Sweat Rate Testing
Sweat rate testing is simple, and any athlete can execute it throughout the season. The sweat testing data is valuable for creating a well-informed hydration strategy for your training or targeted event. Maintaining adequate hydration status during exercise can prevent a drop in performance as your event or session duration continues. At the bottom of this post, I linked a few sweat rate calculators where you can track your own, and immediately use the data to plan your hydration strategy for training sessions and races. I don’t recommend testing every single ride, but checking in for a few days or weeks at a time can provide useful data to apply to your training. You will also learn that drinking 100% to a plan is not always necessary, but a plan is important to get you in the right ball park.
Sweat rate testing involves measuring how much fluid an athlete is sweating out during exercise and is expressed as Liters of fluid per hour (L/h). It is important to know how much an athlete is sweating during exercise because performance begins to decline once a total body sweat loss of more than 2% of an athlete’s initial body weight is surpassed and only further declines once sweat loss exceeds 3%. *Disclaimer, you’ll be okay if you surpass 3% body weight loss, as elite marathoners have been shown to lose up to 10% of their body weight at the finish line, and they are going pretty fast!* Sweat rate testing is accomplished by measuring how much weight is lost during an exercise session, and is accurate because most of the weight lost during a single bout of exercise is water loss.
Ever hear that your body weight is made of 60% water? It’s true. Where is all this water? Well, 2/3rds of all that water is INSIDE of your cells (Called the ICF - Intracellular fluid). The other 1/3rd of it is sitting OUTSIDE of your cells (Called the ECF - Extracellular fluid). Within this 1/3 fluid outside of your cells, 1/4 of that (the ECF) is INSIDE of your blood vessels and cardiovascular system (Intravascular fluid). The other 3/4 of the ECF is the fluid surrounding your cells and blood vessels (Intersitial fluid). When we sweat during exercise, we lose the fluid from the ECF, which means we are sweating out fluid from our blood plasma and cardiovascular system!
Why does performance decline when we sweat too much fluid and don’t replace it during exercise? This has everything to do with blood volume and the hydration status of our cells. As stated above, when we sweat, we are literally losing fluid out of our arteries, veins, and hearts. When the cardiovascular system is low on fluids (blood volume), the heart is not as compliant and does not fill with or pump as much blood per beat. This means that for the same work rate and oxygen demand, the heart must beat faster to push the same amount of blood to the working muscles (our stroke volume decreases). The result is that your heart rate increases for the same given work rate as you become dehydrated, and this phenomenon is called “cardiac drift”.
It is important to know that our blood is responsible for “carrying the heat” of the body, and when we exercise we are generating a ton of heat as our metabolisms turn substrates (carbs, fats, pyruvate, lactate, etc.) into energy (ATP). Roughly 20-25% of the energy we produce goes into powering movement, the other 75-80% is lost as heat in the conversion of chemical energy to mechanical energy. The way the body regulates this heat production is by carrying it via the bloodstream, delivering it to the skin, and sweating to place fluid on the skin for evaporation (which dissipates body heat into the environment as a change in matter occurs from liquid to gas). When our blood volumes run low, we have less capacity to sweat and less capacity to thermoregulate (carry heat). This is a double whammy and dehydration can quickly lead you to overheating and lowered performance.
Also, our muscles are 75-80% water, and it can alter their function when they become dehydrated (can happen to any cell for that matter). It is key to know that the “hydration” of a cell is not just about water, but the concentration of water AND solutes within the cell. The dehydration theory of cramping states that a fluid/solute imbalance inside and outside of cells is what causes cellular malfunctioning and cramping. Another perspective behind the dehydration theory of cramping is the rapid fluid imbalance occurring when we dump fluid out of the ECF at a rate quicker than the ICF can equal out the overall concentration. Water is responsible for nerve transduction, thermoregulation, nutrient and waste transportation, lubrication, blood volume and blood pressure regulation, metabolism, and supports enzymatic reactions. When we are dehydrated, all of these functions are downregulated, and our RPE during exercise increases. (exercising becomes harder!)
When it comes to the testing, the most accurate data for race hydration strategy planning will come from high intensity sessions in similar weather conditions, at a duration of 1.0 hour. The duration is significant because high intensity sessions greater than one hour will have significant glycogen loss, which is extra weight lost that is not fluid. In the end, testing in a variety of conditions and intensities will give you a wide range of data to look back on and help inform you how much to drink each ride.
Sweat test procedure: (Do this within 15-25mins before you start your session)
A.) Use the restroom so you don’t have to during your session (this will mess with the results)
1.) Record your initial body weight unclothed
2.) Record the weight of all your drink bottles, packs, bars, and gels etc.
B.) Dont drink or eat anything that you have not weighed!
3.) Execute your training session as normal, drink and eat consistently (every 20-30mins)
C.) If you have to use the restroom during your session, assume a 0.6L fluid loss
4.) Remove your kit and towel off to remove any sweat
5.) Record your post-workout body weight unclothed
6.) Record the weight of all your drink bottles and bar/gel wrappers.
7.) Subtract your final body weight from your initial body weight to determine weight lost
8.) Subtract your final drink/food weight from your initial drink/food weight to determine how much you ate/drank
9.) Add the weight that you ate/drank to the amount of weight you lost. This is your total sweat loss.
