How Much You Really Need to Drink When Exercising

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YOU’VE HEARD THE WHOLE “8 glasses of water per day” advice, but what about when part of that day is spent exercising … in the heat … under the blazing summer sun?

Well, first of all, it’s worth realizing that the common hydration recommendation of 8 glasses of water per day (8 ounces each) – totaling 64 daily ounces – is actually slightly off. After all, the National Academy of Medicine recommends that women get 2.7 liters (about 91 ounces) of water per day and men get 3.7 liters (about 125 ounces). However, as Joy Dubost, a New York City-based board-certified sports dietitian, notes, about 20 percent of that should actually come from foods. That puts baseline beverage consumption at about 73 ounces for women and 100 ounces for men.

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“We lose fluids during exercise primarily through evaporation via sweat and through respiration,” says board-certified sports dietitian Kelly Pritchett, assistant professor in nutrition and exercise science at Central Washington University and a spokesperson for the Academy of Nutrition and Dietetics. She explains that the average person loses about 17 to 50 ounces per hour when exercising.

That’s a wide range. And your exact fluid losses depend on myriad factors, from your body size and personal sweating style (a 2014 PLoS ONE study suggests that seasoned athletes sweat more than their sedentary counterparts) to your workout type and intensity to the heat, humidity and even elevation of the city where you’re exercising. For example, in one study of National Basketball Association athletes, players lost between 33.8 and 155.5 ounces of sweat during the course of a 40-minute game in which the average playing time was 21 minutes. Again, a huge range.

Obviously, the goal during exercise is to replace any fluids you’re losing. And the National Athletic Trainers’ Association warns that losing 2 percent or more of your body weight in fluids during the course of a workout negatively affects both performance and health. So, for a person who weighs 180 pounds, that would equal 3.6 pounds lost.

“As we become dehydrated, blood plasma becomes more viscous, or thicker, and we may observe an increase in heart rate and in blood pressure,” Pritchett says. A University of North Carolina at Chapel Hill review of the literature on dehydration explains that even mild dehydration can affect physical and cognitive performance as well as gastrointestinal, kidney and heart function. Dubost also notes that dehydration can lead to weakness, muscle cramps, dizziness, confusion and sluggishness.

“With fluid consumption we may observe some benefits including a lower heart rate, higher cardiac output [the amount of blood pumped through the heart per minute], increased blood flow to the skin and lower core temp, which in turn may decrease our perception of effort and improve exercise performance,” Pritchett explains.

Unfortunately, thirst isn’t a very accurate way of gauging your hydration status. In one International Journal of Sport Nutrition and Exercise Metabolism study, drinking according to thirst didn’t adequately hydrate soccer players, and NATA states that when athletes drink to thirst, they only replenish about two-thirds of everything they sweat out.

Your Personalized Hydration Plan

There really are no hydration recommendations that work for every athlete and every workout. However, by taking some baseline recommendations – and tweaking things from there – you can zero in on the right hydration plan for you.

So what’s the baseline? According to the American Council on Exercise, exercisers should consume 17 to 20 ounces of fluid 2 to 3 hours before exercise and another 8 ounces 20 to 30 minutes before starting their workout. Then, during exercise, 7 to 10 ounces every 10 to 20 minutes. And, finally, 30 minutes following exercise, 8 last ounces.

However, ACE also recommends consuming even more depending on weight lost during your workout – and that’s where personalization really comes into play. After all, since the goal is to replace any fluid lost – and definitely not lose 2 percent or more of your body weight from fluids – during exercise, weighing yourself both before and after a workout with an empty bladder (ideally naked, since sweat-drenched clothing can weigh down the scale) can help tell you if you need to further increase fluid intake, Pritchett says. Again, for a person who weighs 180 pounds, the goal is not to lose more than 3.6 pounds during any given workout.

According to the textbook “Essentials of Strength Training and Conditioning,” each pound of fluid lost during exercise represents 16 ounces of fluid. But you may need to drink up to 150 percent of ounces lost to effectively replace fluid losses after exercise, per NATA. For every pound lost during exercise, Dubost recommends drinking 16 to 24 ounces

What’s more, if you lose weight during a workout, it’s your cue to increase how much you drink during your next one. Keep adjusting your fluid intake until you neither gain nor lose any water intake during your workouts. When you do that, you can rest assured that you’ve found the right totals for you.

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