…make sure you have something cold to drink. Because the temperature really does make a difference. In a study from the University of Montana, scientists monitored people walking briskly on a treadmill, for 3 hours, in almost 90-degree heat. And the exercisers either had ice water to drink or lukewarm, room temperature water. The result? Study subjects had to drink twice as much lukewarm water as ice water, to maintain a healthy heart rate, and core body temperature. So if you’ll be exercising in hot weather, carry an insulated sports bottle filled with ice water.