What happens to the body after 20 hours of fasting?

December 20, 2025 •

3 min read

After approximately 18 hours without food, the body's liver glycogen stores are significantly depleted, triggering a major metabolic shift. This phase is key to understanding what happens to the body after 20 hours of fasting, as the body transitions from using glucose to burning stored fat for fuel.

Quick Summary

After 20 hours of fasting, the body undergoes a metabolic switch to burn fat for energy, entering a state of ketosis. Cellular cleanup via autophagy is activated, and favorable hormonal shifts occur, improving insulin sensitivity and potentially promoting weight loss.

Key Points

Metabolic Switch: After about 18 hours, the body depletes its glycogen stores and transitions to burning fat for energy, a state known as ketosis.
Enhanced Autophagy: Cellular cleanup and recycling, called autophagy, is significantly activated around the 20-hour mark, promoting cellular health and longevity.
Hormonal Shifts: Insulin levels drop, while human growth hormone (HGH) and norepinephrine increase, boosting fat burning and preserving muscle mass.
Mental Clarity: The brain's use of ketones for fuel and increased BDNF levels can lead to enhanced focus, concentration, and mood.
Potential Risks: Side effects can include hunger, headaches, irritability, and dehydration, especially for those new to this more intense fasting protocol.
Breaking the Fast: It is important to break the fast gently with easy-to-digest, nutrient-dense foods to prevent gastrointestinal discomfort.

Intermittent fasting, particularly the 20-hour method (often called the Warrior Diet), is an intensive eating pattern that triggers several profound physiological changes. Moving beyond the shorter 12-16 hour fasts, the 20-hour mark represents a crucial point where the body has fully transitioned its primary energy source, ramping up fat-burning and cellular repair.

The Metabolic Shift: From Glucose to Ketosis

During the initial hours of a fast, your body uses glucose from your bloodstream and breaks down stored glycogen in your liver for energy. After approximately 18 hours, these glycogen reserves are largely exhausted. This forces your body to find a new fuel source, initiating a process known as metabolic switching.

Entering Ketosis

By the 20-hour mark, your body begins to significantly accelerate the breakdown of stored fats into fatty acids, which the liver converts into ketone bodies (like beta-hydroxybutyrate).

List of metabolic changes:

Glycogen Depletion: Your liver's glucose reserves are running low, signaling the body to seek alternative fuel.
Increased Lipolysis: The breakdown of fat stores (adipose tissue) for energy is intensified.
Ketone Production: Fatty acids are converted into ketones in the liver, which serve as an efficient energy source for the brain and other organs.
Enhanced Insulin Sensitivity: With low blood sugar, insulin levels drop dramatically, which can improve your body's sensitivity to insulin.

Autophagy and Cellular Repair

One of the most powerful processes activated by a 20-hour fast is autophagy. Meaning "self-eating," autophagy is the body's natural cellular recycling program, where damaged or dysfunctional cell components are broken down and cleared out. This process is vital for cellular health, rejuvenation, and longevity. Studies indicate that while initial autophagy markers can rise after 16 hours, the process is notably up-regulated around the 20-hour mark and peaks closer to 24 hours.

Hormonal Adaptations

Significant hormonal changes occur during a 20-hour fast, optimizing the body's use of energy and promoting cellular health. These include a drop in insulin and an increase in Human Growth Hormone (HGH) and norepinephrine.

Cognitive Effects and Performance

Many individuals report experiencing enhanced mental clarity and focus during the later stages of a fast due to the brain utilizing ketones and potentially increased BDNF.

Risks and Side Effects

A 20-hour fast can be challenging and may not be suitable for everyone. Common side effects include hunger, cravings, fatigue, headaches, irritability, and dehydration. Staying hydrated with water and electrolytes is crucial.

How a 20-Hour Fast Compares to a 16-Hour Fast


Feature	16-Hour Fast	20-Hour Fast
Energy Source	Primarily uses stored glycogen. Starts shifting toward fat.	Primarily uses stored fat, entering deeper ketosis.
Autophagy Activation	Initial markers appear but less pronounced.	Significantly more robust and sustained cellular cleanup.
Fat Burning	Good, but more moderate.	More accelerated and efficient fat oxidation.
Flexibility	Easier for most people to sustain long-term.	More challenging and restrictive, potentially leading to faster results.
Weight Loss Potential	Can lead to gradual weight loss.	May result in more rapid fat loss due to a longer calorie deficit.
Side Effects	Generally milder side effects.	Increased potential for side effects like fatigue and hunger.

Conclusion

After 20 hours of fasting, your body is deep into a powerful state of metabolic and cellular rejuvenation. The body has successfully switched its primary fuel source from glucose to fat, initiating ketosis. At the same time, cellular repair processes like autophagy are in full swing, supported by beneficial hormonal changes. While a 20-hour fast can offer accelerated benefits for weight loss, insulin sensitivity, and mental clarity, it is a more demanding protocol than shorter fasts. Managing potential side effects like dehydration and fatigue requires careful planning and hydration. As always, it is recommended to consult a healthcare professional before starting or significantly altering a fasting routine, especially if you have underlying health conditions. You can read more about the metabolic effects of fasting in research studies, such as this one from the National Institutes of Health.

Frequently Asked Questions

Significant muscle loss is unlikely during a 20-hour fast, especially if you get enough protein during your eating window and incorporate resistance training. The rise in Human Growth Hormone (HGH) during fasting helps to preserve lean muscle tissue.

No, a 20-hour fast is not safe for everyone. It is not recommended for children, pregnant or breastfeeding women, individuals with a history of eating disorders, or those with certain medical conditions like diabetes. Always consult a doctor first.

To break a 20-hour fast, start with small portions of easily digestible, nutrient-dense foods like broth, steamed vegetables, lean protein, and healthy fats. Avoid large, heavy meals, processed foods, and sugary drinks.

Plain black coffee or unsweetened tea without any caloric additives will not break your fast and can help manage hunger. Adding anything with calories will end the fasted state.

To stay hydrated, drink plenty of water. Adding a pinch of salt or electrolyte tablets to your water can help replenish lost minerals and prevent headaches, especially during longer fasts.

Enhanced mental clarity can result from the brain shifting to use ketones for fuel and a potential increase in brain-derived neurotrophic factor (BDNF). This provides a stable, efficient energy source for brain function.

Ketosis during fasting is a temporary metabolic state from calorie restriction, while a ketogenic diet maintains ketosis by severely limiting carbohydrates and consuming high amounts of fat. Fasting-induced ketosis is usually a daily occurrence.