Sweat Testing: How to Personalize Your Hydration Plan
Hydration is critical for daily function and athletic performance. In this post, we’re going to focus on the performance piece and discuss how to plan out your hydration strategy based on how much sweat you lose. For a refresher on general hydration needs for daily life, check out the Hydration 101 for Athletes blog post.
The Science of Sweat
First, a brief physiology lesson. When we exercise, our core temperature rises. Central and skin thermoreceptors sense this shift and transmit that information to a specific area of the brain called the hypothalamus. The hypothalamus essentially helps to act as your body’s thermostat, triggering the sympathetic nervous system to activate your sweat glands. Sweat is released through sweat glands onto your skin surface, which facilitates heat dissipation through evaporative cooling to help lower your temperature and keep you cool (1).
Sweat consists of fluid and electrolytes. The main electrolytes lost in sweat are sodium and chloride (NaCl); however, we also lose potassium, calcium, magnesium, and other trace minerals in sweat too. We’ll focus on the fluid component in this post and save electrolytes for the future.
There are many factors that influence how much you sweat, including your biological sex, genetics, environmental conditions (e.g., heat and humidity), exercise intensity and duration, heat acclimatization, and training status. With this in mind, it’s easy to understand why hydration goals should be individualized.
Planning Your Hydration Strategy
There are two main schools of thought on hydration strategies. One school of thought is to drink ad libitum, i.e. simply drink when you feel thirsty. This can work well in certain circumstances, such as during shorter duration activities in mild weather conditions, and for athletes who start activity well hydrated and are in touch with their thirst cues. The other school of thought is to engage in scheduled or programmed drinking, which is a more structured approach. For example, a runner might plan to drink 4-8 ounces of fluid every ~15-20 minutes or have a general goal of 10-27 ounces per hour and try to adhere to the strategy regardless of whether they feel thirsty or not. This can work well for longer duration activities or stage races, for athletes who may forget to hydrate during activity, and for athletes who are exercising in hot and humid conditions.
For those who are interested in taking a programmed/scheduled drinking approach, the best way to start is to determine your sweat rate. Sweat rate is the volume of fluid lost through your skin’s sweat glands over a given period of time, typically expressed in liters per hour (liters/hr). Knowing this number allows your fluid replacement strategy to be more personalized.
Calculating Your Sweat Rate
You can easily calculate your sweat rate at home using a scale and some simple math. Here’s a simple step-by-step guide:
Weigh yourself (nude) right before a training session.
Perform your training session and write down exactly how much fluid you drank during the session, if any. Avoid using the bathroom during and immediately after your session if possible.
After your session, towel yourself dry to remove excess sweat. Then, weigh yourself again (nude).
Enter your data into the equations below to calculate your sweat rate by subtracting your post-exercise weight from your pre-exercise weight, then dividing by the time spent training. You can use liters and kilograms or pounds and ounces depending on your preference.
Metric System (liters per hour):
Calculate weight lost during training:
Weight lost = pre-exercise weight (kilograms) – post-exercise weight (kilograms)
Convert weight lost (kilograms) to sweat lost (liters). One kilogram of body weight lost equates to 1 liter of sweat lost.
1 kilogram of weight lost = 1 liter of fluid (sweat) lost
To calculate total sweat lost, add any fluid consumed during training.
Total sweat lost = sweat lost (liters) + fluid consumed (liters)
Divide total sweat lost by training duration (in hours) to determine sweat rate (liters/hour).
Sweat rate = total sweat lost (liters) / training duration (hour)
Imperial System (ounces per hour)
Calculate weight lost during training:
Weight lost = pre-exercise weight (pounds) – post-exercise weight (pounds)
Convert weight lost (pounds) to sweat lost (ounces). One pound of body weight lost equates to 16 ounces of sweat lost.
Sweat lost (ounces) = weight lost (pounds) x 16
To calculate total sweat lost, add any fluid consumed during training.
Total sweat lost = sweat lost (ounces) + fluid consumed (ounces)
Divide total sweat lost by training duration (in hours) to determine sweat rate (ounces/hour).
