Proper hydration for runners and endurance athletes

James Winger, MD Assistant Professor of Family Medicine Loyola University Chicago Stritch School of Medicine

Hydration is important during exercise to replenish fluids lost by sweating. Humans are the animal world’s most accomplished sweaters. We manage increased body heat by excreting water (along with very little salt) onto our skins so it may evaporate, taking heat with it. Sweat rate is an adaptation to training and well trained athletes may lose greater than one liter of fluid per hour, depending also on environmental conditions.

Proper rehydration addresses not only replenishment of lost fluid, but can help you avoid performance decrease as well as overhydration-linked illness.


Do drink when you are thirsty

Drinking according to thirst is the only way to avoid both performance deficit as well as to minimize your risk of overhydration illness. When you feel yourself getting thirsty, it’s time to drink some water. Simple enough.

Do use fluid intake as a way to fuel

After approximately one hour of endurance exercise, it is recommended to add carbohydrates to one’s intake to replenish fuel stores. Using a commercially available sports drink, like Gatorade or Powerade, is a good choice, as it provides fluid as well as carbohydrates.

Do practice hydration strategies during training

Whether it is drinking from a disposable cup or handheld bottle while running, or a mounted or free bottle while cycling, at times the mechanics of in-race hydration require practice. Your race experience will be better if you have tried these things before.

Do know that dehydration is normal

Don’t be afraid of dehydration. Dehydration is a normal part of the physiology of exercise. Weight loss is to be expected during a running race and does not lead to performance decrement. The most dehydrated runners in a race are typically the fastest, including the winners. Weight loss of 7% is common in marathons and ultramarathons.


Do not try to match your sweat rate with fluid intake

Most runners given free access to fluid will replete about 50% of what they sweat out. This is due to dehydration being a normal and harmless part of exercise. Due to the metabolic burning of fuel, during a marathon weight loss of 2-3% may indicate normal hydration; thus 0% weight loss would indicate overhydration.

Do not believe that dehydration leads to heat stroke

Laboratory based studies indicate that dehydration leads to a minimal increase in core body temperature versus fully hydrated states. Heat stroke is primarily a metabolic condition in which the body generates excess amounts of heat.

Do not seek to replenish electrolytes, in most cases

Although sweat is salty, athletes lose very little salt in comparison to our bodies’ stores. Supplementation of electrolytes, as is possible with commercially available caplets or tablets, has never been shown to improve performance, nor avoid illness. Replenishment of electrolytes is best done through a normal diet after exercise.

Do not prehydrate

There is no utility in increasing fluid intake for days prior to an event; as this will only increase urination rate. In health, the body’s physiology is finely tuned to maintain blood salt concentrations by altering water and salt intake.

Jumping cartoon

The effects of mild dehydration have been long overstated. Winners of major running races and triathlons are commonly 5 percent dehydrated, with no apparent cost to their performance. Overhydration may be dangerous in the form of exercise-associated hyponatremia, a rare but potentially fatal condition. Your body is finely tuned to obtain more fluid when needed, so drink when you are thirsty.

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James Winger, MDAssistant Professor of Family Medicine

Family Medicine and Primary Care Sports Medicine specialist. ...

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