“Thirst is a very poor indicator of dehydration, especially if you’re up at altitude or training somewhere dry,” says Eric Sternlicht, an associate professor of kinesiology at Chapman State University. A 2016 study published in the International Journal of Sports Nutrition and Exercise Metabolism found that, when allowed to drink freely, 32 percent of collegiate athletes started exercise in a dehydrated state and more than 40 percent finished dehydrated.