Do You Know Your Sweat Rate?

Avoid the performance-limiting effects of dehydration by following these simple steps!

In previous posts on indoor air quality, I made an argument for the importance of monitoring and controlling the potential factors affecting the air we breathe while riding. The adverse health effects of chronic exposure to elevated CO2 and PM2.5 levels, as described in this previous post, Poor Indoor Air Quality – The Invisible Danger to the Virtual Cyclist, are a risk to anyone who exercises indoors.

 

Riding and training in high heat and humidity is a health risk, as well. The performance-limiting effects of dehydration that results are profound and backed up by research. However, there are varied viewpoints, and by reading further, you will learn the multiple factors to consider when setting your hydration strategy.

The key is knowing how much you sweat under certain conditions and developing a practical hydration strategy. The following is an overview of the research supporting the importance of proper hydration for optimal performance and studies that offer different perspectives.  

 

Then we will outline a simple way to determine how much you sweat and the rate under varied conditions. From there, the information will promote an awareness of your state of hydration and develop a basic strategy to maintain it at optimal levels.  The information in this article is meant to be a basic framework and to guide you toward making the best hydration decisions for you.  

The Adverse Effects of High Relative Humidity

2016 study examined the thermoregulatory (ability to control body temperature) and circulatory responses and exercise performance of 11 trained runners during exercise in ~31-degree Celsius heat at varied relative humidity (23, 43, 52, 61, and 71%).  The athletes performed five 60 minute steady-state runs (70% VO2 max) followed immediately by an incremental exercise test to exhaustion.

 

The time to exhaustion was significantly reduced at 61, and 71% relative humidity and a greater thermoregulatory and circulatory stress were evident during steady-state exercise.  The combined thermoregulatory and circulatory strain further limits the capacity to perform all-out exercise to exhaustion.

The Performance Limiting Effects of Dehydration

Several studies support the finding that dehydration impairs cycling performance. A 2018 study examined the effect of dehydration on the performance of seven male cyclists. The authors proved a decrease in cycling performance and impaired ability of the subjects to control their body temperature. The hydrated group cycled faster during a 5-km time trial by producing higher power, and their core temperature was less.

The researchers reported similar conclusions in this 2013 study published in the Journal of Athletic Training. You will find many more like it, although this 2018 study performed by researchers at the University of Arkansas stands out.

Even Mild Dehydration Limits Performance

The study focused on the mild dehydration of 1% to 2% body mass with or without the sensation of thirst.  The group of eleven male competitive cyclists was slower, produced less power, had a higher core temperature, and perceived a higher exertion rate when studied in a mildly dehydrated state.

 

The numbers are there in these studies and others like it, but what does it mean to us?

 

A 2% decrease in body mass is the accepted guideline recognized by researchers and fitness professionals.

 

Basically, for a 140 lb. (63.5 kg) athlete, the difference between pre-and post-exercise body mass is 2.8 lb. (1.27 kg).  For a 175 lb. (79.4 kg) athlete, it is 3.5 lb. (1.59 kg), and for a 200 lb. (90.7 kg) athlete, it is 4 lb. (1.8 kg).

Calculating Your Sweat Rate

You can calculate your sweat rate by following these steps:

1

Determine the amount of bodyweight you lost while riding by subtracting your pre-weight from your post-weight (unclothed).

2

Add the bodyweight you lost to the amount of fluids you consumed (water, gels, etc.) during the duration of the ride to calculate your Sweat Loss.

3

Divide your Sweat Loss by the amount of time you were on the bike to determine your Sweat Rate.

A Practical Example

For example, you ride indoors for 1.5 hours after determining that your pre-ride weight was 140 lbs.  You weigh 139. 5 pounds after the ride. You lost 1/2 pound during your ride.

One pound is equal to 16 ounces of fluid.  You have lost 8 ounces.

You consumed 22 ounces of water during the ride.

Your Sweat Loss is 8 oz. + 22 oz. = 30 oz of fluid loss.

Divide 30 ounces by the length of your ride, 1.5 hours.  

Your Sweat Rate is 20 ounces per hour.

If we also assume that this athlete monitors the conditions in his indoor space and attempts to train under a normalized set of conditions, the rate of fluid loss will be similar.  Calculate a different value for varied exercise intensities, which I recommend breaking down to low, moderate, and high for simplification.  The values you calculate are approximate and used as a guide.

Conflicting Research, Is It?

A reason cited for the contradictory results found in this 2015 study that negated the impact of dehydration was the lack of outdoor wind. Void of the evaporative core and skin temperature decreasing influence of wind, indoor laboratories inflate the dehydration effect.  