Sweat rate = total sweat lost (ounces) / training duration (hour)
Worked Example
An athlete weighs themself before a 90-minute run and they weigh 180 pounds. They drink 12 ounces of fluid during the run. Afterward, they towel off and weigh themself again, measuring 177 pounds. They did not urinate during the run or any time between the pre- and post-exercise weights.
Calculate weight lost during training:
Weight lost = pre-exercise weight (lbs) – post-exercise weight (lbs)
Weight lost = 180 lb - 177 lb = 3 lb
Convert weight lost (pounds) to sweat lost (ounces). One pound of body weight lost equates to 16 ounces of sweat lost.
Sweat lost (ounces) = weight lost (lbs) x 16
Sweat lost (ounces) = 3 x 16 = 48 ounces
To calculate total sweat lost, add any fluid consumed during training.
Total sweat lost = sweat lost (ounces) + fluid consumed (ounces)
Total sweat lost = 48 + 12 = 60 ounces
Divide total sweat lost by training duration (in hours) to determine sweat rate (ounces/hour).
Sweat rate = total sweat lost (ounces) / training duration (hours)
Sweat rate = 60 ounces / 1.5 hrs = 40 ounces per hour
Tips to Simplify Sweat Rate Testing
Try to urinate soon before taking your pre-exercise weight, then wait until after your post-exercise weight to urinate again so that you don’t need to account for urinary losses in your calculations.
Exercise for one hour to make your calculation more straightforward.
Don’t drink anything during your one-hour training session to reduce the complexity of the calculation.
What If You’re Uncomfortable Using The Scale
If you don’t feel comfortable weighing yourself, you can have a friend or family member take a “blind weight” for you. Step on the scale, close your eyes, and have the other person record your pre- and post-exercise weights. The other person can record your data and calculate your sweat rate for you without sharing either of your weight measurements.
If you don’t have access to a scale, you can either drink to thirst, or consider starting with drinking ~4-8 ounces of fluid every ~15-20 minutes and making adjustments as needed.
Interpreting and Using Your Sweat Rate Results
Research indicates that a “typical” sweat rate tends to fall between 0.5 and 2 L/hr; however, there can be extensive variation (2). For illustrative purposes, we can say that an average sweat rate is roughly 1 L/hr. Your sweat rate can vary significantly based on the intensity and duration of your training session, environmental conditions, clothing choices, etc. For this reason, you should test your sweat rate across a range of conditions and training sessions if possible, and write down notes about the conditions under which the testing occurs (e.g. temperature, humidity, etc.).
Either before or while you’re testing your sweat rate, I recommend recording how much you’re drinking across a variety of training sessions. This will give you an idea of your baseline fluid intake, which can be helpful in assessing how to make adjustments based on your sweat rate test results.
Once you have calculated your sweat rate, you can come up with your individualized fluid replacement strategy. We do not need to replace our fluid losses in a 1:1 manner. Replacing 100% of your fluid losses often requires drinking beyond your body’s natural thirst instinct and may increase the risk of exercise-associated hyponatremia, or low blood sodium level, which can be dangerous.
There’s not necessarily a formal consensus on how much fluid to replace during exercise. The American College of Sports Medicine (ACSM) recommends drinking enough to avoid losing more than 2% of your body weight during exercise (3). However, other research has challenged that, citing that high performing marathoners, ultra marathoners, and triathletes have been shown to lose more than 3-4% body weight during competition without ill effects (4,5). While there are certainly people that can withstand greater levels of dehydration, it is probably best to err on the side of caution and try to minimize losses to less than about 2% body weight, especially during longer duration activities (e.g. ultra distance events). We can use that goal, as well as your sweat rate data, to determine approximately how much fluid you should consider replacing during exercise, noting that there is not necessarily a static target to aim for across all training sessions. In general, the longer the event, the greater the replacement of fluids will need to be to minimize dehydration.
For example, if we have a 70-kilogram athlete whose sweat rate is about 1 liter/hour and our goal is for them to finish training with less than a 2% body mass loss, a tiered hydration strategy based on training duration might roughly look like this:
Sessions lasting less than 90 minutes - The amount of fluid that needs to be replaced to prevent dehydration is likely to be minimal to fairly low. For sessions less than 90 minutes, drinking to thirst is likely adequate for many athletes.
