Once you start thinking about a nice, cold glass of water, you're probably already 1 to 2 percent dehydrated, says Jessica Matthews, an exercise physiologist for the American Council on Exercise (ACE).
DO use the "lemonade test." When you peek in the toilet immediately after you urinate, you want to see liquid the color of pale lemonade; if it's a vibrant yellow or looks like apple juice, you need more fluids. ACE's general recommendation is to drink 17 to 20 ounces of water (a typical single-size bottle of water, like you'd buy in a convenience store, is 20 ounces) two to three hours before exercise on a typical day—this takes into account the fact that you'll probably hit the loo before heading outside. Then, during your workout, try to drink an additional 7 to 10 ounces of water every 10 to 20 minutes (this is especially important on sweltering days).
One more thing: Remember that you still sweat even when you're swimming, doing water aerobics or surfing, so leave a bottle of water on the shore or at the side of the pool.
2. Don't Protein-Load Before Your Workout
Some research has suggested that too much protein before a sweat session could elevate your basal temperature, making you feel even hotter. You're better off saving the protein bar for after your workout, when it will help you rebuild muscle.
DO cool off your insides with a slushie. Researchers have known for some time that lowering core body temperature before and during exercise can help athletes perform better, says Rebecca Stearns, director of education at the Korey Stringer Institute at the University of Connecticut—and now we have a tasty, science-backed idea of how to do that. A study published in 2010 in the journal Medicine & Science in Sports & Exercise found that when Australian athletes drank a flavored slushie before running on a treadmill in a hot room, they were able to keep up the pace for an average of 10 minutes longer than when they drank cold, flavored water.
One more thing: Don't plan to stop by the 7-Eleven on your way to the track; Stearns points out that the slushies in the study were more like frozen Gatorades than convenience-store Slurpees. The sodium in sports drinks can help the body retain fluid that is lost through sweat, so for the ultimate workout refresher, try combining your favorite sports drink with some crushed ice.
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