It's an old saw among runners, but it still holds true: if you wait to drink water until you're thirsty, you've waited too long. Thirst is not an adequate indicator for when you need water. Even those who are relatively sedentary need a whopping two and a half quarts of water every day (not that most people actually drink as much as they should). Add a fast run to the mix, especially on a hot day, and that figure can double: the average person sweats one to one and a half quarts of water per hour while running.
At that rate, waiting until your body starts sending out thirst signals puts you in serious danger of falling behind and becoming dehydrated. Everyone who exercises should drink water constantly -- not just while exercising, but all day long. Try to drink four to six ounces of water every hour you're awake, plus a bit more after you run. Don't be shy; it's difficult to drink too much water, since any excess gets (literally) flushed away.
Which brings up the most reliable, though perhaps indelicate, way to see if you are drinking enough water: a visit to the restroom. If your urine is pale or clear, your body has a good fluid balance. If your urine is dark, on the other hand, then you probably should head for the water cooler.
So what's the big deal?
Simply put, water is essential for maintaining almost all of the body's vital processes. It makes up most of your blood. It keeps you cool through perspiration. It enables the storage of glycogen, an essential source of energy, in your muscles.
When you become dehydrated, you not only lose performance as a runner but you also risk becoming seriously ill. For every liter of sweat you lose, your heart rate increases about eight beats per minute (you can lose a liter in as little as half an hour, depending on the conditions). If you don't get some fluids, you add stress to your heart, your core body temperature rises, and your performance goes into the basement. Your body is unforgiving when its water stores dip at all below normal levels. Even slight dehydration will slow you down.
While plenty of water is the best way to avoid dehydration, feel free to wet your whistle with other beverages, too. Sports drinks as well as orange, grapefruit and tomato juices give you water while also fueling you up with energy-supplying carbohydrates and electrolytes. The downside, of course, is that those drinks may have more calories than you might have in mind; water, by contrast, has zero. Too many sodas or juices may have you drinking yourself into an impressive set of love handles. In general, try to drink water for at least half of your liquid intake.
As for the other drinks, some are better than others at staving off dehydration. Some runners, for example, might swear by coffee or soda as a great pick-me-up before a race or a tough training run, while others might complain that it makes them jittery. Both are certain to agree, however, that a cup of joe is likely to send them to the restroom faster than water. The caffeine in coffee and soda is a diuretic, which means that it tricks your body into thinking you need less water than you do. Your body complies by getting rid of "excess" water. Rather than helping you keep hydrated, in other words, caffeinated drinks actually dehydrate you.
Alcohol also is a diuretic. While beer is often provided as the beverage of choice at post-race parties, runners should always be careful to drink enough water, too. One other quick note on beer: quips are often heard from runners quaffing beers that they are carbo-loading. Alas, the carbohydrates in beer are empty calories, more likely to contribute to a larger waistline than to faster race times. Enjoy those well deserved post-race brews, but don't fool yourself into thinking that you're replacing any of the water or nutrients you spent during the race.