Staying Hydrated When Exercising

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How much water to drink during your workout? Find out what the sports nutritionist says

By John Hoeber, MS, RD, CSSD, CPT

Hydration has the biggest effect on performance in endurance events. Water loss influences your race, run, or ride in two ways. First, your blood volume decreases, causing your heart rate to increase. Second, your sweat rate decreases, causing an increase in core temperature. Knowing exactly how much you sweat per hour helps you plan a strategy to prevent water loss.

Losing more than 2% of your body weight in fluid, if not replaced while exercising, will negatively affect your endurance performance. For the 170-pound athlete, a decrease of 2% of body weight is 2.4 pounds. Because sweat rates can range from 0.66 to 5.5 pounds of sweat per hour, staying hydrated can be very difficult without some advance planning.

To calculate your sweat rate, weigh yourself before a workout, including any water bottles you may be drinking on the way, and then weigh yourself when you get back, again with any water bottles, even if they’re now empty. Try not to pee or spit during your workout, as you’d have to collect and weigh that to get the most accurate number. In addition, try to go at your usual pace in typical heat and humidity conditions for where you live.

Here’s the formula to calculate your sweat rate:

pre-exercise weight – post-exercise weight ÷ time in hours = sweat rate

For example, if you weighed 170 pounds — including your full water bottles — before a 90-minute bike ride, and you weighed 165 pounds after the ride — including those same water bottles, whether full or empty  —  your sweat rate per hour is:

170-1651.5 = 3.3 pounds of sweat per hour

Note that sweat rate is usually expressed in liters per hour, and because one liter of water weighs 2.2 pounds, the sweat rate in the example is actually 1.5 liters of fluid per hour (3.3  2.2 = 1.5).

Someone who sweats 3.3 pounds per hour needs to drink at least 50 ounces of fluids per hour. The body doesn’t absorb fluids that quickly, so there will be some amount of dehydration. And because that recommendation is equal to three 16-ounce water bottles that need to be refilled every hour, taking advantage of water stops and refill stations is crucial. You can’t carry enough (or absorb enough) to stay fully hydrated for your entire workout, but as long as you don't lose more than 2% of body weight through sweat, your session won’t suffer.

If you’re exercising in hotter and/or more humid conditions, at altitude, or going at a faster pace than when you calculated your sweat rate initially, adjust by increasing fluid intake. After your workout, a good rule of thumb is to drink 16 to 24 ounces of water for every pound lost during the workout.

Key Points to Keep in Mind

• Losing 2% of your body weight in fluid, if not replaced while exercising, will negatively affect your endurance performance.
• Drink 16 to 24 ounces of fluid for every pound lost during exercise.

• Weigh yourself and all of your water bottles before and after (filled before, empty after) your workout.
• Don’t pee or spit because you will need to weigh that too (and you don’t want to have to do that!)
• Divide that difference by the amount in time (in hours) that you exercised
• Plan on replacing that amount every hour when exercising
• Adjust for heat, humidity, altitude and intensity

Published October 24, 2014.

John Hoeber works with individuals and teams to improve their health and performance through diet with practical and lasting solutions. He is a registered dietitian, certified specialist sports dietitian, personal trainer and wellness coach with more than 26 years of experience. John is available for phone and online consultations at john@bodykineticsmarin.com.