Drinking water at the first sign of thirst seems obvious, but by that time you’ve waited too long — you’re already dehydrated.

“The biggest mistake people make is, ‘I don’t need to worry because my perception of being thirsty will safeguard me and let me know when I need to be replenished,’” said Gail Kaye, program director of OSU Family and Consumer Sciences Extension. “But by then, 1 to 2 percent of body mass and water is already lost.”

The latest statistics from the Institute of Medicine’s Food and Nutrition Board indicate that most people fulfill their daily hydration requirements, but with heat exposure and physical activity increasing in the summer, fluid needs tend to increase while intake does not.

“The key is prevention and beginning hydration before exercise, or pre-hydrating, and keeping hydrated throughout,” said Sean Convery, medical director of the Miami Valley Hospital Sports Medicine Center and the head team physician at the University of Dayton. “If you’re just going by thirst, you’ll never be able to keep up.”

Even those lounging outside or doing yardwork need to be mindful of their fluid intake, too, he added.

“You don’t have to be exercising. If you’re just active — gardening, swimming, cutting the grass — you need to stay hydrated,” Convery said, adding that the same guidelines apply.

He recommends sticking to water when exercising for an hour or less. For physical activity exceeding an hour, a sports drink with electrolytes and carbohydrates may be necessary. To lessen the sugar content, he suggests diluting the liquid by half.

Sports drinks also may be necessary in the event of vomiting or diarrhea, Kaye said, to replace lost fluid and electrolytes.

The people most at risk of dehydration and heat stroke are children and the elderly. Certain medications, including those for blood pressure, ADHD and anxiety, also can increase one’s risk, Convery noted.

Many people may not realize that their choice of drink can actually contribute to dehydration.

“Alcohol can be dehydrating, so it’s good to have some water along with it or have some water in between drinks,” Kaye said. “Coffee has somewhat of a diuretic effect, but not as much as we once thought, but it’s still good to have water along with it.”