Liver Glycogen

Muscle glycogen cannot maintain blood glucose for the brain because muscle cells cannot release glucose into circulation. The liver glycogen floor is the central variable determining when the body shifts toward fat oxidation, gluconeogenesis, and ketone production. When liver glycogen falls low enough, the metabolic switch must occur even if muscle glycogen remains available. Liver glycogen is substantially depleted after about 12 to 16 hours of fasting in a sedentary person eating a mixed diet.