Glycogen is a complex carbohydrate and the primary storage form of glucose, the body’s main energy source. Glucose molecules are linked together to form this highly branched polysaccharide, which is stored mainly in two locations: the skeletal muscles and the liver. Muscle glycogen fuels muscle activity, while liver glycogen plays a systemic role in supplying fuel to the entire body.
Quantifying Liver Glycogen Storage Limits
The adult human liver has a finite capacity to store glucose. Maximum storage in a healthy adult liver ranges from approximately 80 to 120 grams, equivalent to about 320 to 480 calories of energy. This high concentration typically makes up 5% to 6% of the liver’s total fresh weight. While this limit is substantially lower than the 400 grams stored collectively in skeletal muscles (in a 70 kg person), the liver is more efficient on a per-weight basis, as muscle glycogen only constitutes 1% to 2% of total muscle mass.
The Liver’s Essential Role in Glucose Homeostasis
Liver glycogen acts as the central glucose buffer for the entire body, regulating blood sugar levels within a narrow, healthy range. This function is important for organs like the brain, which relies almost exclusively on a steady supply of glucose for fuel. The liver manages this balance through two processes: glycogenesis and glycogenolysis.
After a meal, the liver converts excess glucose into glycogen for storage (glycogenesis). When blood sugar drops between meals, the liver initiates glycogenolysis, breaking down stored glycogen and releasing free glucose directly into the bloodstream. Muscle cells lack the necessary enzyme (glucose-6-phosphatase) to release glucose into the circulation, meaning their glycogen is reserved strictly for their own immediate energy needs.
How Diet and Activity Affect Liver Glycogen Levels
Liver glycogen levels fluctuate in response to diet and physical activity. Following a carbohydrate-rich meal, the liver takes up glucose from the bloodstream, replenishing its stores at an average rate of around six grams per hour in the first few hours after eating. Conversely, fasting or low carbohydrate intake causes depletion as the liver maintains blood sugar. Liver glycogen stores can become almost completely depleted within 12 to 24 hours of continuous fasting, prompting the body to shift its primary energy source to fat breakdown. Intense, prolonged physical exercise also draws upon liver glycogen to release glucose into the blood to fuel working muscles and the brain.