I’ve heard it float around before that one shouldn’t drink water during exercise. It always sounded like a fishy piece of ‘advice,’ it probably comes from those people who come into a gym, run, then weigh themselves right after to see how much weight they’ve ‘lost.’ All they’ve done is lose water weight…

So Dr. Mirkin has a post on the effectiveness of drinking water during a workout:

Anyone who exercises vigorously can increase their endurance by taking in fluids, and competitive athletes can increase their endurance by taking in extra fluids just before the start of their event and drinking fluids regularly during events that last more than an hour. Be sure to replace salt as well as fluid you lose when you sweat. Don’t force yourself to drink large amounts of water.