Hydration & Sweat Rate

by Barry Stokes, USA Triathlon Level II and IRONMAN Certified Triathlon Coach

Hydration is extremely important. The body is approximately 60% water. An athlete who is not fully hydrated is forcing his or her body to work at less than full capacity, especially during physical activity. Dehydration can occur at any time, but it is especially important and beneficial when exercising. Studies have shown that just a small amount of dehydration can have negative effects on athletic performance and may predispose them to heat illnesses. Dehydration occurs when fluid loss (via sweat, urine and through respiration) is greater than fluid intake (via drinking and food).

Sweat rate calculations determine how much fluid an athlete should consume during a training session by calculating the amount of fluid that is expelled through sweat during exercise. Sweat rate is specific to the discipline being tested, so athletes should conduct this test for swimming, biking and running.

To accurately determine the sweat rate of an ahtlete, follow these steps:

1. Athletes should void all urine, then weigh-in wearing little to no clothing, in order to obtain the most accurate reading.
2. Following the weigh-in, an athlete should exercise for at least one hour while keeping track of the quantity of water he or she consumes.
3. After exercise, the athlete should towel off, and step onto the scale again, making sure to wear exactly what was worn before.
4. The athlete’s weight before and after exercise, as well as the amount of fluid that was consumed during the exercise, will be used to determine the athlete’s sweat rate.
5. Subtract the post-exercise weight from the pre-exercise weight in pounds or kilograms, and convert the difference to ounces of fluid loss.
6. Then add to that number the amount of milliliters of fluid that were consumed during the exercise. This will determine how much sweat was lost during exercise.
7. Divide the sweat loss by the duration of the exercise to determine total fluid loss rates per hour during exercise.

*It is best if no food, or semi-solid fueling products are consumed when checking sweat rate, and that the athlete is hydrated prior to conducting the test.

**Athlete should do sweat rate testing in different environments, temperature and humidity conditions, and keep an accurate record of the environmental conditions and related sweat rates in their workout diary. On race day the athlete can reference their sweat rate diary for an accurate measure of how much fluid will be needed during race day conditions.

Sweat Rate Calculator

Here's how to calculate your sweat rate:

1. Empty your bladder and step on the scale. (do this nude or in a swimsuit for accuracy)
2. Record your starting weight:
   Pre-exercise weight=___________ lbs.(A)
3. Record the starting temperature and humidity:
   (for future records and comparison)
   Temperature: ___________ ?
   Humidity: ___________ %
   Dew Point: ___________ ?
4. Do your usual workout. Hydrate like you normally would, and track your consumption.
   Important Note: Hold it! If you go to the bathroom during your workout or prior to your post-workout weigh in, the results will not be accurate.
5. Record the approximate volume of fluid consumed during your workout, as well as the total duration.
   How much you drank=___________ fluid ounces (E)
   Duration of workout: ___________ hours (F)
6. Record the temperature and humidity at the end of the workout. (for your records)
   Temperature: ___________ ?
   Humidity: ___________ %
   Dew Point: ___________ ?
7. Towel dry and then record your weight. (nude or swimsuit)
   Post-exercise weight=___________ lbs.(B)
   Note: If you have long and/or thick hair, blow dry your hair before weigh-in for the most accurate results.
8. Determine the amount of weight you lost during exercise by subtracting your post-exercise weight from your pre-exercise weight.
   Weight Lost=_____lbs.(A)- ______lbs(B)=________lbs.(C)
9. Determine how many fluid ounces of sweat you lost by multiplying the number of pounds you lost by 16.
   _____lbs lost (C)x 16=________ fluid ounces of water you lost (D)
10. To determine hourly fluid replacement needs, add number of fluid ounces you lost (D)to the number of fluid ounces you consumed during the workout (E)and divide by the total duration of the workout (F).
    (_____fluid ounces lost (D)+ ___fluid ounces consumed (E)) ? ___hours (F)
    =_____fluid ounces needed to replace what is lost via sweat each hour

Here is a downloadable worksheet courtesy of Infinit Nutrition:

sweat rate worksheet

Key points to remember:

1. Sweat rates generally increase after 10–14 days of heat exposure, so sweat rate should be calculated following heat acclimatization.
2. Higher sweat rates are generally found in men and those that are highly fit.
3. When first beginning an exercise routine in heat, your body loses more sodium through sweating, so slightly increase the amount of sodium in your diet until you’ve become adapted (after 10–14 days).

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