Sweat rate

Sweat rate is the amount of fluid a person loses as sweat per hour of exercise.^{1Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 503.} Two clients doing the same session under the same conditions can need different amounts of fluid because their sweat rates differ.^{1Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 503.} Observed losses in athletes range from 0.4 to 1.8 liters per hour.^{2USA Triathlon, Complete Triathlon Guide, p. 323.}

To estimate sweat loss in liters, subtract post-exercise body weight from pre-exercise body weight, add fluid drunk and subtract urine passed, counting 1 kg as about 1 L.^{3Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 505.} Divide by exercise time in hours to get liters per hour.^{3Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 505.}

In practice

• Weigh in minimal clothing after drying off and urinating, and remove sweaty clothes.^{4Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 212.} Each pound lost is about 16 ounces (0.5 L) of fluid; losing 2% or more of body weight means the client isn’t replacing enough.^{4Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 212.}
• Reassess periodically under the conditions that matter: intensity, hydration status, environment, heat acclimatization and clothing all affect sweat rate.^{1Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 503.},^{3Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 505.}
• People who drink at will replace only about two-thirds of what they sweat, and thirst is unreliable during heavy sweating.^{5Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 129.}
• Explain to clients pursuing weight loss that an acute drop on the scale after exercise is water loss.^{5Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 129.}

How much fluid to replace

For each pound lost, ACSM recommends 0.5 L (1 pint).^{1Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 503.} The NSCA’s personal training text recommends 20 to 24 ounces (about 600 to 700 mL).^{5Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 129.} Its strength and conditioning text gives about 24 ounces (0.7 L) when dehydration is significant or the next session is under 12 hours away.^{6Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 214.}