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What should you know about hydration when exercising?

What happens when we exercise?

When we exercise, our muscles produce heat that needs to be evacuated to maintain body temperature.

Water acts as a carrier of heat through blood, and as a refrigerant by removing excess heat via sweat evaporation on the skin.¹

Observations of sweat rates in various sports have shown significant variation in mean sweat rates (from 0.29 to 2.37 L/h), depending on the type and intensity (training or competition) of the activity.

For example, during one hour in warm weather, a trained male adult loses approximately²:

  • Swimming: 0.4 L
  • Soccer: 1.5 L
  • Tennis: 1.6 L (during a competition)
  • Cross-country running: 1.8 L

Figures for a female adult will be slightly lower.

Staying well hydrated: before, during and after exercise

The amount of sweat produced increases with the intensity of the exercise, but also with the temperature and humidity of the ambient environment.

Physical activity therefore results in increased water requirements that parallel sweat losses. If these hydration needs for exercise are not met, the body can enter a state of dehydration.

Dehydration during exercise is recognized as having a detrimental effect. Indeed, dehydration has been shown to increase both heart rate and body temperature³. For example, when plasma volume is reduced, the heart has to work faster to maintain delivery of oxygen and nutrients to the muscles.

Numerous studies, reviewed in the American College of Sports Medicine (ACSM)², show that dehydration increases physiologic strain and perceived effort to perform the same exercise task, and that warm-hot weather increases this phenomenon. The ACSM also considers that dehydration above 2% of the body mass can degrade aerobic exercise performance, and therefore endurance performance.

This is why people are advised not only to ensure proper hydration during exercise, but also before and after, in order to match the water losses, without waiting for thirst to appear². For exercise lasting less than one hour, water is sufficient to cover the body’s needs¹.

The American Academy of Pediatrics reminds parents and caregivers that “Water is generally the appropriate first choice for hydration before, during, and after most exercise regimens”⁵.

Understand links between hydration & obesity