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"If the body can no longer cool itself," Roberts tells WebMD, "it starts storing heat inside. The core temperature begins to rise and you put your internal organs and central nervous system at risk."

Signs of heat exhaustion include general fatigue, weakness, nausea, dizziness, muscle cramps, and an increase in body temperature. Temperatures above 104, an inability to sweat, acute respiratory distress, and loss of consciousness can be signs of heat stroke, which is much more severe and can lead to death.

This doesn't mean you have to abandon your quest for a great summer workout. Just follow these nine guidelines to exercise smart in the heat. But make sure to talk to your doctor about starting an exercise regimen and issues about heat and hydration.

Summer Exercise Tip No. 1: Acclimate Yourself

"When the weather warms, you need to be acclimated to the temperature change," says William O. Roberts, MD, FACSM, a family medicine and sports medicine doctor at the University of Minnesota’s Phalen Village Clinic. "Expose yourself regularly."

Branch tells her clients it can take up to 14 days to adjust to temperature changes. When clients are preparing for an event that will take place in the heat of the day, Branch coaches them to be active in the heat ahead of time: "They have to try to get out in the middle of the day when it’s hot and exercise in order to acclimate to the conditions for the event."

But remember, if you are just doing routine exercise, it is better to exercise outside when it is cooler, such as the early morning or evening. (See more about this in tip No. 5.)

Summer Exercise Tip No. 2: Stay Hydrated

When it comes to summer exercise, all our experts agree that the biggest concern is hydration.

Suzanne Girard Eberle, author and sports dietitian in Portland, Ore., says that if you come back from a summer workout 1 to 2 pounds lighter, you’ve got to do a better job keeping up with hydration. You lose 2 1/2 cups of water per pound of body weight lost, she says.

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