(CNN) -- Summer temperatures are soaring, and that scratchy dryness in your throat makes you crave a glass of water.
What you might not know is: If you feel thirsty, you're likely to be already dehydrated.
"If you drink only when you're thirsty, you're probably behind the 8-ball when it comes to drinking water and to staying hydrated," said Dr. Sylvia Morris, assistant professor at the Emory University School of Medicine.
Last week, when Gen. David Petraeus, head of the U.S. Central Command, briefly fainted at a Senate Armed Services Committee meeting, dehydration was cited as the cause. Dehydration is also a challenge for workers cleaning up the oil disaster, as they fight extreme temperatures around the Gulf of Mexico.
Dehydration can lead to your becoming overheated, exhausted and lightheaded, experts say. You could also develop an electrolyte imbalance, muscle cramps and an increased risk of heatstroke.
The old adage recommending "eight glasses of water a day" still holds to some extent -- but people need more than this, some doctors say.
Men should have more than 13 eight-ounce glasses of water a day, and women need nine, Morris said. Pregnant women and nursing mothers need more water than just the eight glasses of eight ounces each, she said.
Sodas do not count toward the eight glasses a day, said Heather Nettle, coordinator of exercise physiology services at the Cleveland Clinic in Ohio.
"Whatever you're drinking, you're going to be adding calories if you're drinking something else besides, and with that, it's going to be negative, particularly if you're doing exercise for weight management," Nettle said.
Fruits and vegetables such as watermelon, tomatoes and celery help give you more water but do not count toward daily recommendations for glasses of water, Morris said.
You can also add lemon, lime, cucumber or mint to your water to jazz it up, Morris said.
Keeping up with your hydration helps quash appetite, which is good for those who are trying to lose some pounds or maintain a healthy weight, she said.
Morris recommends drinking two glasses of water between meals, in addition to one to two glasses with each meal.
You should especially get in the habit of drinking a lot of water for heavy outdoor exercise, Nettle said.
Drink two cups of water before you exercise -- the first one, one to two hours beforehand, then have the second cup up to half an hour before the workout. Have another two cups 10 to 15 minutes before.
During exercise, you should drink on average a half a cup every 15 minutes, Nettle said. Afterward, drink two cups or more if you're thirsty, for every pound you lost through sweat.
"If you know you're going to be exercising heavily outdoors, it's recommended that you weigh yourself before you leave and then weigh yourself after you get back," she said.
If you're going to be exercising vigorously outside for an hour, just drink water, she said. But if you will be out longer than that and you're focusing on performance, consider a sports drink also, such as Gatorade or Powerade. The sugar in juice or soda can cause gastrointestinal distress, causing some people to feel bloated, gassy or nauseated, she said.
"Unfortunately, I think that we are all a little bit dehydrated, and we're not getting nearly as much water as we need, but certainly it's something that is easily rectified," Morris said.