What is the longest you can do intermittent fasting?

December 16, 2025 •

4 min read

In 1966, a medically-supervised patient named Angus Barbieri set a Guinness World Record by fasting for 382 days under medical supervision. For most people, however, the longest you can do intermittent fasting safely is a matter of hours or a few days, depending on the method and individual health. It is crucial to understand the risks and benefits associated with different fasting periods before attempting them.

Quick Summary

The duration of intermittent fasting varies widely, from daily time-restricted eating to occasional 48-hour fasts. The safest and most common methods are shorter, while extended fasts beyond 72 hours should only be attempted under strict medical supervision due to risks like electrolyte imbalance. Individual health, experience, and the specific fasting protocol are key determinants of a safe and sustainable fasting duration.

Key Points

Start Short: Beginners should start with shorter fasting windows (12-16 hours) to allow the body to adjust and to mitigate initial side effects like fatigue.
Prolonged Fasting Risks: Fasting for more than 48-72 hours increases the risk of serious complications like electrolyte imbalances and refeeding syndrome and requires medical supervision.
No Food Is Not Better: For the average person, prolonged fasts (multiple days) are not necessarily better than shorter, consistent intermittent fasting methods for achieving health benefits.
Listen to Your Body: Prioritize safety by paying attention to warning signs like severe dizziness, extreme weakness, or heart palpitations, and break the fast if you feel unwell.
Medical Consultation: Individuals with health conditions, particularly diabetes or a history of eating disorders, should always consult a healthcare provider before starting any fasting regimen.
Nutrient-Dense Eating: The success and safety of any intermittent fasting method depend heavily on consuming balanced, nutrient-dense meals during eating windows.
Hydrate Constantly: Proper hydration is non-negotiable during all fasting periods; drink plenty of water and calorie-free beverages to prevent dehydration.

Common Intermittent Fasting Methods and Their Durations

Intermittent fasting is an eating pattern that cycles between periods of eating and fasting, not a diet focused on what to eat. There are several methods, each with different durations, suitable for various levels of experience.

Time-Restricted Eating (TRE)

This is one of the most popular and sustainable forms of intermittent fasting. It involves fasting for a set number of hours each day.

16:8 Method: This is one of the most commonly practiced methods, involving a 16-hour fast and an 8-hour eating window. Most people include their overnight sleep in the fasting window, making it highly manageable. This can be done daily for an indefinite period as a lifestyle change, assuming it is healthy for the individual.
14:10 Method: A slightly shorter version of time-restricted eating, suitable for beginners or individuals who find the 16:8 method challenging.
20:4 Method (Warrior Diet): A more intense version that involves fasting for 20 hours and eating one large meal within a 4-hour window. This is more restrictive and should be approached with caution.

5:2 Diet

This involves eating normally for five days a week and restricting calorie intake to 500-600 calories on the other two non-consecutive days. This can also be followed long-term with proper nutritional planning.

24-Hour Fasts (Eat-Stop-Eat)

This method involves fasting for a full 24-hour period, typically once or twice per week. It is more challenging than daily time-restricted eating and requires prior experience with shorter fasts. The risks of fatigue, headaches, and irritability are higher with this approach.

Prolonged Fasting (Beyond 48-72 Hours)

Fasting for more than 48 hours is considered a prolonged or extended fast. While some people attempt 48-hour fasts a couple of times per month, and even 72-hour fasts, the risks increase significantly with duration. Fasts exceeding 72 hours are highly discouraged without medical supervision due to risks like refeeding syndrome and severe electrolyte imbalances. Angus Barbieri’s extreme 382-day fast was done under strict medical supervision and is not a safe benchmark for the general public.

Comparison of Intermittent Fasting Methods


Feature	16:8 Time-Restricted Eating	5:2 Diet	24-Hour Fast	Prolonged Fast (48+ hrs)
Typical Duration	16 hours daily	2 days per week	1-2 days per week	1-7 days occasionally
Beginner Friendly?	Yes, very accessible	Yes, moderate	No, requires experience	No, requires supervision
Dietary Simplicity	High, no calorie counting	Moderate, requires calorie tracking	High, zero food intake	High, zero food intake
Long-Term Sustainability	High, often a lifestyle change	Moderate, can be difficult	Low, often used for resets	Very Low, reserved for specific goals
Primary Risks	Initial hunger, minor fatigue	Cravings, poor nutrition on fast days	Fatigue, dehydration, irritability	Electrolyte imbalance, refeeding syndrome, muscle loss

Safely Determining Your Intermittent Fasting Duration

Start with shorter durations

For beginners, it's safest to start with a 12-hour or 14-hour fasting window and gradually increase it. This allows your body to adapt and helps you understand how you feel during different phases of the fast. Starting small also prevents severe side effects like exhaustion and crankiness.

