Summary
With short-term glycogen depletion, there is a progressive decrease in glycogen breakdown
during exercise accompanied by an increase in fat utilization. While this ‘glycogen sparing’ effect
might be expected to increase endurance performance, this does not appear to be the case for
short-term (1-5 days) carbohydrate restriction as endurance time invariably decreases and effort
level increases in the glycogen depleted subjects. This is remarkably consistent with data from
fasted subjects (72) which find that, despite drastically increased utilization of fats for fuel,
performance is still hindered by the lack of muscle glycogen. The effects of long-term adaptation
to carbohydrate restriction are addressed in the next section.
Additionally, there are breakpoints where glycogen utilization during exercise changes. The
first appears to occur around 70 mmol/kg where an initial drop in glycogen utilization (and an
increase in fat use) occurs. The second occurs at 40 mmol/kg which is near the glycogen levels
where exhaustion occurs during continuous exercise. Below 40 mmol/kg, glycolysis appears to be
impaired although the exact mechanism is unknown.
With carbohydrate supercompensation the opposite results are seen. Glycogen utilization
during exercise and at rest increases with a decrease in fat utilization. Exercise heart rate and
oxygen uptake during exercise and an increased in time to exhaustion. Whether this adaptation
would occur following longer term adaptation to a ketogenic diet followed by carb-loading is
discussed in the next section.
Thus, from a purely performance standpoint, at least in the short-term, it appears that
carbohydrates are still the body’s preferred fuel. Fat is simply unable to sustain optimal
performance at high intensities.
Section 6: Long-term ketogenic diet and
endurance activityAlthough many studies have examined the effects of short-term glycogen depletion with a
high fat diet, only a few have examined the long-term effects of a ketogenic diet on endurance
performance. As we shall see, there appears to be a difference between short-term glycogen
depletion and long-term ketogenic adaptation. Please note that these studies generally did not
examine the effects of exercise on fat loss on a ketogenic diet. Rather, they simply examined
what types of exercise could be sustained on a diet devoid of carbohydrates.
Studies on longer term ketogenic (2 to 6 weeks) diets find either a maintenance (73,74) or
increase (75,76) in endurance at low-intensity exercise (75% of maximum heart rate and below).
At higher intensities (around 85% of maximum heart rate which is likely above the lactate
threshold) performance decreases (77). As higher intensity exercise relies proportionally more on
carbohydrate for fuel, this would be expected to occur. A recent review of the ketogenic diet and
exercise literature (78) has criticized the one study showing a performance decrement (77) on the
basis that high-intensity exercise (85% of maximum) was inappropriate for untrained individuals.