Fat Burning Zone

8-16h into your fast

The fat burning zone marks a crucial transition in your metabolism. As liver glycogen depletes, your body increasingly turns to stored fat for energy-a process researchers call the "metabolic switch."

Glycogen Depletion 8-12h

Your liver glycogen stores are beginning to deplete, and your body is transitioning to burning fat for fuel. This is when the metabolic switch begins. Active individuals deplete glycogen faster than those at rest.

Active Fat Burning 12-16h

Your body is now actively burning fat for energy. Ketone production may be beginning as the liver converts fatty acids into ketone bodies. The timing varies widely (12-48 hours) based on metabolism and prior diet.

Key Processes During Fat Burning

Fat mobilization - Stored triglycerides are broken down and released into the bloodstream

Metabolic flexibility - Your body adapts to using fat as a fuel source

Growth hormone increase - Helps preserve muscle mass during the transition

Early ketone production - The liver begins converting fatty acids to ketones

The Metabolic Switch

The "metabolic switch" is a term researchers use to describe the point when your body shifts from using glucose as its primary fuel to mobilizing and burning stored fat. This typically occurs between 12-36 hours after your last meal, depending on individual factors.

This switch represents an evolutionarily conserved mechanism that shifts metabolism from lipid synthesis and fat storage to fat mobilization through fatty acid oxidation and ketone production.
- Research published in Obesity

The metabolic switch is triggered when liver glycogen stores are depleted. Research in Nature Communications demonstrates that this glycogen shortage activates a liver-brain-adipose neural circuit that signals the body to switch fuel sources from liver glycogen to triglycerides stored in fat cells.

Liver Glycogen Depletion

Liver glycogen is your body's readily available glucose reserve. The liver stores approximately 100-120 grams of glycogen, which is used primarily to maintain stable blood sugar levels between meals.

Research shows that within 8 hours of fasting, more than 50% of liver glycogen is depleted. By 12 hours, glycogen content is minimal in most people. The liver plays the greatest role in maintaining blood glucose during the first 24 hours of a fast.

The rate of glycogen depletion varies based on several factors:

Physical activity: Exercise significantly accelerates glycogen depletion
Meal composition: High-carb meals create larger glycogen stores to deplete
Metabolic rate: Higher metabolism uses glycogen faster
Prior diet: Low-carb dieters may have smaller glycogen stores to begin with

Lipolysis: Breaking Down Stored Fat

Lipolysis is the process by which your body breaks down stored triglycerides (fat) into free fatty acids and glycerol, which can then be used for energy.

During the metabolic switch, hormones including glucagon, norepinephrine, epinephrine, cortisol, and growth hormone stimulate hormone-sensitive lipase (HSL) in fat cells. This enzyme breaks down stored triglycerides, releasing fatty acids into the bloodstream.

These free fatty acids are then transported to the liver, where they undergo beta-oxidation-a process that breaks them down into acetyl-CoA, which can either enter the citric acid cycle for energy production or be converted into ketone bodies.

Fun fact: The fat burning process generates more energy per gram than glucose (9 calories per gram vs. 4 calories), which is why fat is such an efficient long-term energy storage system.

Growth Hormone and Muscle Preservation

One of the remarkable adaptations during fasting is the increase in growth hormone (GH) secretion. Research published in the Journal of Clinical Investigation demonstrates that two days of fasting induces a 5-fold increase in endogenous growth hormone production.

This increase occurs through more frequent and larger secretory bursts from the pituitary gland. The primary mechanism involves the reduction of insulin levels during fasting-lower insulin decreases GH inhibition, allowing greater release.

Growth hormone serves several important functions during fasting:

Muscle preservation: GH helps maintain lean muscle mass even while fasting
Fat mobilization: GH promotes the breakdown of stored fat
Blood sugar maintenance: GH helps maintain glucose levels through gluconeogenesis
Tissue repair: GH supports cellular repair and regeneration

Research shows HGH increased 5-fold for males and up to 14-fold for females during a 24-hour water-only fast.

Gluconeogenesis: Making Glucose from Non-Carbs

As glycogen depletes, your body relies increasingly on gluconeogenesis-the creation of new glucose from non-carbohydrate sources. This process ensures your brain and red blood cells continue receiving the glucose they require.

The main substrates for gluconeogenesis include:

Glycerol: Released when fat is broken down (lipolysis)
Lactate: A byproduct of glucose metabolism in muscles
Amino acids: Primarily alanine and glutamine from protein

Research shows that gluconeogenesis contributes approximately 54% of glucose production after 14 hours of fasting, rising to 64% after 22 hours, and up to 84% after 42 hours.

The body has protein-sparing mechanisms to minimise muscle breakdown. As fasting continues, the shift to ketone body utilisation by the brain reduces the need for gluconeogenesis, helping preserve muscle mass.

What to Expect During This Phase

The fat burning zone can feel different for different people. Common experiences include:

Initial hunger: Often peaks around 12-14 hours, then subsides
Increased energy: Many people report feeling more alert as fat burning kicks in
Stable energy: Without blood sugar spikes, energy tends to be more consistent
Mental clarity: Some people report improved focus (individual results vary)

If you're new to fasting, the 16:8 protocol (16 hours fasting, 8 hours eating) is a popular starting point that allows you to experience the fat burning zone while remaining manageable for most people.

Additional Resources

Scientific sources referenced in this article:

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Tip: Most people reach fat burning around 12 hours.

This content is educational and based on our interpretation of published research. See our Educational Content Disclaimer. · Last updated January 2026