Understanding the Metabolic Changes When Fasting for 22 Hours

December 23, 2025 •

4 min read

After approximately 18 to 24 hours of fasting, the body's primary energy source shifts significantly as its stored glycogen reserves become depleted. This metabolic change marks a deeper level of fasting than a standard overnight fast, triggering a cascade of physiological adaptations to sustain energy and promote cellular health.

Quick Summary

A 22-hour fasting window pushes the body past its glycogen reserves, forcing a shift into a fat-burning state and enhancing cellular processes like autophagy. The transition involves significant hormonal changes, which can lead to various potential benefits and side effects. Proper hydration and breaking the fast carefully are key for safety and maximizing positive outcomes.

Key Points

Metabolic Shift: After approximately 18-24 hours, the body depletes its glycogen stores and switches to burning fat for energy through ketogenesis.
Cellular Repair: Autophagy, the process of cellular cleanup and recycling, is significantly heightened during the 18-24 hour window of fasting.
Hormonal Changes: Fasting for 22 hours causes a drop in insulin and a rise in glucagon, HGH, and norepinephrine, optimizing fat breakdown.
Potential Risks: Side effects like headaches, hunger, fatigue, and dizziness are common, especially in the early stages, and serious risks exist for at-risk populations.
Careful Refeeding: Breaking a 22-hour fast requires reintroducing food slowly with light, easily digestible options like broth and lean protein to avoid digestive discomfort.
Medical Consultation: Prior to attempting a 22-hour fast, it is crucial to consult a healthcare professional to ensure safety, particularly for those with underlying health conditions.

The Body's Metabolic Shift: The 22-Hour Mark

By the time a person has fasted for 22 hours, the body has long since moved past the 'fed' and 'post-absorptive' states. The initial 8-12 hours after eating are spent digesting and using glucose for energy, replenishing liver and muscle glycogen stores. As the 12-hour mark passes, the body begins to break down stored glycogen into glucose (glycogenolysis) to maintain stable blood sugar. Around the 18 to 24-hour mark, these glycogen reserves are largely depleted. This forces a critical metabolic shift.

The Rise of Ketogenesis and Autophagy

Once glycogen is scarce, the body turns to its most significant energy reserve: fat. It initiates lipolysis, breaking down triglycerides from adipose tissue into free fatty acids and glycerol. The liver then converts these fatty acids into ketone bodies (ketogenesis) to be used as a primary fuel source by the brain and other tissues. This sustained period of fasting, which includes the 22-hour window, also significantly enhances autophagy, a cellular process of cleaning out and recycling damaged components. This cellular housekeeping is linked to longevity and reduced inflammation.

How Hormones Change During a 22-Hour Fast

Several key hormonal changes occur during this time:

Insulin: Levels drop significantly as blood glucose levels fall, which signals the body to release stored energy.
Glucagon: Rises to promote the breakdown of glycogen (early on) and stimulate gluconeogenesis (the creation of new glucose).
Human Growth Hormone (HGH): Production increases, helping to preserve muscle mass and enhance fat metabolism.
Norepinephrine: Increases, leading to greater alertness and a slight boost in metabolic rate.

Potential Benefits and Risks of 22-Hour Fasting

For individuals seeking specific health outcomes, 22-hour fasting—often referred to as the OMAD (One Meal A Day) protocol—offers a more intense form of intermittent fasting than the more common 16:8 method. However, this intensity also comes with a more pronounced set of potential benefits and risks that must be carefully managed.

Comparison of 16-Hour vs. 22-Hour Fasting


Feature	16-Hour Intermittent Fasting	22-Hour Fasting (OMAD)
Metabolic State	Shifts into fat burning but relies more on consistent metabolic flexibility.	Pushes deeper into ketosis, with glycogen fully depleted.
Autophagy	Occurs at a milder level, typically beginning after 12-16 hours.	Enhanced significantly, especially towards the end of the fasting window.
Weight Loss Potential	Effective due to reduced eating window and calorie intake.	Potentially more rapid due to greater energy deficit, but with higher risk of muscle loss if not managed.
Side Effects	Mild hunger, slight fatigue, and headaches are possible initially.	More intense hunger, fatigue, irritability, and potential dehydration are common, especially early on.
Meal Timing	Allows for multiple meals and snacks within an 8-hour window.	Concentrates all caloric intake into a single, typically large, meal.

