Drink when thirsty.
For years, runners were told to drink enough water to offset all sweat losses during running. If you heed this advice, you have to drink constantly – and may suffer from bloating, nausea, cramps, and in some rare cases, death by hyponatremia. Fortunately, the thinking on hydration has changed. Top coaches now advise that when you run, you should drink only when thirsty. "The science shows that if you drink more or less than your thirst dictates, your performance will be impaired," says Timothy Noakes, author of 'Waterlogged: The Serious Problem of Overhydration in Endurance Sports.' Also note that drinking as you go has little effect on performance in runs lasting less than one hour. So for most races and runs, leave behind the hydration belt.