What is Glycogen?
Glycogen, a polysaccharide, is the main form of carbohydrate storage in animals. Glycogen is stored primarily in the liver and muscle tissues. Glycogen is readily converted to glucose as needed by the body to satisfy its energy needs.
The Difference Between Liver and Muscle Stores
Liver glycogen stores are more readily available for energy and blood sugar maintenance. The central nervous system relies heavily on liver glycogen for energy. Muscle glycogen stores are used solely for muscle fuel because muscle lacks the enzyme G-6-P needed to make glycogen available to other tissues.
Glycogen forms hydrogen bonds with water molecules. This means about 1 gram of carbohydrates is stored with about 3 grams of water. "The 70kg "average man" stores only an 18-hour fuel supply as glycogen compared with a 2-month supply stored as fat" (Mahan). Fun fact: if all human energy stores were glycogen, humans would need to weight an additional 60 pounds!
Glycogen and Exercise
About 150 grams of glycogen is stored in muscle. This number can be increased five-fold with physical training. If carbohydrates are consumed immediately following strenuous exercise, about 50% more glycogen can be stored.
Glycogen and Animals
In a well-rested animal, the glycogen content of the muscle is high and at slaughter residual glycogen converts to lactic acid. This lactic acid causes meat to become tender. With high stress levels during slaughter, epinephrine and stress hormones cause depletion of glycogen stores. The lactic acid level is then reduced and the meat quality is altered.