What Does the Body Eat First When Fasting? A Complete Guide

December 9, 2025 •

3 min read

During the first 24 hours of fasting, your body primarily relies on glycogen stores in the liver for energy before making a metabolic switch. This initial phase ensures a consistent supply of glucose to fuel the brain and muscles until these carbohydrate reserves are depleted.

Quick Summary

The body first burns stored glucose (glycogen) from the liver, then transitions to breaking down fat for energy and producing ketones as fasting continues.

Key Points

Initial Fuel Source: The body's first energy source when fasting is stored glucose, known as glycogen, which is primarily located in the liver.
Metabolic Switch: After about 16-24 hours, as glycogen stores are depleted, the body shifts to burning fat for energy and begins producing ketone bodies.
Fat for the Brain: The brain, which typically runs on glucose, learns to use ketones as a highly efficient alternative fuel source during prolonged fasting.
Muscle Conservation: The body has adaptive mechanisms, including increased growth hormone production, to preserve lean muscle mass and prevent extensive protein breakdown during fasting.
Cellular Recycling: Extended fasting also triggers autophagy, a cellular cleanup process that recycles damaged components, contributing to cellular health and repair.
Re-feeding: When breaking a fast, it is important to eat easily digestible foods to ease the digestive system back into operation.

The Body's Metabolic Journey During a Fast

Fasting initiates a fascinating and highly regulated process within the body, involving several distinct phases and shifts in primary fuel source.

Phase 1: The Fed State (0–4 Hours)

In the fed state after eating, the body digests nutrients. Rising blood glucose leads to insulin release, which helps cells use glucose and stores excess as glycogen in the liver and muscles. Insulin levels are high, promoting storage over fat burning.

Phase 2: The Early Fasting State (4–16 Hours)

As fasting continues, blood glucose and insulin decrease, while glucagon increases, signaling the release of stored energy. The liver breaks down glycogen into glucose (glycogenolysis) to maintain blood sugar, fueling the brain and other organs. This is the main energy source during shorter fasts.

Phase 3: The Fasting State and Ketosis (16–48 Hours)

After approximately 16 to 24 hours, liver glycogen is largely depleted. The body then transitions to burning stored fat. Fat is broken down into fatty acids and glycerol. The liver converts fatty acids into ketone bodies (ketogenesis) to fuel the brain, leading to a state of ketosis. Some protein is used for glucose production (gluconeogenesis), but reliance on fat increases significantly.

Phase 4: Extended Fasting and Protein Conservation (48+ Hours)

During extended fasting, the body efficiently uses ketones for fuel, and the brain adapts to rely heavily on them. This reduces the need for glucose from protein, helping to conserve muscle mass. Human growth hormone (HGH) also rises, further protecting muscle tissue.

A Comparison of Fuel Sources During Fasting


Feature	Glycogen (Stored Glucose)	Fat (Stored Triglycerides)
Primary Use Window	First 4–24 hours of a fast	After ~16 hours, becoming dominant
Energy Availability	Provides quick, readily available energy	A long-term, high-capacity energy reserve
Energy Density	Lower energy density	Higher energy density
Brain Fuel	Readily converted into glucose for the brain	Converted into ketones for brain fuel
Hormonal Regulation	Mobilized by glucagon and low insulin	Mobilized by glucagon and growth hormone
Water Retention	Each gram is stored with water, leading to initial water weight loss	Stored without water, leading to more sustainable weight loss

The Role of Cellular Recycling (Autophagy)

Fasting also promotes autophagy, a cellular process that recycles damaged cell components, contributing to cellular repair and health. Autophagy increases during deeper stages of fasting. For more information, explore the molecular mechanisms of fasting.

Conclusion: The Body's Clever Adaptations

The body's initial response when fasting is to use stored liver glycogen. As this is depleted, it efficiently switches to burning fat and producing ketones as an alternative fuel for the brain. This phased process ensures energy stability and protects muscle mass. Fasting involves an evolutionary sequence of metabolic shifts, demonstrating the body's adaptive capabilities.

What to Eat After a Fast

When breaking a fast, reintroduce food gradually, especially after longer durations, to avoid digestive upset. Recommended options include:

Bone broth: Gentle and provides electrolytes.
Fermented foods: Help reintroduce beneficial gut bacteria.
Cooked vegetables: Easy to digest fiber and nutrients.
Healthy fats: Avocado, olive oil, and nuts.
Lean protein: A small portion to replenish amino acids.

Avoid large, heavy meals, processed sugars, or refined carbohydrates when breaking a fast to prevent digestive issues and rapid blood sugar spikes.

Frequently Asked Questions

The body typically begins transitioning to burning fat and producing ketones after approximately 12 to 24 hours of fasting, once its glycogen stores are mostly depleted.

No, the body primarily burns fat for energy during sustained fasting and works to conserve muscle mass. While minimal protein is used for gluconeogenesis, significant muscle loss is typically not a concern with shorter or intermittent fasts due to hormonal adaptations like increased growth hormone.

Glycogen is the stored form of glucose. It is primarily stored in the liver and muscles, with liver glycogen being the main source used to maintain blood sugar levels during the initial stages of fasting.

Ketosis is a metabolic state where the body uses fat-derived ketone bodies for fuel instead of glucose. Fasting triggers ketosis by depleting carbohydrate stores and forcing the body to use stored fat as its main energy source.

Autophagy is a cellular recycling and cleanup process where the body removes and recycles damaged cell components. It is enhanced during fasting, typically reaching peak activity after about 24 hours of food deprivation.

Extended fasting (beyond 48 hours) should be approached with caution and ideally done under medical supervision, as it is not suitable for everyone and requires specific preparation.

Fasting metabolism moves through stages: the fed state (using recent food), early fasting (burning liver glycogen), the fasting state (burning fat and producing ketones), and extended fasting (high ketosis with muscle preservation).

Regular intermittent fasting can help condition your body to become more 'metabolically flexible,' making the switch from burning glucose to fat more seamless over time.