When calories from food are reduced, the body gets needed energy by releasing its stores of glycogen, a type of carbohydrate found in the muscles and liver.

The glycogen of animal tissues, especially liver is rapidly depolymerized to D-glucose after slaughter, and immediate deep freezing is required to preserve the glycogen.

Carbohydrates were first named according to their natural sources; e.g., beet sugar, cane sugar, grape sugar, malt sugar, milk sugar, cornstarch, liver glycogen, and sweet corn glycogen.