What Does the Body Use When You Don't Eat? The Science of Fasting

December 28, 2025 •

2 min read

According to a 2024 review, fasting has been a practice in diverse cultures for ages, involving a variety of periods of abstaining from food. When you don't eat, the body undergoes a series of metabolic adaptations to maintain energy balance and survive. This intricate process involves switching fuel sources, signaling hormonal changes, and eventually prioritizing vital organs when fuel becomes scarce.

Quick Summary

The body uses stored carbohydrates, then fat, and finally protein for energy during periods without food. This process, which involves glycogenolysis, ketogenesis, and gluconeogenesis, is controlled by hormonal signals like glucagon and insulin to ensure energy homeostasis.

Key Points

Glycogen Depletion: The body first burns stored carbohydrates (glycogen) from the liver and muscles within 24 hours of not eating.
Ketosis Initiation: After glycogen is gone, the body shifts to burning fat, producing ketones for fuel in a process called ketosis.
Fat Powers the Brain: During ketosis, the brain can use ketones as a primary energy source, reducing its need for glucose.
Gluconeogenesis: The liver can create new glucose from non-carbohydrate sources like amino acids during fasting to fuel certain cells.
Protein Sparing vs. Breakdown: Early fasting conserves protein, but prolonged starvation forces the body to break down muscle and organ tissue for energy, leading to wasting.
Hormonal Control: Hormones like glucagon and insulin regulate the switch between fuel sources to maintain stable blood sugar levels and energy balance.
Starvation Risk: Extreme, prolonged fasting can deplete fat stores entirely, leading to dangerous muscle breakdown and potential organ failure.

The Initial Hours: Glycogen is Your First Fuel Source

After eating, the body enters an absorptive state, using and storing nutrients. As blood glucose drops, it shifts to the post-absorptive phase, 4-18 hours after eating. Glucagon is released, signaling the liver to break down stored glucose (glycogenolysis). The liver provides glucose for general use, while muscle glycogen fuels muscles during activity. This provides quick energy for the brain. Liver glycogen depletes in about 24 hours, requiring alternative fuels.

The Shift to Ketosis: Fat Becomes the New Fuel

With glycogen depleted, the body breaks down fat. The liver converts fatty acids from fat cells into ketone bodies (acetoacetate, beta-hydroxybutyrate, acetone). This state, with high ketones in the blood, is ketosis. Ketones are an alternative fuel for many organs, including the brain. Using fat conserves protein stores, vital for organs and muscle.

The Process of Ketogenesis

Fat Breakdown: Triglycerides in fat cells become fatty acids and glycerol.
Liver Conversion: The liver turns fatty acids into ketones.
Brain Fuel: Ketones fuel the brain and other tissues.
Protein Sparing: Ketone use reduces protein breakdown.

Gluconeogenesis and Protein Breakdown During Starvation

Some tissues, like red blood cells, still need glucose even in ketosis. Gluconeogenesis creates new glucose from non-carbohydrates. Short fasts use glycerol from fat. During prolonged starvation, with depleted fat, the body breaks down protein, mainly muscle, for amino acids for gluconeogenesis. This muscle breakdown is a survival tactic but causes wasting. The body slows this by using ketones more, but can't stop it entirely in extended starvation. Vital organs get priority fuel, and the metabolic rate slows.

Comparison of Energy Source Utilization During Fasting


Stage of Fasting	Primary Energy Source	Secondary Fuel Source	Duration (Approximate)
Post-Absorptive (Early Fasting)	Liver Glycogen	Muscle Glycogen	4-18 hours
Early Fasting (Fat Adaptation)	Stored Body Fat	Gluconeogenesis (from glycerol)	18-48 hours
Prolonged Fasting (Ketosis)	Ketone Bodies (from fat)	Gluconeogenesis (from protein)	48+ hours
Starvation (Extreme Fasting)	Protein (Muscle and Organ Tissue)	Minimal Ketones	Weeks to months

The Hormonal Orchestration

Hormones control these fuel shifts. Eating raises insulin, storing glucose. Fasting drops insulin, increases glucagon, releasing stored glycogen. Later, HGH increases, protecting muscle. This system maintains energy homeostasis.

Conclusion: A Masterclass in Metabolic Adaptation

The body's response to not eating shows metabolic flexibility. It starts with glycogen, then shifts to fat via ketosis, sparing protein. Short fasts can offer benefits like improved insulin sensitivity, but prolonged starvation is dangerous, breaking down vital tissues. Consult resources like the National Institutes of Health for more information. Understanding this process highlights the body's resilience.

Frequently Asked Questions

The first thing your body uses is glucose from your bloodstream. As those levels drop, it accesses its readily available stores of glucose, known as glycogen, which are located in your liver and muscles.

The body typically transitions from burning glycogen to burning fat after the liver's glycogen stores are depleted, which can take approximately 18 to 24 hours. This shift leads to the metabolic state of ketosis.

Yes, during prolonged fasting, the brain can adapt to use ketone bodies, which are produced from the breakdown of fat, for up to 70% of its energy needs. This reduces the brain's reliance on glucose and helps spare muscle protein.

In the early stages of fasting, the body works to conserve muscle protein. However, during prolonged starvation, when fat stores are exhausted, the body will begin to break down muscle and organ tissue for energy.

While controlled intermittent fasting can have health benefits and lead to weight loss, prolonged and severe calorie restriction is dangerous. It can lead to severe health issues, including nutrient deficiencies, muscle loss, and metabolic slowdown. Always consult a healthcare professional before significant dietary changes.

Hormones are key regulators. As blood glucose falls, insulin decreases, while glucagon increases to promote glycogen breakdown and gluconeogenesis. Other hormones, like human growth hormone, also increase to help conserve muscle mass.

Glycogenolysis is the breakdown of stored glycogen into glucose. Gluconeogenesis is the creation of new glucose from non-carbohydrate sources, such as amino acids and glycerol. Glycogenolysis occurs first, followed by gluconeogenesis during extended fasting.