Section 5: Effects of short-term carbohydrate
depletion on endurance exerciseAs early as 1967, it was established that overall endurance performance was dependent
on the availability of glycogen in the muscle, finding that exhaustion during aerobic exercise
occurred when muscle glycogen decreased below a certain level (35). At exhaustion, glycogen
levels in the working muscle were almost entirely emptied.
At the same time, it was found that the rate of glycogen used during exercise was related
to the amount of glycogen present in the muscle. When glycogen levels were the highest,
glycogen breakdown was also the highest. As muscle glycogen levels dropped, the rate of
glycogen utilization decreased as well. Since that time, glycogen stores have been assumed to be
the ultimate determinant of endurance exercise performance, but that viewpoint has recently
been challenged (58). Additionally, it is currently unclear why depleted muscle glycogen would
necessitate a reduction in exercise intensity when FFA are so readily available (54).
As discussed in chapter 10, normal glycogen levels are roughly 100-110 mmol/kg. One
interesting observation in the afforementioned study (35) was that glycogen utilization was
severely impaired below a level of 40 mmol/kg of muscle suggesting an impairment in the
glycolytic pathway. Additionally, below 10 mmol/kg, a further decrease in glycogen utilization
was observed suggesting that there may be some critical level below which exercise performance
is severely impaired. This is remarkably consistent with the observation that performance
during a 30 kilometer run was severely impaired when glycogen fell to level of 15-25 mmol/kg (53).
Numerous studies have examined the effects of short-term glycogen depletion during endurance
exercise of various intensities and duration. In general all support the early study described
above: that performance is impaired with glycogen depletion
Endurance exercise below 85% of maximum heart rate
At low exercise intensities, fat is the main fuel for exercise. As intensity increases, muscle
glycogen plays a greater role in energy production. This has led researchers to examine the
effects of both short-term and long-term carbohydrate depletion on endurance performance.
Typically, subjects are tested during exercise at normal glycogen levels and then perform
glycogen depleting exercise followed by 1-5 days of a high fat, carbohydrate restricted diet at
which point they are retested. Following the second test, the subjects are frequently given a diet
high in carbohydrates (causing glycogen super compensation) to examine the effects of above
normal glycogen levels on various parameters of exercise performance. A schematic of this
study design appears in figure 4 on the next page.
Almost without exception, the studies of short-term glycogen depletion on endurance
exercise below 85% of maximum heart rate find similar results. The primary result is a decrease
in glycogen use during exercise (35,59-64). This simply reflects a lack of availability of glucose,
prompting the body to find an alternative fuel source (i.e. FFA). The initial decrease in glycogen
utilization (and increase in fat utilization) during exercise occurs around 70 mmol/kg (65). Other
studies suggest that muscle glycogen breakdown does not decrease until very low glycogen levels
(around 40 mmol/kg) are reached (66).