Fueling around with glycogen

Glycogen is a versatile fuel that provides energy for muscle contraction anaerobically, by substrate-level phosphorylation when glycogen is metabolized to lactic acid, and aerobically, by providing glycogen-derived pyruvate and NADH for oxidative metabolism. Both are required for normal muscle function. Anaerobic glycogenolysis is used to power muscle contractions during maximal exertion since it is able to generate ATP at more than twice the rate of oxidative phosphorylation.¹ Glycogen also becomes a critical oxidative substrate in the transition from rest to exercise and in supporting maximal rates of oxidative phosphorylation.¹ Glycogen depletion during sustained aerobic activities results in fatigue, and is a rationale for “glycogen loading” before endurance exercise. A survival feature of muscle glycogen is suggested by skeletal muscle lacking the enzyme glucose-6-phosphatase, thus preserving intramuscular glycogen for the energetic needs of fight or flight even in a state of severe caloric restriction,² whereas liver glycogen is liberated as glucose to support the metabolic requirements of other tissues.

A high demand for limited muscle glycogen requires tight regulation of glycogen phosphorylase, the rate-limiting …