What Is Your Sweat Rate? - Coach Tip Tuesday

Last week we talked about how it is actually possible to survive and thrive in hot conditions.  Part of how you can thrive in these conditions is by properly hydrating - both in workouts themselves and in your daily routine.  This week, I’m going to talk about a key component of proper hydration: Knowing your sweat rate.

What is your sweat rate?  Simply put, it’s the amount of fluid you are losing through sweat in an hour of exercise.  By determining what your personal sweat rate is, you can fine-tune your hydration strategy for workouts so you minimize blood plasma losses and maintain a good hydration status.

Remember that your body sweats by pulling water from your blood (which is sourced from the plasma part of your blood) to your skin.  Then, the water sits on your skin where it can evaporate into the air around you, thereby cooling you down.  As you sweat more, your blood plasma volume will drop.  If you do not replace that blood plasma volume with hydration, your body will seek to pull water from other sources (such as your soft tissues (read: muscles)).  Your blood will also become thicker, which means your heart will work harder to pump blood to your working muscles.  This means that your heart rate will go up while your power and speed goes down.  All of this combines to lead to fatigue, decreased performance, and the infamous and dreaded wall.

I think it’s safe to say that most of us would prefer to avoid that wall.  Determining your personal sweat rate (and then implementing that knowledge into a hydration plan) is the first step in avoiding it.

Why Sweat Rate Matters

A lot of athletes think that drinking to thirst is sufficient, but quite frankly, it’s not.  Contrary to (very) popular legend, thirst is not an accurate gauge of hydration status.  By the time we feel thirsty, our body is already entering a dehydrated state.  

This is especially true for females.  Both estrogen and progesterone (key female hormones) impact the hypothalamus - the part of your brain that regulates fatigue and fluid balance hormones.  High hormone levels change your body’s signals to respond to a lower plasma volume (up to 8% lower).  Simply put: This means that your level of thirst is impaired and you’re not as thirsty, even though your body is actually dehydrated.  (Also remember that a 2% dehydration status is enough to cause performance to suffer (especially for events lasting longer than two hours), and 5% dehydration causes a performance decrease of 30%.)  So for all you females reading this: drinking to thirst in your daily life and/or in workouts literally puts you at a performance disadvantage right from the very start. 

For both males and females, a lack of thirst only gets worse as we get older.  So, it becomes very important to train yourself to drink - even when you are not thirsty - to maintain proper hydration levels.

How To Determine Your Sweat Rate

Determining your sweat rate is easier than you might think.  Start by planning a one-hour workout of the discipline of your choice.  Ideally, this workout will contain a few different effort levels of intervals so you can mimic harder efforts like you might have in a race.  (If you are a triathlete, you will want to conduct both a Bike Sweat Rate Assessment and a Run Sweat Rate Assessment.)  If you are training for a race, ideally, you should conduct this assessment in conditions that closely replicate the conditions you will be racing in.  For almost everyone, this means that Sweat Rate Assessments should be conducted outdoors.

Sweat rates can vary depending on the ambient conditions (indoors vs. outdoors, cold vs. hot, wind vs. no wind, etc.) and based on what you are doing.  (For instance, your sweat rate while cycling will generally not be the same as your sweat rate when you are running.)  So the specificity here is important, both in sport/discipline and location.

Once you’ve planned your workout, weigh yourself immediately before you begin your workout.  You’ll want to record your nude weight on a digital scale that measures down to at least tenths of pounds.  The scale is a non-negotiable part of determining this, so if you don’t have access to one or prefer not to use one, unfortunately, you will not be able to determine your sweat rate.

Go and do your workout.  Do not consume anything other than liquids during the workout, and keep careful track of exactly how much you are consuming in liquid ounces.  During the workout, voiding yourself (i.e. using the restroom) will make the results of this assessment inaccurate.  (So try to set yourself up for success and take care of that before you start the workout.)  Immediately upon completing your workout, weigh yourself nude again.

The amount of fluid you consumed (in liquid ounces) plus the amount of weight (converted to ounces) that you lost during the workout is the total amount of your sweat rate.  For example, if you consume 12 liquid ounces of fluid during your workout and you lose 0.5 pounds (8 ounces) in that same workout, your sweat rate is: 12 + 8 = 20 ounces

To recap:

1. Plan a one-hour workout.
2. Weigh yourself nude on a digital scale that measures to at least tenths of pounds immediately before you begin your workout.
3. Conduct your workout in conditions that replicate your expected race day conditions as closely as possible.  Do not consume anything other than liquids and do not use the restroom during the workout.
4. When you are done with your workout, immediately weigh yourself nude again on a digital scale.
5. The amount of fluid you consumed (in liquid ounces) plus the amount of weight (converted to ounces) that you lost during the workout is the total amount of your sweat rate.

Implementing Your Sweat Rate Data

Assuming that you conduct your Sweat Rate Assessment in “normal” (temperate) conditions, your sweat rate will determine the amount of fluids that you should be aiming to consume per hour in the discipline that you tested it in.  Ambient weather conditions can change it; for instance, hot and humid conditions will cause your sweat rate to rise, which means that you should be consuming more fluids per hour in those conditions.  

Here are the average consumption rates for temperate based on your sweat rate:

• When your sweat rate is less than 17 ounces per hour, aim to replace 70-100% of your sweat rate
• When your sweat rate is 17-34 ounces per hour, aim to replace 50-75% of your sweat rate
• When your sweat rate exceeds 35 ounces per hour, aim to replace 25-50% of your sweat rate‍

Do not be alarmed if these targets are higher than you are used to consuming.  You don’t need to consume the full amount right away.  You can ease into it; start by consuming a few ounces more per hour than you have been consuming, and build from there until you reach your target hydration rate.

If your sweat rate is higher than 35 ounces per hour (for any sport/discipline), it will be impossible for your body to recuperate those fluid losses, as the gastrointestinal system can only process about 35 ounces of liquid per hour while you are exercising.  If you are someone who this applies to, maintaining adequate hydration status in your daily life and consuming 25-50% of your sweat rate while you are exercising is even more critical for you since you can’t prevent your body from becoming dehydrated.

The Bottom Line

Hydration matters…possibly more than any other single element in your training.  I cannot emphasize this enough, so please don’t make the all-too common mistake of disregarding it.  Determine your sweat rate and set yourself up for success!

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Sources:

Wilson, Patrick B. The Athlete’s Gut : The Inside Science of Digestion, Nutrition, and Stomach Distress. Boulder, Colorado, VeloPress, 2020.