Intermittent Fasting: How long is it best to fast for?

October 15, 2025 •

4 min read

According to a 2023 study published in Nutrients, intermittent fasting for at least 12 hours can cause the body to shift from burning sugar to burning fat for energy. Understanding this metabolic shift is crucial when determining how long is it best to fast for to align with your personal health and wellness goals.

Quick Summary

This article explores various intermittent fasting durations, detailing the metabolic processes and health benefits associated with each. It compares popular schedules like the 16:8 method with more advanced fasts, discussing suitability for beginners versus experienced practitioners. A guide to choosing the ideal duration based on individual goals and health status is provided.

Key Points

Start Slowly for Beginners: If you're new to fasting, start with a simple 12-hour window and gradually increase the duration as your body adapts to minimize discomfort.
Optimal Duration for Common Goals: The 16:8 method, fasting for 16 hours daily, is a popular and effective approach for general weight management and metabolic health.
Extended Fasts for Deep Benefits: Longer fasts, such as a 24-hour fast, can trigger deeper metabolic changes like intense autophagy, but should be done with caution and not too frequently.
Listen to Your Body: Pay close attention to how different fasting durations affect your energy levels, mood, and hunger. Adjust your schedule based on your body's unique response to find a sustainable rhythm.
Refeed Strategically: How you break your fast is critical. Use easily digestible, nutrient-dense foods to ease your digestive system back into action, especially after longer fasts.
Seek Professional Guidance: Always consult a healthcare provider before starting or significantly changing a fasting routine, especially if you have underlying health conditions or plan for longer fasts.

The duration of a fast is a critical factor that influences the specific metabolic changes and health benefits you may experience. From daily time-restricted eating to periodic 24-hour fasts, each approach offers a different set of advantages and challenges. The "best" fasting length is not a one-size-fits-all answer but depends heavily on your goals, experience level, and how your body responds.

The Metabolic Timeline of Fasting

To understand why different durations matter, it helps to know what happens inside your body during a fast.

0-12 hours (Fed state): After your last meal, your body uses glucose for energy. Insulin levels are higher, promoting energy storage and inhibiting fat burning.
12-16 hours (Glycogen depletion): As glucose stores are depleted, insulin levels drop, and the body begins to break down stored glycogen (sugar) from the liver for energy.
16-24 hours (Ketosis initiation): With glycogen stores nearing exhaustion, the body undergoes a metabolic shift and begins producing ketones from stored fat to use as fuel. This is where significant fat burning begins.
24-72 hours (Peak ketosis and autophagy): The body is now firmly in ketosis, running efficiently on fat and ketones. Autophagy—the process of cellular cleaning and regeneration—intensifies, helping remove damaged cells and promote longevity.
72+ hours (Deep cellular repair): In prolonged fasts, autophagy reaches its maximum potential, and stem cell production increases. This can lead to immune system regeneration but requires medical supervision due to health risks.

Popular Fasting Methods and Durations

The 12-Hour Fast (12:12 Method)

This is an excellent starting point for beginners, as the fasting window is relatively short and includes sleeping hours. It allows the body to complete its digestive processes and begin the initial phase of metabolic switching. This method is linked to improved metabolic health, better digestion, and supports the body's natural circadian rhythm.

The 16:8 Method

One of the most popular forms of intermittent fasting, the 16:8 method involves fasting for 16 hours and restricting all eating to an 8-hour window. It provides more potent benefits than the 12:12 method, including enhanced fat burning, improved insulin sensitivity, and the initiation of autophagy. Many find this schedule sustainable as it often involves simply skipping breakfast.

The 18:6 Method

For more experienced fasters, the 18:6 method offers a longer fasting window for a deeper metabolic effect. It can lead to more significant improvements in insulin sensitivity and fat utilization, pushing the body further into ketosis. The shorter 6-hour eating window requires more planning to ensure adequate nutrient intake.

The 20:4 Method (The Warrior Diet)

This method involves a 20-hour fasting period, often with a 4-hour eating window in the evening. It is a more intense approach that can significantly boost fat-burning and cellular repair processes. However, it is more restrictive and may not be suitable for beginners or individuals with certain medical conditions.

Extended Fasting (>24 hours)

This can range from a weekly 24-hour fast to prolonged multi-day fasts under strict supervision. Extended fasts maximize autophagy and immune regeneration but carry greater risks, including electrolyte imbalances and nutrient deficiencies. These fasts are not for everyone and should always be discussed with a healthcare professional beforehand.

