Understanding Your Energy Reserves: How long do glycogen stores last in starvation?

October 16, 2025 •

5 min read

For a healthy adult, liver glycogen stores, which are crucial for maintaining blood sugar, can be almost entirely depleted within 24 hours of fasting. A deeper understanding of how long do glycogen stores last in starvation provides insight into the body's remarkable and complex metabolic adaptations for survival.

Quick Summary

Glycogen reserves serve as short-term energy stores, depleted in roughly 12 to 24 hours of fasting. The body then undergoes a metabolic shift, transitioning from carbohydrate burning to producing alternative fuel from fat stores through ketosis to sustain energy needs.

Key Points

Liver Glycogen Depletion: A healthy adult's liver glycogen stores, used to maintain blood glucose, are typically exhausted within 12 to 24 hours of fasting.
Muscle Glycogen Use: Muscle glycogen is primarily for local muscle activity and not directly used to regulate overall blood sugar levels, though it contributes to total body energy expenditure.
Metabolic Transition to Ketosis: After 24-48 hours, with glycogen depleted, the body transitions to burning fat and producing ketones for energy, a process called ketosis.
Survival Mechanism: Ketosis is an evolutionarily crucial adaptation that allows the brain and other tissues to use fat-derived ketones as fuel, sparing muscle tissue from being broken down for glucose.
Individual Variation: The exact timeline for glycogen depletion and the onset of ketosis varies based on factors such as starting glycogen levels, activity level, and dietary history.
Fuel Source Hierarchy: The body preferentially burns glucose first, followed by a transition to fat and ketones, and finally relies on protein for energy only in the most prolonged periods of starvation.

The human body is an efficient energy-management system, utilizing a hierarchy of fuel sources to power its functions. The primary and most readily available source is glucose, derived from dietary carbohydrates and stored as glycogen. However, during periods of prolonged fasting or starvation, this rapid-access fuel is quickly exhausted, forcing the body to adapt its energy strategy. Understanding this intricate process is fundamental to grasping how our bodies survive and adapt under extreme conditions.

The Body's Primary Energy Sources

To power its cells, the body relies on three main macronutrients: carbohydrates, fats, and proteins. Each is utilized differently depending on availability and the body's immediate needs.

Carbohydrates: These are the body's preferred source for quick energy. Once consumed, carbohydrates are broken down into glucose, which is used immediately or stored as glycogen in the liver and muscles for later use.
Fats: Stored in adipose tissue, fats are the body's long-term energy reserve. They are broken down into fatty acids, which can be converted into ketones to fuel most tissues, including the brain during starvation.
Proteins: While essential for building and repairing tissue, proteins from muscle and other tissues are broken down for energy only when carbohydrate and fat reserves are low. This process, known as gluconeogenesis, can supply some glucose to the brain.

The Glycogen Depletion Timeline

The timeline for glycogen depletion is not instantaneous but follows a predictable, multi-staged metabolic process.

The Post-Absorptive Phase (6–24 hours)

This initial stage begins after a meal has been digested and absorbed. Blood glucose levels start to fall, signaling the pancreas to decrease insulin and increase glucagon. The liver then begins to break down its glycogen stores (glycogenolysis), releasing glucose into the bloodstream to maintain a stable blood sugar level for the brain and red blood cells. The liver's glycogen stores can sustain this function for about 12 to 24 hours. Muscle glycogen, while a larger reserve, is primarily used by the muscles themselves and cannot directly contribute to systemic blood glucose levels.

The Gluconeogenic Phase (24–48 hours)

As liver glycogen becomes nearly exhausted around the 24-hour mark, the body transitions to its next metabolic strategy: gluconeogenesis, or the creation of "new glucose". During this phase, the liver synthesizes glucose from non-carbohydrate sources. The primary substrates are amino acids released from the breakdown of muscle protein and glycerol, a byproduct of fat breakdown from adipose tissue. While necessary for producing glucose for the brain, this protein breakdown is unsustainable and potentially harmful over the long term, which is why the body initiates the next phase.

The Ketogenic Phase (2–3+ days)

After approximately 2-3 days of starvation, low insulin levels trigger widespread lipolysis, the breakdown of fat stores. The liver takes up a large number of fatty acids and begins converting them into ketone bodies (acetoacetate, beta-hydroxybutyrate, and acetone) in a process called ketogenesis. These ketones become the brain's primary energy source, reducing its need for glucose and effectively sparing the body's remaining muscle protein. This metabolic shift is a crucial survival mechanism that allows the body to rely on its much larger fat reserves for an extended period.

