What are the fuel sources during starvation?

December 9, 2025 •

3 min read

Within 24 hours of starting a fast, the human body exhausts its primary carbohydrate reserves. In response, it initiates a series of complex metabolic shifts to find alternative sources of energy, revealing precisely what are the fuel sources during starvation.

Quick Summary

The body uses glycogen stores for the first 24 hours of starvation before shifting to fat for energy. As fasting continues, the liver produces ketones from fatty acids to fuel the brain, conserving muscle protein. Eventually, the body catabolizes protein from muscle tissue for glucose, leading to muscle wasting.

Key Points

Initial Glycogen Use: The body first burns stored glycogen in the liver and muscles for approximately 24 to 48 hours to maintain blood glucose levels.
Fat Becomes Primary Fuel: After glycogen is depleted, the body transitions to burning stored fat via lipolysis, using fatty acids for energy and conserving vital protein.
Ketone Bodies Fuel the Brain: During prolonged starvation, the liver converts fatty acids into ketone bodies, which can cross the blood-brain barrier to serve as the brain's main fuel source, replacing glucose.
Protein Sparing Mechanism: The shift to fat and ketones is a crucial survival tactic, as it prevents the rapid breakdown of muscle protein that would otherwise be needed for glucose production.
Final Stage Protein Catabolism: Only when fat reserves are exhausted does the body resort to breaking down its own muscle and organ protein for energy, leading to severe tissue degradation.
Metabolic Rate Reduction: An adaptive drop in the basal metabolic rate helps conserve energy during prolonged caloric deprivation, further prolonging survival.

The Initial Phase: Glycogen Depletion

In the immediate hours following a meal, the body enters a post-absorptive state where it uses circulating glucose for energy. Once this glucose is depleted, the body turns to its stored carbohydrates, primarily glycogen, which is stored in the liver and muscles.

Liver Glycogen: The liver contains approximately 100 grams of glycogen, which it can rapidly convert into glucose and release into the bloodstream to maintain stable blood sugar levels for tissues, especially the brain and red blood cells.
Muscle Glycogen: Muscles store 300–400 grams of glycogen, but this is reserved for the muscle's own use and cannot be directly released into the bloodstream for other organs.

This initial phase, known as the glycogenolytic phase, lasts for about 24 to 48 hours. After this period, the body's glycogen reserves are largely exhausted, and it must find more durable fuel sources to sustain itself.

The Transition to Fat: The Ketogenic Phase

Once glycogen stores are depleted, the body shifts its primary fuel source to its vast reserves of fat, stored as triglycerides in adipose tissue. This metabolic phase, called the ketogenic phase, is a key survival mechanism designed to spare muscle protein.

How the body burns fat and produces ketones

Lipolysis: Fat cells break down stored triglycerides into fatty acids and glycerol.
Fatty Acid Oxidation: Most tissues, including skeletal and cardiac muscle, can directly oxidize fatty acids for energy. However, fatty acids cannot cross the blood-brain barrier.
Gluconeogenesis from Glycerol: The small amount of glycerol released during lipolysis travels to the liver, where it is converted into glucose through gluconeogenesis. This process helps provide some glucose for the brain, which still requires a baseline amount.
Ketone Production: To provide fuel for the brain, the liver converts fatty acids into ketone bodies (acetoacetate, β-hydroxybutyrate, and acetone) via a process called ketogenesis. These water-soluble molecules can cross the blood-brain barrier, offering a vital alternative fuel source.

As starvation progresses over several weeks, the brain can derive up to 75% of its energy from ketone bodies, significantly reducing its dependence on glucose and conserving the body's limited protein.

The Final Stage: Protein Catabolism

When the body's fat reserves are exhausted, a final, desperate stage of starvation begins. The body has no choice but to break down its own functional proteins to provide glucose for the brain and other glucose-dependent tissues.

Proteolysis: Muscle and other tissues are catabolized, releasing amino acids into the bloodstream.
Gluconeogenesis from Amino Acids: These amino acids travel to the liver and kidneys, which convert them into glucose through gluconeogenesis.
Widespread Tissue Degradation: This process leads to severe muscle wasting and impairs the function of vital organs. Ultimately, the progressive degradation of critical tissues and electrolyte imbalances lead to death.

Comparison Table: Fuel Source Transition During Starvation


Feature	Phase 1: Glycogen Depletion (0–48 hours)	Phase 2: Fat & Ketone Utilization (Days to Weeks)	Phase 3: Protein Catabolism (Late Stage)
Primary Fuel Source	Stored glycogen in liver and muscle	Fat (fatty acids and glycerol)	Protein from muscle and organs
Key Process	Glycogenolysis	Lipolysis and ketogenesis	Proteolysis and gluconeogenesis
Brain's Fuel	Glucose	Mix of glucose and ketones (primarily ketones later)	Glucose derived from amino acids
Protein Sparing	Minimal sparing	Significant sparing	None; protein is actively broken down
Survival Strategy	Short-term energy provision	Long-term adaptation	Last-resort survival mechanism
Body Weight Change	Initial water and glycogen loss	Steady weight loss	Rapid, severe weight and muscle loss

Conclusion

Survival during starvation is a meticulously orchestrated biological process that shifts the body's energy sourcing from carbohydrates to fat and ultimately to protein. Beginning with the rapid consumption of glycogen, the body quickly adapts to rely primarily on its extensive fat reserves, producing ketone bodies to preserve the brain's function. This fat-based metabolism is an efficient, protein-sparing adaptation crucial for extended survival. However, when fat stores are exhausted, the body's last line of defense is the breakdown of its own muscle tissue. This terminal phase of using protein for fuel highlights the extreme physiological stress of prolonged starvation and ultimately leads to critical organ failure. Understanding this metabolic journey reveals the remarkable yet fragile adaptability of the human body in the face of caloric deprivation.

For more information on the body's stress response during starvation, you can visit a source like the Life in the Fast Lane medical resource site for clinical context.

Frequently Asked Questions

The body can typically survive on its initial glycogen stores for about 24 to 48 hours before needing to tap into its fat reserves.

The brain cannot directly use fatty acids for energy because they cannot cross the specialized blood-brain barrier. Instead, the liver must convert fatty acids into ketone bodies, which the brain can use.

The liver is a central player during starvation, managing multiple processes. It releases glucose from glycogen, converts glycerol into glucose, and produces ketone bodies from fatty acids to fuel the brain and other tissues.

The body begins breaking down muscle and other proteins for energy only after its fat reserves are largely exhausted. This marks the final, most damaging stage of starvation.

Ketone bodies are water-soluble molecules produced by the liver from fatty acids. They are critical because they provide an alternative fuel source that can cross the blood-brain barrier, allowing the brain to function even when glucose is scarce.

No, a short fast (e.g., overnight) and true starvation are distinct metabolic states. In a short fast, the body primarily relies on liver glycogen. Starvation begins only after these stores are depleted and the body begins using fat and protein for fuel.

Yes, during prolonged starvation, the body's basal metabolic rate can decrease by up to 30%. This is an adaptive response to conserve energy and prolong survival.