Glycogen

Glycogen is the critical fuel supply for endurance running.

• Glycogen is formed primarily from the carbohydrates we consume and is stored in our livers and muscles.
• The glycogen in our livers can leave the liver and flow via our blood to our muscles, brains and other organs.
• The human liver typically stores between 90 and 160 grams of Glycogen, or 350 to 650 Calories.
• Blood typically contains less than 20 calories of glucose.
• The glycogen in our muscles can only be used by those muscle fibers.
• Glycogen can also be created from protein via a process called gluconeogenesis, but not from fat.

Further reading

For more details see http://en.wikipedia.org/wiki/Glycogen