How to Do Your Own Sweat Testing

When it comes to race nutrition, there are two key elements that will determine your race outcome: carbohydrate capacity and hydration plan. We will focus here on hydration, and what you can do to develop an individualized hydration plan for your next race. Your plan will depend on a few key elements:

• Defining and understanding sweat rate
• Calculating your sweat rate
• Understanding how to minimize dehydration
• Being able to execute an effective hydration plan in a race situation

Defining & understanding sweat rate

For many athletes, the first step to developing a hydration plan is understanding their sweat rate and how much they’re losing during a race. Yet, those same athletes don’t know where to start and often ask me, “Where should I go to get a sweat rate test?” To each of them, I respond with the same answer: “At home!”

One of the biggest misconceptions in performance nutrition and sport is that you need special equipment or an expensive lab environment to determine your base-level sweat rate.

By definition, your sweat rate is the amount of sweat you lose during exercise for a given period of time—usually, one hour—denoted as fl oz/hr or L/hr. The sweat rate does not involve sodium or electrolytes lost during the activity, merely the volume of bodily fluids lost. Through repeated tests in varying environmental conditions and intensities, you can determine your sweat rate range across swim, bike, and run. You can then use these fluid loss estimates to determine a target rate of fluid consumption per hour in training and racing.

RELATED: How Sweat and Hydration Needs Differ for Men and Women

Calculating your sweat rate

Your sweat rate is not static and will change depending on key environmental factors, the type of activity you’re doing, and the intensity level. I recommend performing multiple sweat tests across a variety of training sessions (45 minutes or longer) and in different climatic conditions in order to determine your sweat rate range.

Follow these steps to calculating sweat rate:

1. Record weight before session (without clothes)
2. Record weight after session (without clothes, towel dry)
3. Record duration of session
4. Record temperature of session
5. Record type of session, e.g., bike or run
6. Record intensity of session, e.g., zone 2 or zone 4
7. Record total fluid consumed during session (fluid consumed during the session is sweat loss that you are replacing concurrently)
8. Make a note of how many times you urinate (if any) during the session*
9. Subtract end weight from starting weight, add fluids consumed, then divide by 60 to get fluid rate loss per hour. This is your sweat rate.
10. Subtract end weight from starting weight & divide by starting weight to calculate % body weight loss
11. Record average and peak sweat rates from all sessions you do this for.

*Assume 200-300ml per urination and add to ending body weight before calculating sweat loss.

The Fuelin app also calculates sweat rate for you or you can head to fuelin.com for a sweat rate worksheet. 

I recommend that athletes aim to replace about 70% of sweat losses during sessions. Therefore, to calculate your initial fluid requirements for racing based on your sweat loss, you can multiply the average hourly loss of fluids by 0.7.

Pro tip: Each pound of body weight lost is equivalent to 16 ounces of sweat loss, and similarly for those that prefer metric, one-kilogram weight loss is equal to 1000 ml of sweat.

Understanding how to minimize dehydration

Finishing a race slightly dehydrated is generally not a major issue; however, the level of dehydration needs to be managed. Bodyweight loss of approximately 2-3% at the end of a long endurance activity is acceptable, whether this is a race or training scenario.

You should also be aware of the dangers of overconsuming fluids and the resulting condition of exercise-associated hyponatremia (EAH). This can be far more dangerous than dehydration. Hyponatremia can occur when fluids are overconsumed and an electrolyte imbalance occurs, due to low sodium in the blood (below 135mEq/L), and the body becomes hyperhydrated. Cellular swelling results and can be especially dangerous, even leading to death.

In order to fine-tune your understanding of your sweat rate, you can repeat testing on multiple occasions for each discipline. Record and collate data points until you see a clear pattern in body weight loss and fluids required to minimize dehydration without overdoing it. The ideal amount of body weight loss is often reported as <2%; this is based on clinical studies rather than field testing. It is possible to race and lose more than 2% and still perform at your best. Typically I recommend a loss no greater than 3%, although some specialists and coaches may set their limits lower depending on practical experience and others may find it is acceptable to have a higher loss and still perform well. This is an individual variable and one that will only become clear with repeated testing.

Despite what you find yourself comfortable with losing during training and racing, understanding sweat losses is also important for recovery. The general rule is to replace at least 150-200% of the body weight lost over the 12 hours post-session in order to fully recover. The period of time to replace this amount may be reduced if a second session is planned for the day or timing before the following day does not allow for it.

RELATED: Dehydration Makes Any Effort Feel Harder

Executing an effective hydration plan

A key factor in managing your training and race hydration plan is recognizing that flexibility is required. While the repeated collection of data in multiple sessions, across a range of varied temperatures and intensities, is important to gain an insight into your fluid loss and hydration requirements, this information is not absolute and should be used to guide your target range for consumption. Once you have a target range of fluids required, you are able to plan the amount you need to plan on consuming, whether a bottle should be carried on the run, or the number of cups to drink at aid stations on the run.

Thirst is still a very important element in effectively managing hydration status and should also be utilized when considering fluid intake. As noted, there is often a far greater risk of over- than under-consuming fluids. Appreciating what you’re experiencing “real-time,” along with understanding your historical hydration needs, is key to appropriate hydration management. Listen to your body, acknowledge thirst, consider your experience from multiple sweat rate tests, and execute your hydration plan accordingly. Above all, be flexible and not rigid in the amounts to be consumed. There will be days when you require more and days when you require less. Calculating, understanding, and establishing your target range is a great place to start.

RELATED: Everything You Ever Need to Know About Electrolytes

Scott Tindal is a nutrition coach, with 20+ years of experience in professional sports. He sits on an Ironman advisory board as a performance nutrition expert, and holds an MSc in Sports and Exercise Medicine from the Queen Mary University of London. He is also the founder of Fuelin, a personalized nutrition program that syncs directly with Training Peaks and Today’s Plan.