Protein
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.
