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Your playlist is loaded with the newest releases. Your workout journal is up to date. You’ve got your favorite gym shorts and a workout shirt on the ready in your gym bag.

What could you possibly have forgotten?

Oh yeah. You were so busy finalizing that spreadsheet and emptying your inbox that you didn’t drink a sip of water all day. And quickly drinking a few ounces of Gatorade right now would be like cramming for a calculus test without studying all semester. Dehydration has already set in, and it’s setting you up to fail.

“Dehydration causes overheating, decreased endurance and increased heart rates — it makes exercise seem harder,” says Illinois-based sports nutritionist Monique Ryan, MS, RD, author of Sports Nutrition for Endurance Athletes. “But you can learn how to hydrate better — it’s a skill you can practice.” (See “What Happens to Your Body When It’s Underhydrated” for more the side affects of dehydration.)

Below, Ryan and other top sports nutritionists offer some advice:

1. Know the dangers of dehydration

John Ivy, PhD, coauthor of Nutrient Timing: The Future of Sports Nutrition, compares a dehydrated athlete to a stalled car on the side of the road, hissing smoke. In a car, the water system should cool the engine and transfer heat to the radiator, but if the cooling system fails, the engine conks out.

“The same thing happens when you reduce the water in the body,” says Ivy. “The circulatory system sends water by the muscles, which are generating heat. The muscles release the heat as sweat on the skin, which then dissipates through evaporation and wind conductivity.” (That’s why you feel extra hot on humid days — water vapor in the air slows evaporation and, thus, the cooling process.)

When you get dehydrated, you cause a drop in blood volume, so the heat doesn’t transfer properly, explains Ivy. Overheating then causes muscles to cramp and the heart to beat faster in an effort to move blood. You tire faster, you lose balance and coordination, and your brain feels like Cream of Wheat. (See “All About Hydration” for more.)

2. Stay up-to-speed on the latest standards

Eight glasses of water a day? That’s old school. The latest standards recommend at least 91 ounces for women and 125 ounces for men, according to the Institute of Medicine in Washington, D.C.

But what about those electrolytes, carbohydrates and protein now added to sports drinks? Most nutritionists agree that plain water is sufficient unless you’re working out for more than an hour.

The latest standards [to maintain hydration levels] recommend at least 91 ounces for women and 125 ounces for men.

So if you’re going to a 90-minute group-cycling class or on a two-hour cross-country ski, for example, you’ll want some sugars to help fuel muscles. Research by Dutch nutritionist Asker Jeukendrup, PhD, indicates that concoctions with two different types of carbs may deliver even more energy than those with a single source. Most ready-made drinks fit the bill. Or, with powdered fructose, glucose and other types of carb-laden sugars available in nutrition stores, you can mix your own sports drinks to suit your tastes — without the freaky neon colors. If you’re going for a major endurance session lasting three hours or more, you’ll need to include sodium in your fluid stores to help replace the salt lost by sweat.

Many dietitians have also started pushing protein as part of a hydration plan for long workouts. “One of the things we do know now is that the addition of protein to a sports drink really helps to maintain fluid retention,” says Ivy.

3. Familiarize yourself with the other fluids

Urine and sweat are the best ways to gauge whether you’re consuming the right amount of fluids. If your pee looks like apple juice, for instance, you need to start drinking — pronto. “Urine should be pale yellow to clear,” says David Sandler, president of StrengthPro, a Las Vegas–based sports-performance consulting group, and author of Sports Power. “It’s a good indicator because all fluids are processed through the kidneys, and excess water and used metabolites are transferred to the bladder for urinary excretion.”

To maintain hydration, says Ryan, you’ve got to know your personal sweat rate. To do that:

  • Weigh yourself immediately before and after a one-hour workout.
  • The amount of weight lost is your sweat rate.
  • Although 1 pound equals about 16 ounces of water, Ryan recommends consuming 20 to 24 ounces of fluid for every pound of weight lost after training to replace both sweat and urine losses.

A weight gain, on the other hand, could indicate hyponatremia, a condition in which too much water floods the body and dilutes blood salts. This is mainly a concern for endurance exercisers, and it’s the reason sports drinks include electrolytes.

4. Prevent dehydration

Most experts agree on the 2 percent rule when gauging the effects of dehydration. “If you’re dehydrated by 2 percent or more — that is, if you lose 2 percent of your body weight during a workout session — your performance could be affected,” says Ryan.

It takes less than 30 minutes for fluids to reach the muscles, speed the onset of sweating and help cool you down, last-minute guzzling can cause the stomach to slosh and cramp during exercise.

And while it takes less than 30 minutes for fluids to reach the muscles, speed the onset of sweating and help cool you down, last-minute guzzling can cause the stomach to slosh and cramp during exercise.

So instead of waiting until you’re thirsty or on your way to the gym, you should be hydrating during all your waking hours to maintain a constant fluid balance all day, every day, says Sandler.

Even if you’re not exercising, you lose about 10 cups of water each day, according to the Mayo Clinic in Rochester, Minn. If plain old H2O bores you, find other forms of fluids: Fruits and vegetables (especially lettuce, watermelon, broccoli and grapefruit), dairy products, and even decaffeinated coffee and tea can hydrate you. (On average, 20 percent of people’s hydration needs are met through food.)

5. Outsmart the elements

In the winter, it’s not just the shorter days and bitter weather that make us weary: We also may be dehydrated. The cold can shut off the body’s thirst mechanism and trick you into thinking you’re not sweating as much, says Ivy. Research compiled by sports scientists Ed Burke, MD, and John Seifert, MD, indicates that a 150-pound skier could lose as much as 6 pounds of fluids while on the slopes for three hours. A 2004 study by University of New Hampshire researcher Robert Kenefick, PhD, found fluid deficits equal to 3 to 8 percent of body mass in individuals who did moderate-intensity exercise in the cold. It also showed that the cold reduced thirst by up to 40 percent both while at rest and during exercise.

Indoor environments can also cause athletes to ignore hydration, says Ryan, who suggests checking your sweat rate in varying conditions and seasons to avoid such pitfalls.

Once you’re in the habit of staying hydrated, it won’t seem like an unmanageable task. And since exercise feels easier when your tank is on full, any lingering reservations you have about quaffing your quota every day just might evaporate.

Thirsting for Knowledge

General Hydration Guidelines:

  • When: Throughout the day
  • What: Water, milk, fruits, vegetables, yogurt, decaf tea and coffee
  • How Much: At least 91 ounces for women and 125 ounces for men

How to Hydrate for a Workout:

  • When: Two to four hours prior
  • What: Water
  • How Much: As needed, until urine is pale or clear

How to Hydrate During a Workout:

  • When: Every 15 minutes
  • What: Water (if you’re exercising an hour or less)
  • Water + Carbs (one to three hours)
  • Water + Carbs + Sodium (three hours or more)
  • How Much: 8 ounces

How to Hydrate After a Workout:

  • When: Within 45 minutes and over the next eight to 12 hours
  • What: Milk or other carbohydrate and protein drink; water, juice, fruits, vegetables, yogurt, decaf tea or coffee
  • How Much: 1.5 ounces for every ounce of body weight lost

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