The Gatorade Sports Science Institute offers the following facts on hydration:
Research has shown that losing as little as 2% of your body weight through sweat can hurt athlete performance by causing fatigue and affecting physical and mental skills.
Studies show that soccer players, on average over the first 60 minutes of exercise, will lose more than 1 quart of sweat.
Athletes showing an increase in body weight are over hydrating. Athletes recording a loss in body weight should look to replenish fluids, carbohydrates and electrolytes lost in sweat within one hour after exercise.