Safely Reintroducing Food After a 22-Hour Fast

Breaking a 22-hour fast requires a gentle approach to avoid overwhelming the digestive system, which has been resting. Rushing into a heavy, large meal can cause gastrointestinal discomfort, bloating, and nausea. It's crucial to reintroduce food in a measured and thoughtful way.

Here is a recommended approach for breaking a 22-hour fast:

Start with hydrating fluids: Begin with water, broth, or an electrolyte drink to replenish fluids lost during the fast.
Ease in with light, easy-to-digest foods: Simple options like bone broth, vegetable soup, or a small portion of cooked, non-starchy vegetables are ideal.
Include healthy fats and protein: After a gentle warm-up, introduce a balanced meal containing lean protein (fish or chicken), healthy fats (avocado), and low-glycemic carbohydrates (berries).
Avoid heavy, processed foods: Steer clear of fried, sugary, or highly processed foods that can spike insulin and cause digestive upset.

For more detailed guidance on breaking a fast according to specific dietary preferences, the blog from Zero Longevity Science provides valuable insights on keto, high-protein, and plant-based options.

Considerations and Conclusion

While fasting for 22 hours offers potential benefits such as enhanced ketosis, autophagy, and weight management, it is a significant strain on the body that should not be taken lightly. The experience varies widely between individuals, and potential side effects like fatigue, dizziness, and irritability are common. This approach is not suitable for everyone, particularly those with pre-existing medical conditions, a history of disordered eating, or who are pregnant or breastfeeding. For healthy individuals considering this regimen, starting with shorter fasts like 16:8 is recommended to gauge the body's response. Always consult with a healthcare professional before embarking on a new or longer fasting protocol to ensure safety and appropriateness for your individual health profile.

Ultimately, understanding the physiological processes at play during a 22-hour fast is key to practicing it safely and effectively. The switch from glucose to fat for energy, coupled with enhanced cellular repair, explains the purported benefits. However, managing the risks and carefully breaking the fast are just as important for a positive and sustainable experience.

Frequently Asked Questions

No, a 22-hour fast is not safe for everyone. It should be avoided by individuals who are pregnant, breastfeeding, underweight, under 18, or have a history of eating disorders. People with diabetes or other chronic medical conditions should consult a doctor before attempting any form of prolonged fasting.

The OMAD diet is a form of intermittent fasting that involves fasting for 23 hours and consuming all of your daily calories within a single one-hour window. This is very similar to a 22-hour fast with a two-hour eating window.

To help manage hunger, ensure you stay adequately hydrated by drinking plenty of water, black coffee, or herbal tea throughout the fasting period. Staying busy and keeping your mind distracted can also be effective.

Yes, a 22-hour fast can promote weight loss by naturally reducing overall calorie intake and shifting the body into a fat-burning state. Combined with a healthy diet during the eating window, it can help create the calorie deficit necessary for weight loss.

Your metabolism shifts significantly. Your body transitions from relying on stored glucose (glycogen) to using stored fat and producing ketones for energy, a state known as ketosis. Your metabolic rate may also increase due to hormonal changes.

Break your fast with small portions of easily digestible foods, such as broth, steamed vegetables, or a small smoothie. Focus on nutrient-dense options like lean proteins and healthy fats, while avoiding high-sugar, high-fat, or overly processed foods initially.

The body prioritizes preserving muscle mass by increasing Human Growth Hormone (HGH) and utilizing fat stores for energy. Significant muscle loss is more of a concern during prolonged 'starvation mode' lasting several days or weeks, not typically a 22-hour fast. Adequate protein intake during your eating window is crucial for muscle preservation.