Glycogen is a storage form of glucose.
While we can store an almost limitless amount of fat in our adipose tissues, the glycogen storage is limited to about a pound. Our muscles store about 75% of glycogen and the rest of it is stored in the liver. Liver glycogen is an important source of energy for our brain while muscle glycogen supplies an immediate source of energy during exercise.
The issue with eating carbs during and for intense exercise is that blood flow to your stomach is restricted as it’s being directed to the muscles being worked. The energy you’re consuming is not immediately available for those exercising muscles. That’s why it’s helpful for athletes to have a large storage.
We also know that the higher the exercise intensity, higher the portion of energy coming from glucose. For example, at rest, as much as 70% of energy is derived from fat, but when the maximum intensity is reached, it is 100% glucose, not fat!
In elite athletes, glycogen storage size determines the intensity they can obtain. It’s no wonder why Michael Phelps switched to a very high carbohydrate diet where about 75% of total calories came from carbohydrates while training. This is also the reasoning behind why endurance athletes like to carb-load before a big race.
According to the Biochemistry Primer for Exercise Science (Michael Houston, Human Kinetics), the amount of glycogen you can store is reduced to negligible to less than half on a low carb diet while it can be almost doubled on a high carb diet. However, eating more is not enough. One needs to use it up by training to maintain a large storage of glycogen.
Therefore, if a person is a couch potato, the max amount of glycogen is not important, but for an active person, it does make a huge difference. This explains why some Kenyan runners can run for a long time without consuming energy drinks as their storage may be larger than American runners who eat more protein, but not enough carbs and antioxidants which come from plant-based foods.
Written from an interview with Kazuko “Kaz” Aoyagi, PhD, who founded the KAZ Personal Fitness Program in 1996. Answers were edited for length and clarity.