Factors Influencing Glycogen Depletion

The speed and duration of glycogen depletion are not uniform across all individuals. Several factors play a significant role:

Initial Glycogen Stores: A person's initial glycogen levels are influenced by their body mass, diet, fitness, and recent exercise. An athlete following a high-carbohydrate diet will start with higher stores than a sedentary individual on a low-carb diet.
Physical Activity: Any level of physical exertion during a period of calorie restriction will accelerate glycogen depletion. Intense exercise can deplete muscle glycogen stores in as little as 45-120 minutes, depending on the intensity and duration.
Insulin Sensitivity: Individuals with higher insulin sensitivity may utilize glycogen more efficiently, but the overall timeline for depletion remains similar under strict caloric restriction.
Dietary History: Someone already adapted to a ketogenic or very-low-carbohydrate diet will likely transition into ketosis much faster because their body is already accustomed to fat-burning for fuel.

Comparison of Metabolic States


Feature	Fed State	Early Starvation (12-24 hours)	Prolonged Starvation (2-3+ days)
Primary Fuel Source	Dietary Glucose	Liver Glycogen	Ketones and Fatty Acids
Key Hormonal Signal	High Insulin	High Glucagon	Low Insulin, High Glucagon
Brain Fuel	Glucose	Glucose	Ketones (major) & Glucose (minor)
Protein Sparing	N/A	Low (Some Gluconeogenesis)	High (Protein is conserved)
Metabolic Process	Glycogenesis	Glycogenolysis, Gluconeogenesis starts	Ketogenesis, Gluconeogenesis continues

A Deeper Look at Ketosis and Adaptation

Ketosis is more than just an alternative fuel system; it is an evolutionarily conserved survival mechanism. When ketones become a major fuel source for the brain, it significantly reduces the need for glucose, allowing the body to conserve its limited protein reserves. This metabolic flexibility ensures that critical tissues like the brain can continue to function efficiently during prolonged periods of food scarcity. The process is tightly regulated by hormones like insulin and glucagon, which orchestrate the shift from glycogen utilization to fat- and ketone-based metabolism. This adaptive response helps explain how humans can survive for weeks without food, relying heavily on the energy stored in adipose tissue, with minimal loss of lean muscle mass until fat stores are near exhaustion.

Conclusion: The Body's Adaptive Survival Mechanism

In conclusion, how long do glycogen stores last in starvation depends on the specific store. Liver glycogen, which regulates blood sugar for the whole body, is exhausted within 12 to 24 hours. The much larger muscle glycogen reserves are less accessible for systemic use but are depleted over a few days, depending on activity level. Once these readily available carbohydrate stores are depleted, the body embarks on an impressive metabolic journey, transitioning through gluconeogenesis and into a state of ketosis to utilize its abundant fat reserves. This strategic shift is a testament to the body's robust ability to prioritize function and survival under stress. The timeline varies slightly based on individual factors, but the underlying mechanism remains a powerful evolutionary tool for enduring famine and caloric restriction.

For more in-depth information on the physiological changes during fasting, consult the NCBI Bookshelf article on Physiology, Fasting.

Frequently Asked Questions

Glycogen is a stored form of glucose. It is primarily stored in the liver (about 100g) and in the skeletal muscles (about 400g), serving as a readily available energy reserve.

Liver glycogen stores, which are used to regulate blood sugar, can become almost completely depleted within 12 to 24 hours after you stop eating.

Yes, physical activity significantly accelerates the rate of glycogen depletion. The more intense and prolonged the exercise, the faster your muscle and liver glycogen stores will be used.

After glycogen is depleted, the body begins to produce glucose from non-carbohydrate sources like protein (gluconeogenesis) and switches to burning fat and producing ketones for energy (ketosis).

Physiological ketosis, which occurs during fasting, is a normal and beneficial metabolic state for survival. It should not be confused with diabetic ketoacidosis, a dangerous condition in which blood sugar and ketone levels are both dangerously high.

Once glycogen is depleted and ketosis is established, the body primarily runs on its fat reserves. This allows for survival for many weeks, and sometimes months, depending on the individual's initial body fat and overall health.

The body is very efficient at preserving muscle mass. Significant muscle breakdown for energy typically only occurs during the later stages of prolonged starvation, when fat stores are nearly exhausted.