Adapted from Matt Fitzgerald
Hydration during running is not as complicated as you may have been led to believe.
When you run, you sweat. The more you sweat, the more your blood volume decreases. The more your blood volume decreases, the harder your heart has to work to deliver oxygen to your working muscles.
Sounds dangerous, but it’s really not. Runners almost never experience dehydration levels sufficient to cause major health consequences. But normal levels of dehydration will make you feel uncomfortable and cause you to slow down.
Drinking while you run will limit these negative effects of dehydration. But what should you drink, how much, and when?
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In the past, athletes were encouraged to drink as much as possible during exercise, or at least to drink enough to completely offset dehydration (that is, to drink enough to prevent any decrease in body weight during exercise). However, it is now understood that this is bad advice, for two reasons.
Firstly, it is possible to drink too much during exercise. Forcing yourself to swallow more fluid than your body really needs while running may cause gastrointestinal distress, and in extreme cases it can cause a dangerous condition known as water intoxication, or hyponatremia. Secondly, research has shown that drinking to completely offset sweating offers no advantage with respect to performance or body temperature regulation compared to drinking by thirst.
The new exercise hydration advice is in fact to drink according to your thirst. As long as you keep an adequate supply of a palatable drink accessible during your runs, you will naturally drink enough to optimize your performance if you just drink as often and as much as your thirst dictates.
Dehydration only affects performance in workouts lasting longer than an hour, so you don’t have to drink during workouts that are shorter than an hour. However, you can if you like.