 

Does this sound familiar? Properly positioned fans and an efficient air conditioner and dehumidifier are a necessity. Can they replace the cooling, environmental effect of wind?

 

It’s better than nothing and feels pretty good when you are riding indoors. 

How Much Do We Sweat?

Use this as a rough estimate, but the average person sweats approximately 0.8 to 1.4 liters or up to 48 oz. during an hour-long ride.  Between one to two large bottles per hour.

 

Of course, there is tremendous variation among athletes and conditions, indoors vs. outdoors.  That is why having a general idea about your Sweat Rate is vital.

Develop Basic Practical Hydration Guidelines

The Adverse Effects of Dehydration Are a Topic of Debate

When I asked Professor of Exercise Metabolism Dr. Adam Upshaw his take on the issue, he was quick to point out the controversial nature of the topic.  The following is Dr. Upshaw’s reasoning for considering the multiple factors involved before making a decision on this issue.

“It should be noted however that the relationship between thermal stress, hydration, and performance is not very strong. Perceived thermal cooling via menthol or ice slurry rinses has been shown to have a beneficial effect on time to exhaustion performance. (Jeffries et al. 2018)

 

However, data is equivocal when it comes to hydration levels and performance. Lee et al. 2010, did not find any association between fluid balance, body core temperature (up to 39.8 degrees Celcius), and performance, indicating that the most trained individuals, competing at high intensities and in warm environments, do not suffer from heat-related illnesses despite physiological conditions of hypohydration.

 

Many factors could be at play in these studies as the results do not translate into real-world scenarios. Many, if not all, elite marathoners and triathletes complete their races in a state of mild dehydration with little to no impact on performance or heat-related illness. (Tan et al. 2021) 

 

In addition, change in body weight during activity, especially long-duration high-intensity exercise, is primarily the result of fuel utilization. Specifically, stored carbohydrates in the form of glycogen are oxidized during such activity and can thus easily account for 1 to 3 % percent changes in body weight.  Pre and post-weighing provide no real indication of hydration status or susceptibility to heat-related illness in this case. (Maughan et al. 2007)

Over-Hydration Has Significant Negative Consequences

Improper hydration strategy implementation doesn’t always mean drinking too little.  Drinking fluid in excess has significant consequences and an adverse effect upon performance, according to Dr. Upshaw.  Here are his words of warning.

“It is often strict adherence to traditional hydration strategies that give rise to in-performance muscle cramping, bloating, feeling of illness, low urine output (somewhat surprising, I know), and hyponatremia.  The low blood sodium levels associated with hyponatremia can lead to headaches, confusion, and dizziness.  

As a result, I would advise athletes to drink to thirst primarily. There is no need to drink large volumes of fluid leading up to race day, and in most cases, there is certainly no need to drown the body in fluids and salt tablets during endurance competition. For most, the simplest (and often safest) recommendation for maximizing performance as it relates to hydration is to drink to thirst.”

Conclusion

An awareness of how much we sweat and the rate at which we lose fluids under different conditions is essential to our health and performance.  To optimize your cycling performance without the limitations of dehydration, a few simple calculations, and a basic drinking strategy is a great way to start.  If you know your sweat rate, you will be confident that you are in the ballpark when replenishing your fluids.

Use the above information as a basic guide, but keep in mind that the topic is controversial.  One thing that athletes and sports professionals agree upon is that hydration is an important topic.  Finding the right hydration strategy is vital to performance and is as simple as drinking when you are thirsty for most.  

Seek the guidance of a fitness professional for exact guidelines and specific advice about the other aspects affecting hydration, like electrolytes, fluid osmolarity, carb intake, etc.  Be careful to avoid the risks of overhydration.

Water is essential to life and drinking enough of it, but not too much, while riding is vital to success. Follow the simple steps noted above to ensure that your body is in the best position to succeed!

What is your strategy?

Do you have any hydration tips or tricks that work for you?  Your fellow virtual cyclists want to know!

For more performance tips and helpful information to ensure that your body is in the best position to succeed check out the Training & Peformance page of The ZOM!

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Justin F
1 month ago

I’ve been tracking my sweat rate for a while now. Garmin added in estimated sweat loss to the platform and I find that it is generally half of my actual sweat loss. I keep hoping it will learn from my corrections, but that is probably a bit much to ask.

I recently I added a CORE body temperature sensor to my stable of devices. I’m hoping I can find a relationship between power, body temperature, and sweat rate.

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