Muscle Glycogen Use During Exercise
Exercises 3.8 shows that the rate of muscle glycogen utilization during exercise is greatest when prerace muscle glycogen levels are highest. Thus it follows that if one has taken the trouble to carbohydrate load before exercise, that athlete must also try to prevent those stores from depleting too rapidly.
A technique that might prevent this rapid depletion is to elevate preexercise blood free-fatty-acid levels by ingesting caffeine, a high-fat meal, or both, 3 to 5 hours before exercise. The use of heparin injections to elevate blood free-fatty-acid levels should be considered purely experimental because of the risks involved. The experimental evidence supporting these practices is, as described, quite slim (Ravussin et al, 1986; Weir et al, 1987).
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Liver Glycogen Use During Exercise
Failure to maintain liver glycogen levels during exercise will cause blood glucose levels to fall. Because the brain depends on an adequate glucose supply, falling blood glucose levels (hypoglycemia) ultimately lead to exhaustion due to impaired brain functioning.
The typical symptoms of hypoglycemia are incoordination, inability to concentrate or to think clearly, and extreme physical weakness leading to collapse.
Probably the first report of hypoglycemia in marathon runners was that provided by a group of doctors from the Peter Bent Brigham Hospital in Boston (Levine et al, 1924). These doctors studied six runners competing in the 1924 Boston Marathon and found that postrace blood glucose levels were decreased in all runners. The doctors also noted a strong correlation between the conditions of the athletes at the end of the race and their blood glucose levels; athletes with low blood glucose levels showed asthenia, pallor, and prostration. For the next race, these researchers encouraged those runners who had developed hypoglycemia in the 1924 race to eat high-carbohydrate diets for the last 24 hours before the race and to start eating candies after they had run about 24 km (Gordon et al, 1925).