From the BarryS Coaching Training Library

Hydration & Sweat Rate

by Barry Stokes

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

This works best if converted into kilograms (kg) and milliliters (mL)

  1. Body Weight pre-exercise ___________________  [lb/2.2&= kg]
  2. Body Weight post exercise  ___________________  [lb/2.2&= kg] (A-B)
  3. Change in Body Weight  ___________________  grams [kgx1000=g]
  4. Volume of fluid consumed ___________________  mL [ozx30=mL]
  5. Sweat Loss ___________________  mL [ozx30=mL] (C+D)
  6. Exercise time  ___________________  [min or hr]
  7. Sweat Rate ___________________  [mL/min or mL/hr] (E/F)

The final number (G) is your sweat rate, or the amount of fluid that you lose through sweat during a specific amount of exercise (usually expressed at liters per hour). This should help you determine the amount of fluid you should be drinking during and after your workouts.

Key points to remember:

  • Sweat rates generally increase after 10–14 days of heat exposure, so sweat rate should be calculated following heat acclimatization.
  • Higher sweat rates are generally found in men and those that are highly fit.
  • 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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