Monitor your body’s response

Pay attention to physical and mental symptoms. Temporary side effects like headaches, mild hunger, and fatigue are common initially, but they should not be severe. If you experience persistent dizziness, extreme weakness, or heart palpitations, end the fast and seek medical advice. Listen to your body and recognize the signals that you need to stop.

Seek medical supervision for longer fasts

Fasting for periods exceeding 24 hours can be risky without medical guidance, especially if you have underlying health conditions. Your doctor can help determine if a longer fast is appropriate and supervise it safely, including monitoring your electrolytes and overall health. Certain populations, such as pregnant or breastfeeding women, individuals with a history of eating disorders, or those with diabetes, should avoid fasting altogether.

Prioritize nutrient intake during eating windows

Regardless of your fasting duration, focus on consuming nutrient-dense foods during your eating period to prevent deficiencies. A balanced diet rich in protein, fiber, healthy fats, and micronutrients is essential for health and satiety, and it prevents overeating to compensate for fasting.

The importance of proper refeeding

Breaking a prolonged fast incorrectly can lead to severe health complications, most notably refeeding syndrome, caused by a rapid shift in fluids and electrolytes. For fasts longer than 24 hours, and particularly for prolonged fasts, food should be reintroduced gradually. Start with small portions of easily digestible foods like bone broth before moving to more solid foods.

Hydration is non-negotiable

Adequate hydration is critical during any fast. Drink plenty of water and other calorie-free beverages like black coffee or unsweetened tea. Electrolyte supplements, particularly for fasts longer than 24 hours, can help prevent imbalances.

Conclusion

There is no single answer to what is the longest you can do intermittent fasting. It varies greatly depending on the individual, their health status, and the specific fasting method. For most people, daily 12- to 16-hour fasts can be a safe and sustainable long-term practice with proper nutrition. Longer fasts of 24 to 48 hours are possible for experienced fasters but carry increased risks and should be done less frequently. Fasts exceeding 72 hours should only be undertaken with professional medical supervision. Safety, preparation, and listening to your body are paramount, and shorter, more consistent fasting methods are often the most effective and sustainable for long-term health benefits.

Key considerations for choosing a fasting duration

Your current health status and any underlying medical conditions.
Prior fasting experience and tolerance.
Personal goals, whether for weight loss, metabolic health, or general wellness.
The level of medical support available for prolonged periods.
Your commitment to prioritizing nutrient-dense foods during eating windows.

For more detailed information on specific methods and safety protocols, consult reliable health resources like the Johns Hopkins Medicine guide on intermittent fasting: https://www.hopkinsmedicine.org/health/wellness-and-prevention/intermittent-fasting-what-is-it-and-how-does-it-work.

Frequently Asked Questions

While daily methods like the 16:8 protocol can be followed long-term, fasting continuously for 30 days without food is highly dangerous and requires strict medical supervision due to risks of nutrient deficiencies, muscle loss, and electrolyte imbalance.

Daily time-restricted intermittent fasting methods like 16:8 can be adopted as a long-term lifestyle, and many people sustain them indefinitely. However, more extreme methods like prolonged fasting are not sustainable and are only for occasional use.

The longest medically supervised fast was by Angus Barbieri in the 1960s, who fasted for 382 days. This was an extreme, medically-monitored case and should not be attempted by anyone without intensive, professional medical oversight.

A prolonged fast is typically defined as lasting longer than 24 hours. Fasts of 48-72 hours are sometimes practiced by experienced fasters, but medical supervision is highly recommended for any fast beyond 72 hours.

For shorter fasts (under 24 hours), supplements are typically not necessary if you maintain a balanced diet during your eating window. However, during prolonged fasts (over 24 hours), electrolyte supplements (sodium, potassium, magnesium) are essential to prevent dangerous imbalances.

Some evidence suggests that going without food for a very long time can cause the body to conserve energy and potentially slow the metabolism. However, shorter intermittent fasting periods can actually boost metabolism by promoting the use of fat stores for energy.

When breaking a fast, especially a longer one, it is important to reintroduce food slowly to prevent digestive issues and refeeding syndrome. Start with small, easily digestible foods like bone broth, fermented foods, or cooked vegetables before gradually incorporating other foods.