10.) Divide your total sweat loss by the duration (in hours) of your session. This is your SWEAT RATE (L/h)
Excel Spreadsheet Sweat Rate Calculators:
Sweat Rate Calculator (Most basic)
Sweat Rate Calculator + Sodium (Includes sodium losses)
Hydration Needs Planner (Use this to plan your hydration strategy for any training session or race and see how much you need to drink to stay within 1% (very safe zone), 2% (safe zone), and 3% (performance still manageable) body weight loss
You can use these calculators above to track and measure your sweat rate. To make things simple, I color-coded them so you only have to enter data into the YELLOW columns to get your data. The planner is designed to be used AFTER you’ve collected some sweat rate data.
The main things to look at using these spreadsheets:
1.) % Body weight loss: Aim to have this number less than 3%. Performance begins to decline around a 2% loss (cardiac drift)
2.) Sweat Rate per hour (L/h): This is an objective number of how much fluid you sweat per hour at that intensity and temperature
3.) % Fluid Replaced: This tells you what percentage of your total fluid loss you are replacing during exercise (a 70% or greater fluid replacement may warrant greater emphasis on sodium replacement)
4.) Temperature: Recording the temperature makes your sweat testing more insightful to inform future hydration decisions in training and races. This gives you the ability to predict what your sweat rate will be at many different temperatures.
5.) Fluids to Drink After Your Ride: After we complete a training session and lose body water, it is essential for health and recovery to replace these fluids. The research proves it is best to drink back 100% to 150% of your total fluid loss. So, if you lost 1L of sweat, drink 1.0-1.5L of fluid after. Drinking more that 100% offsets the further fluid loss you’ll experience as you continue to sweat and urinate after your ride.
For the sheet that includes sodium loss:
5.) 30% Na Replaced: This column states roughly how much sodium per hour you would target if you are drinking back 70% of your total sweat losses. If your “% Fluid Replaced” cell has 70% or more, refer to this column to see your sodium needs.
A note on sweat sodium concentration:
For the calculator that includes sodium losses, you will need to do a sweat sodium test to determine how salty your sweat is using a Precision Sweat Test. Precision Hydration offers medical-grade sweat sodium testing to determine how much sodium you lose in your sweat, expressed as mg/L of sweat. They use a piece of equipment that many hospitals use to test patients for cystic fibrosis, which is a disorder of the sweat glands related to excessive sodium loss in sweat. Individuals’ sweat sodium concentrations are largely genetic and only change by roughly 20% in response to diet (McCubbin, A. 2019). On average, athletes have a sweat sodium concentration of 1000mg/L.
For endurance events, less than 4 hours, excessive sodium replacement is barely beneficial. While the basic recommendation to consume 300-700mg/h of sodium during exercise is practical and beneficial, Alan McCubbin’s research proved that sodium replacement is only necessary for shorter events if you are drinking back 70% or more of your sweat losses (your calculator will tell you if you are doing so). When you are drinking back 70% or more of your fluid losses, you only need to replace 30% of your sodium losses (your calculator will tell you this once you enter your unique sweat sodium concentration). Sodium replacement is also more beneficial towards the later stages of exercise, as your blood sodium concentration actually rises initially as you lose more fluids than you do sodium in the early stages of exercise.
Our blood sodium concentration is very tightly regulated and does not leave a tight range of 135-145mmol/L (~3500mg/L) very easily. If your blood sodium concentration leaves this range, you will experience negative symptoms. Often, establishing a rough plan of how much to drink, and then drinking to thirst is a safe bet. Also consider that over-consuming fluids can be as detrimental as under-consuming fluids, so don’t go too crazy with drinking plain water.
Keep in mind that sodium is the main electrolyte we lose in sweat. Sodium plays a massive role in fluid retention in the body, and the amount of sodium sitting around the cells (in the ECF) dictates how much fluid sticks along with it. This is why you see electrolyte drinks advertise that their product hydrates you better than plain water…in a way, it does by helping keep that water inside the body.
What to do with the data:
Once you have collected some sweat testing data, you now know what your sweat rate is for various intensities, durations, and weather conditions. The way in which you can use this data is by looking ahead at the training on your calendar and planning how much fluid to bring. For example, if your sweat rate has consistently been 1.5L/h when it is 70 degrees and you plan to do a 2 hour ride, you will sweat roughly 3.0L in that session. If you weigh 70 kilograms, that is a 4.3% fluid loss, which would negatively impact your performance. You would need to drink at least 2.3% of your body weight to stay within the 2% weight loss range, which means you would need to drink roughly 1.6L during this 2 hour ride to stay within 2%. I provided a calculator below that does this for you once you know your sweat rate data, where you plug in the duration of your ride, weight, and estimated sweat rate. It will tell you how much you need to drink to stay within a 1%, 2%, and 3% body weight loss.
SUMMARY:
Sweat testing tells you how much fluid you sweat per hour at a given work rate and temperature
Performance can begin to decline after a 2% body weight loss due to cardiac drift and a lowered stroke volume (blood pumped per beat)
Weigh yourself before and after sessions, weigh what you drank before and after your sessions to determine your sweat rate in L/h
Use the hydration planner to get a rough idea of how much you need to drink to stay within 1-3% body weight loss
Track your sweat rate in many different temperatures, weather conditions, and intensities to have a strong database to refer to for future sessions
Sodium losses become more important for events lasting more than 4 hours, or if you are replacing more than 70% of your fluid losses. If you are replacing 70% or more of your fluid losses, you only need to replace ~30% of your sodium losses.
Sweat sodium concentrations are highly individual and tests can be done here
Drinking to thirst is often effective in most cases. Planning can give you a rough idea of how much to drink, but don’t think you MUST drink to an exact plan