Dehydration

Dehydration is a loss of body water, usually defined in exercise as losing water equal to 2% or more of body weight.^{1International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 506.} Sweating causes large losses of water and electrolytes; a client dehydrates if those losses aren’t replaced.^{1International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 506.} Even a 2 to 3% loss can raise core temperature and reduce strength, power, muscular endurance, accuracy and motivation.^{2Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 212.} Dehydration also lowers stroke volume, blood pressure and blood flow to muscle, and raises heatstroke risk.^{2Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 212.} It impairs endurance in any climate, especially in the heat.^{3Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 503.}

In practice

• Weigh clients before and after a session, after they’ve urinated and dried off, in minimal clothing; a loss of 2% or more means they aren’t replacing what they sweat.^{2Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 212.}
• Don’t rely on thirst when sweat losses are high.^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 129.} Drinking at will, most people replace only about two-thirds of the water they sweat off.^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 129.}
• A post-workout drop on the scale is water loss, even if a client trying to lose weight reads it as fat loss.^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 129.} Replace it, ideally with sodium-rich food or an electrolyte drink.^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 129.}
• Before exercise, about 5 to 7 ml/kg of fluid at least 4 hours ahead is a reasonable start.^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 129.}

Where sources differ

ACSM recommends drinking 0.5 L for each pound lost, while NSCA’s personal training text gives 600 to 700 ml per pound.^{3Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 503.},^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 129.} ACSM treats darker first-morning urine as a sign of dehydration.^{3Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 503.} NSCA’s strength and conditioning text calls that link very subjective and notes that some foods and medications change urine color.^{2Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 212.}