As the hike starts those places where you store excitement are stuffed as full as your pack. And so are those places where you store glycogen, the first fuel your body burns for endurance. Glycogen was once the food you ate, derived from carbohydrates and strung now, like pearls, into chains of glucose molecules. Built from your diet of the last 24 hours or so, glycogen waits, packed into the cells of your muscles and liver, ready to provide energy. What you have eaten prior to the hike, and your level of physical fitness, determine how long you will endure.

DIET AND DOING
Energy production takes place within each of your muscle cells. For the first 20 to 30 minutes of your hike, stored muscle glycogen provides almost your entire source of energy, with the addition of a little glucose from your blood which has been released from the stores in your liver. But the drain on your glycogen starts to tell. Hormone levels in your blood change, insulin lowering and epinephrine rising, and stored fats begin to play an increasing role in providing energy. At the 90-minute mark for the average backpacker, fats and blood glucose have become a major supplier of endurance. With two hours behind you, glycogen has almost been used up. Fortunately, your fat supply is virtually inexhaustible. Unfortunately, fats won't burn unless carbohydrates are present, and continued exercise depends on your muscles taking glucose from your blood. You must take on more carbos or you start to poop out.

Replenishing carbos is most effective with a drink containing a six to seven percent concentration of carbohydrates (e.g. Gatorade, Exceed, Sytomax). Choose the drink that tastes best to you. Within minutes glucose will be spilling into your bloodstream. Higher concentrations are absorbed more sluggishly, sometimes causing an upset stomach. For optimum endurance, drinks should be taken in small swallows consistently throughout periods of exercise at a rate of about a liter per hour. Start sipping around 30 minutes into your hike to avoid carbo depletion.

If you don't like carbo-replacement drinks, endurance can be maintained with energy bars (e.g. PowerBars, Exceed Sports Bars). They'll take 30 to 60 minutes to go into action, and they must be followed with the same drinking regimen using plain water. Even if you're using drinks for carbohydrates, you should still start munching something about 90 minutes into your exercise, to ensure your blood glucose stays high enough to prevent exhaustion. When energy stores are exhausted, they take a long time to rebuild, up to a day.

Carbohydrates can be replaced with regular old food (such as breads and sweets) and water, but they just aren't as efficient. Besides energy bars are lightweight and compact, with a lot of calories, and little garbage to pack out.

Another reason exists for keeping your carbo intake adequate. After two hours of strenuous backpacking without taking the time to replenish carbohydrates, your brain, which feeds almost entirely off blood glucose, may begin to complain with headaches and dizziness. Your ability to think things through carefully will rapidly diminish. You may find yourself in serious trouble.

To maintain energy stores within your muscles the normal American diet of approximately 46 percent carbohydrates is not enough. "A diet containing 70 percent carbohydrate is recommended when you're exercising hard," writes Ellen Coleman in her book Eating For Endurance. Ms. Coleman suggests balancing your diet from the four food groups -- dairy, protein, fruit-vegetable, and grain -- then doubling your intake of fruits and vegetables and tripling your intake of grains to achieve a high carbo diet. Primary carbohydrate sources are cereals, breads, pastas, muffins, pancakes, rolls, rice and other grain products, fruits and vegetables.

Carbohydrate loading, achieving maximum glycogen storage, is a combination of diet and exercise that endurance athletes sometimes use (See Ms. Coleman's book for complete details.) But, since exercise stimulates high muscle glycogen storage, you can't load more carbos unless you're involved in endurance training.

TRAINING
For food to be burned into energy, oxygen must be present in each cell. The harder you work, the more oxygen you use. But you, and everyone else, will reach a place during exercise beyond which your ability to utilize oxygen will not increase, even should the intensity of your exercise continue to rise. When you are using oxygen at your maximum capacity, you have reached an exercise plateau of 100 percent aerobic capacity. You can only function at maximum for about five minutes. Then you've reached another poop-out point.

Training for endurance will increase your ability to utilize oxygen. That, in practical terms, means you'll be able to cover the same distance on the trail with less effort, or a greater distance with the same effort.

"We define endurance exercise," writes Coleman, "as exercising for three to five days per week, for 20 to 60 minutes, at 50 to 80 percent . . . of aerobic capacity . . . (or 60 to 90 percent of maximum heart rate)." Find your maximum heart rate by subtracting your age from 220. On this training regimen, you can maximize your aerobic capacity in six months to two years, depending on the intensity of your exercise and your genetic predisposition.

HYDRATION
"Water," says Ms. Coleman, "is the most commonly overlooked endurance aid." Even very mild dehydration produces a loss of efficiency. Before heading out along the trail, drink a half-liter of cold water. Cold fluids empty from the stomach quicker and cool the engine, preparing it for the heat stress of the exercise to come. Then keep up the consistent drinking -- plain water or energy drink -- at the liter per hour rate.

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