Staying Hydrated during Exercise


Summary of ACSM Position stand for fluid intake and exercise

George Cho, MFSc, CEP, CSCS

Maintaining hydration is crucial to perform optimally during exercise and to avoid unwanted health consequences. The American College of Sports Medicine has a position paper that outlines general guidelines for physically active individuals regarding adequate hydration for exercise. The summary is below: 


How does dehydration influence performance?

Dehydration of >2% of body weight decreases aerobic performance in warm and hot environments. The physiological reasons for this is likely because dehydration raises core body temperature, increases cardiovascular strain, alters the central nervous system, and increases glycogen utilization. However, dehydration of 3-5% of body weight does not seem to negatively impact muscular strength and anaerobic performance.


What activities lead to the most fluid loss?

Ranking of a selection of activities from fastest sweat rate to slower sweat rates

  1. Tennis (cramp-prone males): 1.79 - 3.41 L/h
  2. Squash: 1.49 - 3.25 L/h
  3. American football: 1.1 - 3.18 L/h
  4. Summer rowing: 0.88 - 2.92 L/h
  5. Tennis (males): 0.62 - 2.58 L/h
  6. Cross-country running: 0.99 - 2.55 L/h
  7. Basketball: 1.23 - 1.97 L/h
  8. Soccer: 0.99 - 1.93 L/h


Should I over-hydrate (hyperhydration) to prevent dehydration?

Hyperhydration may help delay dehydration but there are two things worth pointing out. First, large volumes of fluids may cause hyponatremia (diluting blood sodium levels), a potentially dangerous situation. Also, drinking large volumes of fluids will stimulate increased urine production bringing the body back to normal hydration within a several hours. So, hyperhydration may help, but timing the intake of fluids would be important.



What are negative health affects of dehydration?

  • Heat exhaustion and heat stroke
  • Muscle cramps are associated with dehydration, electrolyte deficits, and muscle fatigue
  • Rhabdomyolysis refers to release of skeletal muscle contents. It can also lead to renal failure.
  • Hyponatremia refers to a drop in the blood’s sodium concentration. When this concentration drops below 125 mmol/L, it results in vomiting, headache, swollen hands and feet, restlessness, fatigue, confusion and disorientation, and wheezing. Further decrease in the concentration to <120 mmol/L increases the chance of cerebral edema, seizure, coma, brain stem herniation, respiratory arrest and death.


Are there sex and age differences in hydration needs?

Women generally have lower sweat rates and electrolyte losses compared to men, likely due to a smaller body size and lower metabolic rates when performing similar activities. However, in events like marathons, it seems women are at greater risk for symptomatic hyponatremia. The cause for this is unclear. 

Older adults (>65 years old) need to especially be cognizant of the risk of dehydration. Older adults are more susceptible to becoming dehydrated. They are slower at restoring body fluid homeostasis and making the situation worse is that older adults have a blunting of their thirst response. It should also be noted that older adults excrete fluids slower than younger persons. This means that with rehydration, sodium retention increases meaning potential increases in blood pressure. The slower responses to water or saline loads and dehydration is likely attributable to the fall in the number of nephrons in the kidneys which lowers the organ's filtration rate. 


Is diet important for rehydration?

Yes, meal consumption is critical  for day-to-day hydration. Eating foods promotes fluid intake and retention. Food intake will also help replace electrolytes. It is unlikely that caffeine consumption during exercise will cause dehydration during exercise. However, since caffeine is a diuretic, it should be consumed in moderation, especially after the exercise period where rehydration is crucial. 


How much fluid should I take in?

Before exercise
Slowly take in ~5-7 ml/kg of body weight at least 4 hrs before exercise.

  • 130 lb individual: 295 ml - 413 ml
  • 150 lb individual: 340 ml - 477 ml
  • 170 lb individual: 385 ml - 539 ml 

If the urine is dark or highly concentrated, then slowly drink more (~3-5 ml/kg of body weight about 2 hrs before the exercise.

Taking fluids with glycerol solutions will increase chances that an individual will need to urinate during exercise.


During exercise
This would be vary depending on the exercise, the individual and the environment. However, a broad, generalized recommendation for marathon runners would be:

  • 400 - 800 ml/h.
    (Note: Roughly speaking, that would mean aiming to finish the equivalent of a typical water bottle in 1 hr)

Weighing oneself during training may help individuals ascertain how much fluid they typically lose and give them a rough estimate of how much fluid to take in. 


After exercise
Consuming a meal and water or other fluids with sodium makes for an excellent rehydration strategy. Rehydration would be more aggressive if the time until the next workout is short (<12 hours) or severe dehydration has taken place. A general rule of thumb would be:

  • drink ~1.5 L of fluid for each kilogram of body weight that was lost. 


Are carbohydrates during exercise helpful?

If an aerobic type exercise session is greater than an hour long, ingesting about 30 - 60 g/h of carbohydrate has been demonstrated to maintain blood glucose levels and sustain exercise performance. But in a beverage designed to replace fluid and provide carbohydrate, the concentration of carbs should not exceed 8%. More than 8% would slow the rate at which the stomach will empty its contents. This would not be beneficial to an exerciser.

Sawka, MN et al. American College of Sports Medicine Position Stand. Exercise and Fluid Replacement. Med Sci Sports Exerc. 2007 Feb; 39(2): 377-90

Click here for the full text of the position paper

George Cho