Sessions lasting ~2 hours - The athlete might aim to replace ~25-30% of hourly sweat losses.
Sessions lasting ~3-4 hours - The athlete might aim to replace ~50-60% of hourly sweat losses.
Sessions lasting ~5-6 hours - The athlete might aim to replace 75% of hourly sweat losses.
The examples above are adapted from a framework shared by Alan McCubbin, PhD, Accredited Sports Dietitian and researcher at Monash University, during a Q&A with Fuelin (watch it here). Dr. McCubbin's research focuses on sodium and hydration requirements in athletes.
Sweat Rate Calculations Made Easy
If you’re confused or just looking to make things easier, you can use the sweat rate calculator below. Enter your pre- and post-exercise weights, how much fluid you consumed, and the session duration in the appropriate boxes. There are options for the metric (liters/hour) and imperial systems (ounces/hour) depending on your preference. Once you’ve entered your data, the calculator should share your sweat rate, as well as general fluid replacement goals to consider trialing for training sessions/events up to ~10 hours in length.
Note that this calculator and the suggested fluid replacement targets are intended as a starting point only and do not replace individualized guidance from a sports dietitian or other healthcare provider. The calculator assumes a constant sweat rate across all session durations, which may not reflect real-world conditions, particularly for longer efforts when sweat rate may change throughout an effort. Individual fluid needs vary based on sweat rate, environmental conditions, exercise intensity, and other factors. If you have a medical condition that affects fluid or electrolyte balance, consult your healthcare provider before making changes to your hydration plan.
Considerations
While it’s great to know your average sweat rate, there are additional factors that can impact your hydration plan. Here are some things to consider:
Our access to fluid and ability to carry a certain amount of fluid can play a role (often, a limiting role) in our hydration plan. Consider if you’ll be using a bladder, bottles, stream access with a filter, aid stations, etc., and how that may impact how much fluid you can take in.
Athletes with a high sweat rate (greater than about 1.5 liters/hour) will have a harder time replacing a large amount of their sweat losses simply due to the volume of fluid required to do so and the risk of gastrointestinal upset with higher fluid intake. These athletes will likely be limited by the rate of gastric emptying and therefore may not be able to replace as much of their losses as would be suggested by the sweat rate calculator and generated fluid targets.
Sweat rates vary with intensity, heat, fatigue, acclimatization status and more. For this reason, it’s ideal to test your sweat rate multiple times across a variety of conditions so that you can either generate an average or at least understand how to make adjustments for certain types of conditions, like a really hot and humid race day.
In this post, we’ve exclusively addressed fluid losses and have not mentioned electrolytes. In reality, fluid and electrolyte intake during exercise should be coordinated. We will discuss electrolytes in more detail in a future blog post.
Calculating your sweat rate is a great first step and applying those numbers is likely to take some experimentation. If you want guidance on testing your sweat rate and analyzing your numbers to build a customized hydration plan, book a 1:1 appointment.
References
Baker LB. Physiology of sweat gland function: The roles of sweating and sweat composition in human health. Temperature (Austin). 2019;6(3):211-259. Published 2019 Jul 17. doi:10.1080/23328940.2019.1632145
Smith JW, Bello ML, Price FG. A Case-Series Observation of Sweat Rate Variability in Endurance-Trained Athletes. Nutrients. 2021;13(6):1807. Published 2021 May 26. doi:10.3390/nu13061807
American College of Sports Medicine, Sawka MN, Burke LM, et al. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007;39(2):377-390. doi:10.1249/mss.0b013e31802ca597
Zouhal H, Groussard C, Minter G, et al. Inverse relationship between percentage body weight change and finishing time in 643 forty-two-kilometre marathon runners. Br J Sports Med. 2011;45(14):1101-1105. doi:10.1136/bjsm.2010.074641
Beis LY, Wright-Whyte M, Fudge B, Noakes T, Pitsiladis YP. Drinking behaviors of elite male runners during marathon competition. Clin J Sport Med. 2012;22(3):254-261. doi:10.1097/JSM.0b013e31824a55d7
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