How to Choose Your Ideal Fasting Length

Choosing the right fasting duration is a personal journey. Here are some factors to consider:

Goal: For weight loss, moderate fasts like 16:8 or 18:6 are often effective and sustainable. For deeper cellular repair (autophagy), longer fasts may be necessary but should be approached cautiously.
Experience Level: Beginners should start with shorter, more manageable fasts (12-14 hours) and gradually increase the duration as their body adapts.
Health Status: Individuals with underlying medical conditions, such as diabetes, should consult a doctor before starting any fasting regimen. Long fasts are not suitable for those who are underweight or pregnant.
Sustainability: The best fasting method is one you can stick with consistently over the long term. A less intense but consistent schedule often yields better results than an extreme one that is quickly abandoned.


Fasting Method	Fasting Duration	Ideal For	Key Benefits	Considerations
12:12 Method	12 hours	Beginners, circadian rhythm alignment	Improved digestion, metabolic support, beginner-friendly	Milder benefits; may not trigger significant ketosis
16:8 Method	16 hours	General weight loss, most people	Enhanced fat burning, insulin sensitivity, moderate autophagy	Some adjustment needed; may cause initial hunger
18:6 Method	18 hours	Experienced fasters, accelerated fat loss	Stronger fat-burning, deeper metabolic effects	Shorter eating window requires careful planning
24-Hour Fast	24 hours (1-2x per week)	Advanced users seeking specific benefits	Significant impact on blood lipids, inflammation, and autophagy	Potential fatigue, irritability; not for everyone
Prolonged Fast	48-72+ hours	Medically supervised therapeutic use	Maximum autophagy, immune regeneration	Significant health risks; requires medical supervision

Breaking Your Fast Safely

Equally important to the duration of the fast is how you reintroduce food. After a prolonged fast, your digestive system is sensitive and needs to be eased back into action. Breaking your fast with nutrient-dense, easily digestible foods like bone broth, cooked vegetables, and fermented foods is recommended. Avoiding processed foods and high-carbohydrate meals immediately after a fast is crucial to prevent digestive distress and blood sugar spikes. For a gradual and safe reintroduction of food, especially after longer fasts, consider following a specific refeeding protocol.

Conclusion

The question of how long is it best to fast for has no single answer. The ideal duration is a dynamic choice that depends on individual health, lifestyle, and goals. For most people, a time-restricted eating schedule like the 16:8 method provides a safe and sustainable entry point with significant health benefits. Longer fasts, while offering deeper metabolic and cellular effects, require greater caution and often professional medical oversight. The key is to listen to your body, start slowly, and prioritize a balanced, nutrient-dense diet during your eating window to maximize the positive effects of fasting while minimizing potential risks. For further guidance, reputable resources like Johns Hopkins Medicine offer valuable insights into intermittent fasting.

Frequently Asked Questions

Beginners should start with a 12-hour fasting window, as it is the most gentle and can be easily incorporated into an overnight sleep schedule. This helps the body adapt to metabolic switching without causing excessive stress.

The body typically starts to enter ketosis and begins burning stored fat for energy between 16 and 24 hours into a fast, after its glycogen stores have been depleted.

Not necessarily. While longer fasts can trigger more intense processes like autophagy, they also come with a higher risk of side effects like electrolyte imbalances. The best results often come from a sustainable and consistent regimen that fits your lifestyle.

Fasting for more than 24 hours carries greater risks and should ideally be done under medical supervision. Extended fasts are not suitable for everyone, particularly those with underlying health conditions or who are underweight.

The 16:8 method is a great starting point for moderate fat burning and insulin sensitivity. If you have adapted to this and are seeking stronger effects, the 18:6 method offers a longer fasting window for deeper fat utilization.

Feeling dizzy or lightheaded can be a sign of dehydration or electrolyte imbalance, especially during longer fasts. Ensure you are drinking plenty of water and, if necessary, consider a calorie-free electrolyte supplement to replenish lost minerals.

Yes, you can have black coffee, unsweetened tea, or water during your fasting window without breaking your fast. These beverages have minimal calories and won't trigger an insulin response.

Autophagy is the body's natural process of cellular cleansing and regeneration. Significant autophagy is believed to start around 16 hours of fasting and intensifies during longer fasts, reaching higher levels between 24 and 72 hours.