Chris Kouwenhoven


Muscle fatigue (the "bonk" in cycling, "hitting the wall" in running) generally occurs when the body's internal carbohydrate stores are depleted and there is a shift towards fat metabolism as the prime energy source for the exercising muscle (with maximum energy output limited to approximately 50% VO2 max.). It would be logical to assume that if adequate carbohydrates (to offset those expended) were replaced during a ride, the cyclist could maintain his or her pace indefinitely. Unfortunately this is not the case. Studies on cyclists with low muscle glycogen stores but high blood glucose levels still experienced fatigue at some point, even though the time to onset of fatigue was delayed by taking the carbohydrate supplements. Unknown factors, perhaps related to physical changes in the muscle cell itself, are thought to be responsible. This type of fatigue is more common in the untrained athlete.

Carbohydrate is stored as glycogen in muscle cells and in the liver. On a normal diet there is enough glycogen to support 2 hours of aerobic exercise before the bonk occurs. These internal stores can be extended by using oral carbohydrate supplements for events expected to last more than 2 hours. It is best to begin the carbohydrates at the start of the event as they are much less effective after the bonk has occurred. A well trained cyclist will need slightly more than 1 gram of carbohydrate per minute to sustain maximum performance, and oral supplementation (started at the beginning of the exercise, not after glycogen depletion has occurred) should replace carbohydrate at that rate.

In addition to extending the time to fatigue in longer, moderate activity events, one study has also suggested that maximal performance in a 1 hour, high intensity (time trial, ~80% VO2max) event can be improved with oral carbohydrate supplementation. Drinking a total of 1 liter of a 7% carbohydrate solution at the beginning and during the event improved times by 2%.

Skeletal muscle oxidizes carbohydrate in the form of glucose, and other sugars must be converted to glucose by the liver before they can be used as fuel by the muscle. Studies have demonstrated no additional benefit for glucose polymers, fructose, or sucrose (common table sugar) which is a dimer of glucose and fructose, for carbohydrate replacement - aside from palatability. In large amounts, fructose can cause diarrhea.

Although carbohydrates are superior to fats in supporting maximal performance, there is some controversy over the relative benefits of simple vs complex carbohydrates as the ideal supplement to be used during prolonged exercise. Examples of complex carbohydrates are rice (200 Cal per cup), spaghetti (180 Cal per cup), and baked potatoes (140 Cal per large spud). Examples of other carbohydrates.

Protein is a maintenance material being used to repair muscle (and other) cell injuries - including the micro trauma that occurs with exercise. It is NOT used by the body as an energy source except in very malnourished states. Even in endurance activities such as the Tour De france, protein needs of 1.5 gms protein/kg body wt/day were easily met by a normal (unsupplemented) diet that replaced the total Calories expended. A review of the literature failed to demonstrate any benefits for protein supplements (assuming an adequate daily protein intake) over pure carbohydrates alone. And one study actually demonstrated a DECREASE in overall performance from the appetite suppressing effects of a high protein diet, decreased carbohydrate intake, and as a result diminished pre event muscle glycogen stores.

The average 70 kg (154 pound) cyclist will need from 70 to 100 grams of protein per day. Lean beef, skinless chicken, and fish will provide about 7 grams of protein per ounce. Beans will provide 6 grams per 1/2 cooked cup, and rice (and other cereal grains) about 3 grams per 1/2 cup serving. A cup of milk or yogurt supplies 8 grams of protein. So it's relatively easy to meet your basic protein requirements from 6-8 ounces of meat, 2-3 servings of dairy products, and 6-10 servings of cereal per day.

Is there an optimum diet for the cyclist?? There is overwhelming evidence that adequate dietary carbohydrates are needed for maximum performance. At least 10 grams per kilogram of body weight per day. What is unclear is whether more than that will add any benefit. So high Calorie diets, in training athletes, may not improve performance if more that 3000 Calories are carbohydrate Calories. So if you hold carbohydrates to a maximum of 3000 Cal per day and increase total Calories, there will be a decreasing % of total daily Calories as carbohydrates as the total Caloric expenditure (replacement needs) per day increase. And Fat?? If you are interested in multiday endurance events, there may be some advantage to several weeks of a moderate fat intake equivalent to 30% of total Calories. But there is no evidence this helps in single day, high performance (%VO2max greater than 60%) activities and there may be long term health consequences. As total Caloric needs increase, the only reason I'd consider a high fat (more than 15 to 20% of total Caloric needs) diet would be maintenance ofCaloric balance IF carbohydrates alone were not meeting the challenge. And finally, there is NO evidence tha more than 2 grams per day of protein are beneficial in endurance, sprint, or power training/performance.

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