Glycogen is essential for high intensity exercise performance. A review concluded that elevated glycogen concentration can improve performance by 2-3% and endurance capacity by 15-25%. Muscle glycogen concentrations can be increased by eating a diet that is rich in carbohydrate. However, studies in the 70s suggested that extreme glycogen loading protocols resulted in very high muscle glycogen concentrations. These protocols employed combinations of high carbohydrate days, low carbohydrate days and extreme exercise to achieve this (see a previous blog). Athletes successfully used these carb loading protocols.
Extreme glycogen loading protocols
The protocols that were initially used were extreme, with glycogen depleting exercise 7 days before competition, 3 days of no carb intake, 3 days of extremely high carb intake, no training for 6 days before the race. Needless to say the protocol was not practical and had a lot of disadvantages. But the undeniable end result was glycogen supercompensation on the day of competition.
There are a few questions though that have not been answered. These early studies compared a low glycogen situation with a high glycogen situation. To the best of my knowledge, studies have not investigated normal muscle glycogen with high muscle glycogen or high versus very high muscle glycogen concentrations.
If we modify substrate (fuel) availability through dietary manipulation (such as carbohydrate loading regimens) this will affect the regulation of metabolism during endurance exercise. A number of metabolic pathways and control points are involved. Most notably, increasing the muscle glycogen concentration will also enhance the speed with which muscle glycogen is broken down to pyruvate during exercise (a process called glycogenolysis) (1). The enzyme responsible for the breakdown of glycogen (phosphorylase) is more active with higher glycogen concentrations.
Increasing muscle glycogen concentration will also enhance the speed with which muscle glycogen is broken down to pyruvate during exercise.
There is another effect. In addition to glycogenolysis, muscle glycogen also appears to be a strong regulator of another key enzyme in glycogen metabolism: pyruvate dehydrogenase (PDH). This enzyme breaks down pyruvate to acetyl-CoA and is a rate limiting step for carbohydrate oxidation. Indeed, starting exercise with higher muscle glycogen results in a greater exercise-induced increase in PDH activity (2), and starting with lower glycogen reduces it.
So, are extreme glycogen loading protocols necessary?
As I mentioned, there are very few studies that have directly addressed the differences between high and very high muscle glycogen. In a study by Melissa Arkinstall and colleagues (3) it was observed that glycogen utilisation was higher during exercise at 45% VO2max that was commenced with high glycogen as opposed to exercise at 70% VO2max commenced with low glycogen concentration, despite the higher intensity.
From a very practical point of view. Imagine running a marathon in 3-4 hours and starting with either high or very high muscle glycogen concentrations. The infographic is a theoretical modelling of what would happen with muscle glycogen concentrations. The first hour there would be a difference in the concentration but in both conditions there would be sufficient glycogen to run at high pace. After 2 hours in both conditions the glycogen concentrations will become depleted and the difference between conditions is now minimal. Towards the end of the marathon glycogen concentrations are probably similar. So is there really an advantage of a more extreme carb loading protocol?
Conclusions
One could argue that there still might be a small difference because in the first hour running would be more economical. The higher carbohydrate oxidation in the first hour will require less oxygen. The counterargument would be that oxygen uptake is never a limiting factor in this scenario. So until someone carefully conducts the study where high glycogen and very high glycogen are compared, we may never know, but it seems that any advantages of extreme carb loading might be outweighed by the disadvantages of such regimens in the week leading up to a race.
References
Hargreaves M., McConell G., Proletto J. Influence of muscle glycogen on glycogenolysis and glucose uptake during exercise in humans. J. Appl. Physiol. 1995;78:288–292
Kiilerich K., Gudmundsson M., Birk J.B., Lundby C., Taudorf S., Plomgard P., Saltin B., Pedersen P.A., Wojtaszewski J.F.P., Pilegaard H. Low muscle glycogen and elevated plasma free fatty acid modify but do not prevent exercise-induced PDH activation in human skeletal muscle. Diabetes. 2010;59:26–32
Arkinstall M.J., Bruce C.R., Clark S.A., Rickards C.A., Burke L.M., Hawley J.A. Regulation of fuel metabolism by pre-exercise muscle glycogen content and exercise intensity. J. Appl. Physiol. 2004;97:2275–2283
