Proper fueling prevents fatigue during long workouts

"I'm at the gym from 5:30 to 7:00 pm and feel exhausted by the end of my workout. What can I do to prevent fatigue?"

"I'm training for a marathon ... I dread the long runs. I'm dragging after 12 miles. Any suggestions for how to boost my energy?"

"I'm whipped by the end of my after-school soccer practices ..."

Sound familiar? Preventing fatigue is the No. 1 concern of active people who exercise for more than an hour.

This article can help you enjoy high energy and enhanced stamina during long, hard exercise sessions. (For shorter exercise sessions, a pre-exercise snack and some water should fuel you well.)

To prevent fatigue during extensive exercise that lasts for more than 60 to 90 minutes, you have two nutrition goals:

1. To prevent dehydration

2. To prevent your blood sugar from dropping

The following tips can help you reach those goals.

Sweat and dehydration

When you exercise hard, you sweat. Sweating is the body's way of dissipating heat and maintaining a constant internal temperature (98.6F).During hard exercise, your muscles can generate 20 times more heat than when you are at rest.

You dissipate this heat by sweating. As the sweat evaporates, it cools the skin. This in turn cools the blood, which cools the inner body. If you did not sweat, you could cook yourself to death.

A body temperature higher than 106F damages the cells. At 107.6F, cell protein coagulates (like egg whites do when they cook), and the cell dies. This is one serious reason why you shouldn't push yourself beyond your limits in very hot weather.

When you sweat for more than an hour, you lose significant amounts of water from your blood. The remaining blood becomes more concentrated and has, for example, an abnormally high sodium level. This triggers the thirst mechanism and increases your desire to drink.

To quench your thirst, you have to replace the water losses and bring the blood back to its normal concentration.

Unfortunately for athletes, this thirst mechanism can be an unreliable signal to drink. Hence, you should plan to drink before you are thirsty. By the time your brain signals thirst, you may have lost 1 percent of your body weight, the equivalent of 1.5 pounds (24 ounces) of sweat for a 150-pound person.

This 1 percent loss corresponds with the need for your heart to beat an additional three to five times per minute. This contributes to early fatigue.

Thirst sensations change with age and older people, even athletes, become less sensitive to thirst. For example, 56-year-old hikers became progressively dehydrated during 10 days of strenuous hill walking. The younger, 24-year-old hikers remained adequately hydrated. This means older people, in particular, should carefully monitor their fluid intake.

Light-colored urine, in significant volume, is a sign of adequate hydration.

Most athletes voluntarily replace less than half of sweat losses; thirst can be blunted by exercise or overridden by the mind. To be safe, always drink enough to quench your thirst, plus a little more.

If you know how much you sweat, you can then replace those losses according to a plan. To learn your sweat rate (and fluid targets), weigh yourself naked before and after a workout. For every pound (16 ounces) you lose, you should strive to replace 13 to 16 ounces (80 to 100 percent of that loss) while exercising.

This requires training your gut to handle this volume. Do not drink more water if your stomach is already sloshing; enough is enough!

You might find it helpful to figure out how many gulps of water equate to 16 ounces, and even set an alarm wristwatch to remind you to drink on schedule. You'll also need to plan on having the right quantity of enjoyable fluids readily available. Do not be in such a rush to start your workout that you fail to bring with you the sports drinks and fluids that will enhance your efforts.

Carbohydrates and blood sugar

As Ive mentioned above, you can significantly increase your stamina by consuming a pre-exercise snack that provides fuel for the first hour of the workout and by drinking adequate fluids during exercise.

The third trick to enhancing endurance is to consume carbs after an hour of exercise. Depending on your body size and ability to tolerate fuel while you work out, you'll want to target 100 to 250 calories of carbohydrates per hour of endurance exercise.

The larger you are, the more calories you need. For example, if you weigh 180 pounds, you should target about 250 calories per hour, such as 8 ounces of a sports drink every 15 minutes, or a 250-calorie energy bar plus water.

During a moderate to hard endurance workout, carbohydrates supply about 50 percent of the energy. As you deplete carbohydrates from muscle glycogen stores, you increasingly rely on the carbs (sugar) in your blood for energy. By consuming carbohydrates such as sports drinks, bananas, or energy bars during exercise, you can both fuel your muscles as well as maintain a normal blood sugar level.

Because your brain relies on the sugar in your blood for energy, keeping your brain fed helps you think clearly, concentrate well, and remain focused. So much of performance depends on mental stamina; maintaining a normal blood sugar level is essential to optimize your workouts and boost your stamina.

Your body doesn't care if you ingest solid or liquid carbohydrates both are equally effective forms of fuel. You just have to learn which sports snacks settle best for your body gels, gummy bears, dried figs, animal crackers, defizzed cola, whatever.

Despite popular belief, sugar can be a positive snack during exercise and is unlikely to cause you to "crash" (experience hypoglycemia). That's because sugar feedings during exercise result in only small increases in both insulin and blood glucose. Yet, too much sugar or food taken at once can slow the rate at which fluids leave the stomach. Hence, "more" is not always better.

Because consuming 100 to 250 calories per hour of exercise (after the first hour) may be far more than you are used to taking in during exercise, you need to practice fueling while exercising to figure out what foods and fluids settle best.

You'll learn through trial and error which snacks help prevent fatigue, boost performance and contribute to enjoyment of your long, hard workouts.

Nancy Clark, MS, RD, nutritionist at SportsMedicine Associates in Brookline MA (617.739.2003), teaches casual and competitive athletes how to win with good nutrition. For her best-selling Sports Nutrition Guidebook ($23) and her Food Guide for Marathoners: Tips for Everyday Champions ($20) send a check to Sports Nutrition Services, 830 Boylston St. #205, Brookline MA 02467 or obtain via

Copyright: Nancy Clark 1/03

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