Your muscles refuse to share their stored carbohydrates
When your liver stores glycogen, it breaks it down into free glucose to stabilize blood sugar and feed your brain. But skeletal muscle lacks an essential enzyme called glucose-6-phosphatase. Once glucose enters a muscle cell and becomes glycogen, it is permanently locked inside. Your muscles can burn this fuel for their own contractions, but cannot release a single molecule back into the bloodstream to help starving tissues elsewhere in your body.
Two Storage Depots, Two Opposing Purposes
Carbohydrates consumed in the diet are broken down into simple sugars, entering the bloodstream primarily as glucose. To prevent excessive surges in blood sugar and to preserve fuel for periods between meals, the body converts surplus glucose into glycogen, a densely packed, branched polymer. Glycogen is deposited almost exclusively in two tissues: the liver and skeletal muscle. While both tissues assemble the exact same polymer from identical building blocks, their physiological reasons for doing so could not be more distinct.
The liver acts as a central metabolic buffer for the entire organism. Hepatocytes store glycogen when nutrients are plentiful and steadily dismantle it during fasting, releasing free glucose into systemic circulation to maintain a steady baseline. This steady supply is indispensable for obligate glucose consumers, such as mature red blood cells, which lack mitochondria, and the central nervous system, which relies on a constant flow of blood-borne sugar to function. The liver's glycogen pool is a public utility, designed specifically to be shared with any organ in need.
Skeletal muscle, by contrast, operates under a strictly localized mandate. Together, the muscles of the human body contain a larger total quantity of glycogen than the liver simply because muscle comprises a much greater share of overall body mass. Yet none of this substantial reservoir exists to support other tissues. Muscle glycogen is synthesized and held solely to fuel muscular work, serving as a private, high-speed energy cache that is spent exclusively on mechanical contractions.