Carbohydrates during Exercise

Now that you understand your hydration needs, the next order of business is your carbohydrate requirements. Most of us have about two thousand calories' worth of stored carbohydrates among what's stored in our liver, muscles, and blood to use during exercise. That will last about sixty to ninety minutes, depending on how big you are and the intensity of the exercise. An hour of high-intensity exercise has been shown to burn through 55 percent of liver glycogen, and in two hours, that glycogen is kaput. This is why athletes can get by with only water when exercising less than an hour; but go longer, and they'll need additional carbohydrates to keep the pace and intensity levels high.

Hundreds of studies have shown that carbohydrates consumed during exercise delay fatigue in endurance athletes, but carbohydrates during exercise also are important for athletes in high-intensity stop-and-go sports such as soccer, ice hockey, tennis, basketball, baseball, and football, as well as in precision sports. Carbohydrate consumption during all sports can help ward off fatigue within muscles but also the mental fatigue that can be associated with sports requiring lots of concentration. Remember, the brain is the hungriest of all organs for glucose. In fact, the brain is twenty to thirty times more metabolically active than muscles, and unlike other tissues, the brain can utilize only glucose for fuel.

Studies also have found that there is an upper limit to how much carbohydrate will be burned as fuel, even when more carbs are ingested. The rate of stomach emptying and intestinal absorption of carbohydrates appears to have limits as to how much carbohydrate can be absorbed per minute. It appears that 1 gram per minute is the upper limit, but newer studies suggest that this may be increased substantially with training and by the delivery and type of carbohydrate consumed.

As a general rule, sports nutritionists and the American College of Sports Medicine recommend the following: 30 to 60 grams of carbohydrates per hour 120 to 240 carbohydrate calories per hour This recommendation, as you can see, is fairly general and equals a 120-calorie difference per hour. For athletes exercising at high intensity and burning through carbs at a rate of 4 grams per minute or 240 grams an hour, you can see it doesn't take too long to burn through all the available carbs the body has stored and then some. When exercising for more than an hour, it's essential to take in additional carbohydrates to maintain the speed or power or intensity.

There are also newer data questioning if 60 grams of carbohydrates per hour is really the upper limit that athletes can burn. Despite the concern that more carbs could delay stomach emptying and increase GI discomfort, the glucose/fructose blend appears to be faring quite well—similar to water. As a general rule, athletes exercising at lower intensities can tolerate higher amounts of carbohydrates because there is more blood flow to the GI tract and less in the muscles compared to individuals working out at higher intensities.

The best way to determine what works best is to start with 30 to 60 grams of CHO per hour and increase or decrease intake according to how you feel and how well you perform. If you are a larger athlete, you require more energy and should strive for the higher carbohydrate goals compared to a lighter athlete. Use the table on page 118 as a general guideline to determine how much carbohydrate to strive for during exercise. We have broken it into categories based on body weight, as that is one way to better tailor carbohydrate guidelines. However, we have seen female Ironman competitors who weigh a feathery 110 pounds (soaking wet) who can tolerate more than 90 grams of carbs per hour on the bike portion of the triathlon. That said, use this as a starting point and tweak your carbohydrate intake upward or downward based on how you feel and perform during exercise.

We suggest keeping a workout log when dialing in your carbohydrate needs. In it, track the workout and everything you ate and drank during the workout. Record your times, splits, how you felt on a scale of 1 to 10, and any GI issues. Repeat that same workout once a month, change the carbohydrate intake, and record the same measures as before. Many elite-level athletes we work with do this to dial in their nutrition before key events. They also tell us that their body adapts to higher carbohydrate intake as the season progresses and they practice consuming more in training.

To get those carbs in you, it's best to stick with the most digestible carb-rich options available. Your sports nutrition arsenal can include sport bars, sport drinks, gels, and real food. Use the chart on page 119 for the best ways to use different sports nutrition products.

Duration of Exercise/Event Carbohydrate Requirements

Under 1 hour

Water or sports drinks only; additional carbohydrates unnecessary

1 to 2 hours

Carbs at 30 to 60 grams per hour <130 pounds, 30 grams >150 pounds, 60 grams

2+ hours

Carbs at 45 to 90 grams per hour <130 lbs, 45 grams >150 lbs, 90 grams

4+ hour

Carbs at 45 to 100* grams per hour

'Research reveals that by consuming multiple sources of carbohydrates, with primary sources being glucose, maltodextrin, and other simple sugars, and fructose as the secondary carbohydrate source in a 2:1 ratio, you can take in more grams of carbohydrate and your body is able to use more carbohydrate per hour. It is suggested that the body uses dual transport mechanisms to absorb fructose compared to other simple sugars and therefore can burn substantially more than 60 grams of carbohydrates per hour. Some studies have found that a glucose-fructose combination can boost carb combustion by 20 percent or more compared to a glucose-only feeding.

Since the inception of the PowerBar Performance Bar in the 1984, there are now hundreds of energy bars available, many of which claim to have miraculous ingredients to enhance performance. There are "women-only," high-protein, meal-replacement bars and endurance-focused bars or those for power athletes, so it's hard to determine what you really need.

Look for a bar that is high in carbohydrates relative to the protein and fat content. Since protein and fat slow digestion, only individuals exercising in longer-duration, lower-intensity events tend to be able to eat protein- or fat-rich bars while exercising. Some bars, such as PowerBar Performance Bars, provide 60 percent or more of their calories from carbohydrates. Other bars are too high in protein or fat.

Since the goal of an energy bar should be to help you meet your carbohydrate goals per hour, add the carbs from the bar you choose to the carbs consumed with a sports drink to calculate your total carbohydrate consumption per hour. Generally, the higher the intensity of exercise, the more you will rely on fluid sources of carbohydrates. Individuals who rely on bars for most of their carbohydrates tend to be active people who may be exercising at a lower intensity, or doing endurance events where they need more variety in their fueling options. To enhance gastric emptying of the bar and to minimize GI distress, be sure to drink plenty of water with

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