Most adults lose about 10 cups of water every day through normal body functions, including perspiration and breathing. When you're hiking you may lose one to two liters of fluids an hour. Warm weather, high humidity, and a high level of fitness can all increase the amount you perspire. If you don't replenish those lost fluids, you'll become dehydrated. Dehydration has a serious effect on the body's functions.

When fluid levels are low, your blood volume decreases, which means less oxygen reaches your muscles, individual cells become dehydrated and don't process energy as efficiently as they normally do, and you don't sweat as much, so your body temperature rises.

It doesn't take long for fluid loss to diminish your performance. Exertional heatstroke usually occurs within the first two hours of exercise.

The consequences of not drinking enough are made worse by not eating (see hyponatremia). As you exercise, your body burns both fat and carbohydrates for energy, but glycogen, the sugar that muscles store as fuel, runs low after about 90 minutes.

It is recommended that you drink 16 ounces of fluids two hours before you take off down the trail. Plain water, watered-down fruit juices, and glucose-filled sports drinks are good choices.

Dehydration can occur in any season, not just in the summer, and not just on hot humid days. In the winter, heated air evaporates moisture on your skin, and although you may not feel thirsty, you need to replace fluids. Exercising in cold weather can cause you to perspire and become dehydrated as well. If you are going to be physically active, drink fluids on a schedule before, during, and after the activity.

The best way to know if you're adequately hydrated is to check your urine. It should be clear and copious.

Heat and humidity mean sweat. You can measure your own fluid loss by weighing yourself before and after you hike. One pound of loss equals 16 ounces of water. Fluids should be replaced with 150% of the volume you've lost, within 48 hours.

Here is a chart based on a healthy person weighing 180 to 210 pounds doing 90 minutes of moderate to intense exercise. Your own fluid loss could vary by 10% or more from these amounts, depending on your own weight. The bottom line is, if it's hot and you're out hiking then you'd better consume lots of liquids before, during, and after your hike.

If you're going out for more than two hours, start eating and drinking within 30 minutes of beginning. If you start early you'll feel much better at the two, three, and four hour marks than if you had waited.

During exercise we can absorb a maximum of one liter of fluid per hour. So even if you drink as much as your body can use, you could still dehydrate by 2 to 6 percent of your body weight when you hike in the heat.

It is recommended that you drink five to 10 ounces of liquid every 15 minutes during the hike, depending on intensity, duration, and the conditions. Water is generally fine, but pack a sports drink for hikes of longer than 90 minutes. Sports drinks help get more energy back into the muscles and will make you feel better.

Most trails or trailhead have water available. Avoid soft drinks with caffeine, coffee, and sugar. They actually dehydrate you.

Sports drinks also help add calories to your fuel intake. Aim for 100 to 200 calories of carb-rich food every hour. Eat whatever you like, but be sure there's variety in the food you carry, like carbs, protein, and fat.

Eat a meal of 300 to 800 calories one to four hours before you head out. The sooner you plan to start, the smaller your meal should be. If you're going out for less than 90 minutes, drinking your calories is fine, but if you're starting on a long day, you'll feel better if you eat some solid food. What you eat is less important that how the food will agree with you when you're on the move. Stick with what you know will work. Eat a meal that is balanced but high in carbohydrates. Something like a bagel with peanut butter or a sports bar and a banana.

Even if you've taken care of yourself during your hike you're probably still somewhat dehydrated and have depleted the energy stored in your muscles.

If you haven't urinated in the last six to eight hours you're dehydrated. If your urine is clear, you're in the clear. If it's not, drink more. You'll lose about a pound for every 16 ounces of fluid lost. Drinking 150% of the lost weight within three hours of the completion of your hike is the best way to rehydrate.

When it comes to food, refuel with carbs and protein within 30 minutes of exercise.