But when is a good time to start the hydration process? Experts run down the signs of dehydration and what happens if it turns to heat stroke.
On a hot day, that build-up you see on your workout clothes after a jog around the Rose Bowl could be a concerning sign that your body is heading toward dehydration.
"We can actually look at our clothing as it dries, and there's white stuff, well that's sodium and potassium," said Dr. Collin Kitchell with Cleveland Clinic.
While you may not see the same residue or perspiration in swimmers, they can lose up to a quarter gallon of fluid during a two-hour workout.
"That's why electrolytes are important to go along with the free water we're drinking," he said.
In extreme heat, Kitchell said whether you're in water or on land, fluid loss can happen quickly.
"We can lose a large amount of fluid in our body, pounds of fluid just exercising for an hour in the heat. and we have to replace that, and that's paramount," he said.
Signs of dehydration include feeling thirsty, having dark-colored urine, increased heart rate, muscle cramps, headache and fatigue.
Dehydration can lead to heat exhaustion and the risk for heat stroke - which is when your body can't regulate its temperature and needs to be treated immediately.
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The advice for people who have to work out on hot days: start hydrating and pumping up your body's cells well ahead of time.
"I always tell my athletes that are preparing that it actually starts three days before the event," Kitchell said.
Whether you're a student, amateur or professional athlete, hydrating during a practice or game is too late.
"Leading up to the event, two hours before is a good time to really start hydrating and helping with fluids, as well as throughout," he said.
Drink 7 to 10 ounces of a sports drink every 10 to 15 minutes during exertion and continue to drink up to two hours afterward to help rehydration.
If you are planning to buy any sports drinks with electrolytes, Kitchell said check the sugar content because you don't want to have too much. And avoid caffeinated energy drinks, because they can increase your heart rate and your blood pressure, which can increase your risk for heat stroke.
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