Drinking cold fluids lowers body temperature. More than 70 percent of the calories that you use to convert food to energy are lost as heat. So the more intensely you exercise, the more heat you produce. A rise in body temperature slows you down because the heart has to work harder to pump extra blood from your hot muscles to your skin to dissipate the heat. Seven studies show that cold beverages lower body temperature and improve performance by an average of 10 percent (International Journal of Sport Nutrition and Exercise Metabolism, April 2010). How much water do you need?