How Glycogen works and stores..

The Basics

Glycogen is a branched polymer of glucose stored in the liver and muscles that acts as a fuel source for exercise. Glycogen metabolism is the process by which these stored carbohydrates are used as fuel, involving many enzymes with chemical compositions that fill a quarter of a page. While the glycogen-to-fuel process is complex enough to fuel many PhD dissertations, the basic takeaway is that excess carbohydrates are stored as glycogen in the liver and muscles, turning pasta into performance.

Among other things, the breakdown of glycogen is used in ATP synthesis, which is essentially how energy transfer happens in cells. Even though glycogen only accounts for a minimal amount of total stored energy in the body, lower-burning fat takes too long to go through the same process to fuel sustained moderate exercise, so at around 55 to 65 percent of VO₂ max for most athletes, glycogen stores become essential to maintain performance levels (though exact intensity depends on many athlete-specific factors).

Think of fat and carbohydrate burning on a spectrum, with high-intensity efforts involving mostly carbohydrates and low-intensity efforts involving mostly fat. Both energy sources are important for running.

According to cycling expert Dr. Iñigo San Millán, at race pace, most athletes will burn two to three grams of carbohydrate per minute. Even at lower intensities, most athletes will burn one to two grams of carbohydrate a minute (though this rate can be adjusted with training). Most athletes store 300 to 500 grams of glycogen when fully fuelled, equating to about 90 to 120 minutes of intense exercise.

Glycogen burns rapidly but is refilled at a drip, usually replenishing at a rate of two to five percent per hour after exercise. Empty glycogen stores can take a full day or more to restore.

Important 

During workouts, replenish glycogen as you go.

As glycogen levels drop, so, too, does performance for most athletes. For workouts long enough to begin burning stored glycogen (usually 60 to 90 minutes or longer), practice refuelling as you go. For most athletes, 200 to 300 calories per hour of mostly carbohydrates—like gels or sports drink—is a safe bet, adjusting for body type and background.

QUESTION:

How much glycogen the liver and muscles can store?

In humans, glycogen is made and stored primarily in the cells of the liver and skeletal muscle. In the liver, glycogen can make up 5–6% of the organ's fresh weight, and the liver of an adult, weighing 1.5 kg, can store roughly 100–120 grams of glycogen.