What Does the Body Feed on When Fasting? The Metabolic Shift Explained

December 11, 2025 •

3 min read

Within the first 12-24 hours of a fast, the body's primary glucose reserves stored in the liver are depleted. This triggers a remarkable metabolic adaptation to ensure continuous energy, providing the answer to what does the body feed on when fasting by shifting fuel sources.

Quick Summary

The body transitions through a series of energy phases when food is restricted, moving from glycogen stores to stored fat for fuel. This metabolic process, called the glucose-to-ketone switch, allows for sustained energy and initiates cellular cleanup processes.

Key Points

Glycogen depletion: In the initial 12-24 hours of fasting, the body uses glucose from stored glycogen, primarily located in the liver, as its main energy source.
Metabolic switch to ketosis: Once glycogen is depleted, the body switches to burning stored fat. The liver produces ketone bodies from fatty acids to fuel the brain and other tissues.
Protein sparing: During prolonged fasting (beyond 72 hours), the body's efficiency at using ketones increases, reducing the need to break down muscle protein for energy.
Cellular recycling (Autophagy): Fasting activates autophagy, a process where cells clear out damaged components, which supports cellular health and can reduce inflammation.
Hormonal shifts: A drop in insulin and a rise in glucagon, growth hormone, and norepinephrine orchestrate the transition from a fed state to a fasted, fat-burning state.
Brain fuel flexibility: The brain, which typically relies on glucose, adapts to efficiently use ketones as an alternative fuel during extended fasting, offering potential neuroprotective benefits.

The Initial Phase: Relying on Stored Glucose

After consuming a meal, your body's preferred source of energy is glucose, derived from the carbohydrates you eat. Any excess glucose is stored in your liver and muscles in a complex carbohydrate form called glycogen. During the initial hours of fasting, typically 4 to 18 hours after your last meal, your body relies on this readily available glycogen for energy.

Under the influence of the hormone glucagon, the liver breaks down its glycogen stores in a process known as glycogenolysis, releasing glucose into the bloodstream to maintain stable blood sugar levels. This is a crucial mechanism, especially for the brain, which relies heavily on glucose for fuel. However, these glycogen reserves are finite and, depending on activity level, are typically exhausted within first day of fasting.

The Shift to Fat and the Onset of Ketosis

As glycogen stores dwindle, the body transitions to its vast fat reserves for fuel, a state known as ketosis. Fat is stored in adipose tissue as triglycerides, which are broken down into fatty acids and glycerol through a process called lipolysis.

Fatty Acids: The liver converts these free fatty acids into ketone bodies (acetoacetate, $\beta$-hydroxybutyrate, and acetone), which can be used by most tissues in the body, including the brain.
Glycerol: The liver can use the glycerol backbone of triglycerides to produce a small amount of new glucose via gluconeogenesis.

Ketosis typically begins after about 12 to 24 hours of fasting and becomes more pronounced with longer fasts. The efficiency with which the body utilizes fat for energy is a key feature of metabolic flexibility.

The Role of Autophagy

Beyond simply providing energy, the fasting state activates a powerful cellular recycling process called autophagy. Autophagy is a mechanism where cells break down and recycle damaged or unnecessary components, a kind of cellular 'spring cleaning'. This process is fueled by the recycled materials and helps maintain cellular health and resilience. It's enhanced during periods of calorie restriction and is thought to contribute to various health benefits associated with fasting, such as reduced inflammation and improved neurological health.

Extended Fasting and Protein Sparing

For extended fasts lasting more than 72 hours, the body becomes highly efficient at using ketones to protect muscle tissue. While some protein breakdown occurs initially to support gluconeogenesis, the body's reliance on ketones reduces the need to break down muscle for energy. This mechanism, known as protein sparing, ensures that essential structural proteins are preserved as much as possible. However, in extreme and prolonged starvation where fat reserves are depleted, the body will catabolize skeletal muscle for energy, leading to muscle mass loss.

The Key Hormonal Changes

Fasting is orchestrated by a precise shift in hormones:

Insulin: Levels drop significantly, which signals the body to stop storing glucose and start mobilizing stored energy.
Glucagon: Rises to promote the breakdown of glycogen and later, the conversion of fatty acids into ketones.
Norepinephrine: Increases, contributing to alertness and stimulating the release of fatty acids from adipose tissue.
Growth Hormone: Levels increase, helping to preserve lean muscle mass and enhance fat breakdown.

Comparing Glucose and Ketones as Fuel


Feature	Glucose (Fed State)	Ketones (Fasted State)
Primary Source	Carbohydrates from diet, liver glycogen	Stored fat (triglycerides)
Main Use	Immediate energy for most cells	Sustained energy, preferred by brain during prolonged fasting
Metabolic State	Insulin-dominant	Glucagon-dominant
Availability	Rapidly available, short-term storage	Gradual release, long-term storage
Brain Fuel	Primary fuel source	Adaptable fuel source, offers neuroprotective effects
Fuel Efficiency	High ATP yield, but higher reactive oxygen species	Efficient, produces fewer reactive oxygen species

Conclusion

Understanding what the body feeds on when fasting reveals a remarkable system of metabolic adaptation and resourcefulness. The body is not a static machine but an intricate network capable of switching its primary fuel source from readily available glucose to more sustainable fat-derived ketones. This metabolic flexibility, driven by hormonal shifts, ensures that the body can maintain essential functions during periods of food scarcity. From utilizing glycogen to activating cellular repair through autophagy and preserving protein during extended fasts, the process showcases the body's innate intelligence in maintaining homeostasis. While the benefits of these adaptive mechanisms are increasingly studied, it is essential to approach fasting with a complete understanding of its physiological effects. For more detailed clinical insights, refer to sources like this study from the National Institutes of Health.

Frequently Asked Questions

The body typically begins transitioning to using stored fat for fuel, a process called ketosis, after about 12 to 24 hours of fasting, once its glycogen stores are mostly depleted.

For shorter fasts, significant muscle loss is unlikely due to the body's protein-sparing mechanisms. The body prioritizes using fat and becomes highly efficient at it, especially in prolonged fasting (over 72 hours), to protect lean muscle mass.

Ketone bodies are an alternative energy source to glucose. They are produced by the liver from fatty acids and can fuel the brain and other tissues during prolonged fasting.

The liver plays a central role in fasting metabolism. It first breaks down its stored glycogen for glucose and later converts fatty acids from fat tissue into ketone bodies to supply energy to the body.

Yes, fasting causes significant hormonal shifts. Insulin levels decrease, while glucagon, norepinephrine, and human growth hormone levels increase to promote the mobilization of stored energy and preserve muscle.

Autophagy is a cellular process where the body cleans out and recycles old or damaged cell components. It is activated during fasting and can promote cellular health and longevity.

From a cellular health perspective, ketones are often described as a 'cleaner' fuel than glucose. While glucose provides rapid energy, ketones produce fewer reactive oxygen species, which can reduce oxidative stress and potentially support cellular longevity.