature
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)
Do the math using the equation below
For every 1 lb that you lose, that equates to 16oz of sweat lost
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!
FYI, I want you to make note of the weather and exercise intensity because it's helpful to know if you're testing in similar or 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).
What to do with your sweat test results
I 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 it's important to test and individualize fluid strategies!
Note that the goal here isn't to replace 100% of your losses during exercise. Usually, you will aim to replace closer to 50-75% of losses, 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 (if you lose >1L per hour) may not be able to tolerate much above 1L/hr, especially over extended periods of time. This is a great example of why you need to start all exercise well-hydrated!
Learn more about what to do with your sweat test results, and how to begin all sessions hydrated and rehydrate afterwards in my post Hydration for Endurance Athletes.
How to estimate hydration goals without a scale
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. You can determine estimated fluid needs in other ways.
First, ask yourself if you think you are a heavy sweater. Then, you can use a more general fluid goal range of 12-28oz/hr during exercise, making adjustments based on estimated sweat amounts, weather, and intensity. Hydration for Endurance Athletes will help you make a plan!
Need help with your fluid or overall fueling plan?
Check out my 1:1 nutrition coaching and small-group coaching services
Book a free 10-min call to see if we're a good fit for 1:1 coaching
I also offer a self-paced comprehensive sports nutrition course, a library full of FREE nutrition resources and low-cost nutrition mini guides, and a podcast
Questions? Email me at claire@eatforendurance.com.
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