What happens when glucose is needed by the body?

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When the body needs glucose, glycogenolysis is activated, mainly in the liver, to release free glucose into the blood and keep blood sugar levels steady during fasting or activity. Glycogen phosphorylase cleaves α-1,4 bonds in glycogen to produce glucose-1-phosphate, which is converted by phosphoglucomutase to glucose-6-phosphate. In the liver, glucose-6-phosphatase removes the phosphate to yield free glucose that exits the cell via GLUT2 transporters and enters the bloodstream. Muscle tissue also breaks down glycogen, but the glucose-6-phosphate it produces is used locally for glycolysis to fuel contraction, not released into blood, since muscle lacks glucose-6-phosphatase.

Other ideas don’t fit this immediate need for glucose: glycogen isn’t directly turned into fat in the short term—lipogenesis can occur with excess calories but isn’t the direct outcome of glycogen breakdown; glycogen isn’t secreted as a protein, and glycogenin is a protein primer used to start glycogen synthesis, not a breakdown product.