The body stores carbohydrates as glycogen, a large, branching polymer of glucose molecules. Liver glycogen (hepatic glycogen) functions as the body’s readily accessible glucose reserve, used to maintain stable blood sugar levels. The total amount stored in the liver is relatively small, typically around 80 to 100 grams in a healthy adult. How long these stores last before exhaustion is highly variable and depends on an individual’s metabolic state, recent diet, and physical activity level.
Depletion Time During Rest
During rest or complete fasting, the liver’s glycogen reserves serve as the primary fuel source for glucose-dependent tissues. The liver continuously breaks down this stored glycogen into glucose via glycogenolysis, releasing it into the bloodstream to feed the brain and red blood cells. This mechanism is primarily stimulated by the hormone glucagon as blood glucose levels begin to drop.
The initial hours of fasting show the most rapid use of stored glucose. Within the first four to eight hours after a meal, liver glycogen breakdown contributes significantly to maintaining blood sugar. For a typical adult, liver glycogen stores are depleted, or nearly depleted, within 12 to 24 hours of complete calorie restriction. This time frame marks a significant metabolic shift, as the body can no longer rely on these quick-access carbohydrate reserves to stabilize blood glucose.
The glycogen stored in skeletal muscles is metabolically separate from the liver’s reserves. Muscle glycogen is reserved for the energy needs of the muscle cells themselves and is not released into the bloodstream to support blood sugar maintenance. Once the hepatic stores are exhausted, the body must activate a new, more complex energy production pathway to prevent a drop in blood glucose.
How Exercise Accelerates Glycogen Use
Physical activity accelerates the rate at which the body consumes its stored energy, including liver glycogen. The intensity and duration of the exercise are the main factors that determine the speed of depletion. During moderate- to high-intensity exercise, the body’s demand for immediate glucose fuel increases substantially, leading to a faster breakdown of hepatic reserves.
For an athlete engaging in prolonged, high-intensity endurance activities, such as a marathon run or intense cycling, liver glycogen can be depleted at a rapid rate. If such activity is performed in a fasted state, these stores may be reduced to nearly zero within 90 minutes to two hours. This rapid exhaustion is a primary factor contributing to the sudden onset of fatigue described by endurance athletes.
In contrast, lower-intensity activities, such as a brisk walk, rely more heavily on the breakdown of fat stores for energy. This reliance on fat spares the limited glycogen reserves in the liver and muscles, prolonging the time before depletion occurs. Even low-intensity activity increases overall energy expenditure compared to rest, meaning the total time to deplete liver glycogen will still be shorter than during complete rest or sleep.
The Shift to Alternative Fuel Sources
Once the liver glycogen stores are exhausted, the body initiates a metabolic transition to ensure that blood glucose levels remain stable. This adaptive process is called gluconeogenesis, which translates to the creation of new glucose. The liver becomes the primary site for this process, synthesizing glucose from non-carbohydrate sources.
The precursors for this new glucose include lactate (a byproduct of muscle activity), glycerol (released from the breakdown of fat tissue), and certain amino acids derived from protein breakdown. Gluconeogenesis begins to increase as soon as liver glycogen starts to drop, but it peaks in activity after about 24 hours of fasting, producing the glucose needed to sustain the brain and other glucose-dependent cells. This process prevents severe hypoglycemia, which would otherwise impair brain function.
As fasting or carbohydrate restriction continues, the body further adapts by increasing the breakdown of fat into free fatty acids and then into ketone bodies, a process called ketogenesis. Ketone bodies become an alternative fuel source, particularly for the brain, which significantly reduces the body’s dependence on glucose. While gluconeogenesis maintains a minimal glucose supply, the initiation of full nutritional ketosis typically takes two to four days, providing a sustainable, long-term energy source derived from fat.
Restoring Glycogen Reserves
The process of restoring liver glycogen is efficient once carbohydrate intake resumes. The speed of replenishment depends mainly on the amount and type of carbohydrates consumed and the degree of initial depletion. Following a period of intense exercise or fasting, the body is primed to rapidly absorb and store incoming glucose.
Consuming carbohydrates immediately after a period of depletion significantly speeds up the restoration process. The liver, which prioritizes its own reserves, can be fully repleted in as little as six hours post-exercise if optimal amounts of carbohydrates are ingested. Combining glucose with fructose is particularly effective for liver glycogen restoration, as fructose is preferentially metabolized by the liver.
Complete restoration of both liver and muscle glycogen reserves, especially after an endurance event, often requires 24 hours or more of consistent carbohydrate intake. The body uses the incoming glucose to rebuild these stores, ensuring that the readily available fuel is in place for future energy demands. This replenishment phase returns the body to a state of full energy readiness.