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Fluid Fundamentals

Rita Maas; Gerri Williams
The new message in a bottle: Let thirst be your guide to staying hydrated.

Water makes up 85 percent of our blood, 80 percent of our muscles, and 75 percent of our brains. It regulates body temperature, transports nutrients, and removes impurities. We can't survive without it.

It comes in seemingly endless variations―tap, bottled, bubbly, lightly sweetened, fruit-flavored, mineral-fortified, and electrolyte-enhanced. We also obtain it in the many other beverages we drink, such as juices, colas, teas, and coffees, not to mention various foods.

But how much fluid do you need, what are the best sources, and what special considerations are important for exercisers?

"There are so many misconceptions, such as the old adage about drinking eight eight-ounce glasses of water per day," says Samuel N. Cheuvront, PhD, RD, principal investigator with the U.S. Army Research Institute of Environmental Medicine in Natick, Massachusetts.

To help clear up the confusion, we turned to the experts for answers that hold water.

How much fluid do I need each day? Optimal hydration varies depending on activity level, body size, sweat rate, and climate. Because of that, the Institute of Medicine, the group that provides scientific research to formulate public health policies, such as the U.S. dietary guidelines, recommends an Adequate Intake (AI) rather than a specific daily amount.

AI is a more flexible measure; a strict recommendation does not account for individual needs. For a sedentary woman living in a moderate climate, the AI is 2.7 liters or 91 ounces of fluid a day. A man's AI is 3.7 liters or 125 ounces of fluid per day.

Is drinking water the only hydration option? To stay hydrated, you can consume any beverage, including milk, juice, soda, and even coffee and tea (their diuretic role has been exaggerated). Also, food helps you meet daily fluid needs. "Soup, fruit, vegetables, and dairy products are all more than 80 percent water," says Roberta Anding, MS, RD, a clinical dietitian at Baylor College of Medicine in Houston and spokesperson for the American Dietetic Association. For example, a typical wedge of watermelon contains approximately 9 ounces of water. Food also helps the body hold on to fluids because of the electrolytes it contains―minerals like sodium, potassium, and chloride that bind with water. Drinking beverages with meals and snacks helps your body retain water longer.

If I only drink when I'm thirsty, will I consume enough? "Generally, yes," Cheuvront says. We often hear that thirst is a poor indicator of hydration, but this advice really only applies to endurance athletes, people who work outdoors in hot climates, or those who may risk dehydration due to illness. For sedentary and moderately active people, thirst is a good guide. According to the U.S. Department of Agriculture, the combination of thirst and usual drinking behavior, especially the consumption of fluids with meals, is sufficient to maintain normal hydration.

Are there any special considerations for exercisers? "How much additional fluid you need when you exercise depends almost entirely on how much you sweat," Cheuvront says. "Typical sweat rates for people exercising are on the order of half to one liter per hour," he says. "For competitive athletes who exercise at high intensity, it is not unusual to see sweat rates as high as one-and-a-half liters per hour. Sweat rates can vary even for the same person under different conditions―running slow or fast, in hot weather or cool weather."

The best way to calculate your sweat rate is to weigh yourself without clothes before and after your workout. Then, for each pound lost, you'll need 16 ounces of fluid to fully compensate. "Most people, if left to their own devices, will naturally replace the sweat they lose during exercise over the next 24 hours," Cheuvront says. Determining if you're well hydrated is simple; nearly clear first morning urine is a good way to gauge if you're getting enough fluid.

What about sports drinks? Sports drinks contain electrolytes that are helpful to replace when you exercise for an hour or longer or in hot weather, Anding says. But if you exercise for less than an hour a day, the meals you eat afterward will naturally restore lost fluids and electrolytes.

Sports drinks have other helpful benefits. They provide carbohydrates for energy during intense exercise. "Also, since sports drinks are flavored, many people drink more than they would if they just drank water," Anding says. Achieve a similar effect by adding a splash of fruit juice to your water.

Should I drink fluids during exercise? You only need to consume fluids during exercise if you're likely to become dehydrated (that doesn't happen until you lose more than two percent of your total body weight, or about three pounds). If you routinely lose more than three pounds during exercise, sipping fluids during your workout can help prevent dehydration. The fluid you choose is up to you.

Does bottled water offer advantages over tap? What about fortified waters? Bottled water is popular―more than half of us drink it. It's portable, so you can drink it on the go. And it offers consistent taste. The taste of tap water, on the other hand, can vary from place to place. Tap water is usually treated with fluoride, which helps maintain strong teeth and bones. Fortified waters confer the same advantages as other bottled waters, and contain additional vitamins and nutrients. They're fine in moderation, though "you can obtain plenty of vitamins and minerals from eating a balanced diet," Anding says.