How long are you allowed to fast safely?

October 14, 2025 •

4 min read

While a notable case in 1965 saw a man fast for 382 days under medical supervision, this extreme is highly dangerous for the general public and not recommended. For most healthy adults, shorter periods are safer, so the key is understanding how long are you allowed to fast based on the specific method and your health status.

Quick Summary

Safe fasting durations vary significantly depending on the method, and shorter fasts of 12–24 hours are generally considered safe for healthy individuals. Longer fasts, especially beyond 72 hours, carry increased health risks and must be medically supervised. The duration is influenced by factors like overall health, fitness, and goals. Always listen to your body and consult a healthcare professional before starting.

Key Points

Start Short: Beginners should begin with shorter intermittent fasting periods, such as a 12-hour or 16-hour daily fast, to help the body adapt safely.
Extended Fasts Require Caution: Fasting for longer than 24 hours increases potential health risks, with fasts over 72 hours requiring mandatory medical supervision.
Stay Hydrated: Always drink plenty of water and zero-calorie fluids during a fast to prevent dehydration, which can lead to serious side effects.
Electrolyte Balance is Critical: For fasts longer than 24 hours, supplementing with electrolytes like sodium, potassium, and magnesium is necessary to prevent imbalances.
Consult a Doctor: Before attempting any fasting, especially if you have pre-existing health conditions, always speak with a healthcare professional.
Refeed Slowly: After a prolonged fast, reintroduce food gradually to avoid overstimulating your digestive system and prevent refeeding syndrome.

Understanding the Different Fasting Durations

Fasting is an eating pattern that cycles between periods of voluntary food abstinence and eating. The safe duration of a fast depends largely on the type of fasting method you follow and your individual health profile.

Intermittent Fasting (IF) Protocols

Intermittent fasting is the most popular form of fasting and typically involves daily or weekly cycles. These are considered the safest for most healthy adults.

12-Hour Fasting (12:12): This is a beginner-friendly method where you fast for 12 hours and have a 12-hour eating window. It's often achieved by simply extending the natural overnight fast, for example, fasting from 7 p.m. to 7 a.m.. This is a low-risk starting point for many individuals.
16-Hour Fasting (16:8): This method involves fasting for 16 hours and eating during an 8-hour window. Many people find this sustainable for the long term by skipping one meal, such as breakfast.
5:2 Diet: With this approach, you eat normally for five days a week and restrict your caloric intake (typically to 500-600 calories) on two non-consecutive days. The fasting period here is less about a time frame and more about calorie reduction on specific days.
Eat-Stop-Eat (24-Hour Fast): This involves a full 24-hour fast, once or twice per week, for example, from dinner to dinner. While safe for many, it can be challenging and may cause fatigue and irritability.

Extended Fasting (EF) Protocols

Extended fasting refers to any fast lasting longer than 24 hours. These methods should be approached with extreme caution and, for longer periods, under medical supervision.

36- to 48-Hour Fasts: Fasting for a full 1.5 to 2 days is a more advanced technique. The body has already depleted its glucose stores and is fully in a ketogenic state, burning fat for fuel. Potential side effects like fatigue, headaches, and dizziness are more common. Hydration and electrolyte intake are crucial.
72-Hour Fasts (or longer): Fasting for three days or more should only be done under the strict supervision of a healthcare professional. Risks increase significantly with longer durations and include electrolyte imbalances, nutritional deficiencies, and loss of lean muscle mass. The reintroduction of food after a prolonged fast, known as refeeding, must also be done slowly and carefully to avoid refeeding syndrome.

Comparison of Fasting Durations


Feature	Intermittent Fasting (e.g., 16:8)	24-Hour Fast (Eat-Stop-Eat)	Extended Fasting (72+ Hours)
Recommended Duration	12-16 hours daily, long-term	Up to 24 hours, 1-2 times weekly	Short-term, only under medical supervision
Primary Goal	Weight management, metabolic health	Weight loss, autophagy induction	Weight loss, deeper cellular repair
Typical Side Effects	Mild hunger, crankiness during adjustment period	Fatigue, headaches, hunger	Significant fatigue, dizziness, nausea, electrolyte issues
Risk Level	Generally low for healthy adults	Moderate for beginners	High risk, requires professional monitoring
Who Should Avoid?	Pregnant, breastfeeding, underweight, history of eating disorders	Same as IF, plus those with pre-existing conditions	Nearly all individuals without strict medical oversight

Health Considerations for Any Fasting Duration

No matter the duration, there are critical considerations to ensure safety and effectiveness.

Consult a professional. Always talk to a doctor before starting a new fasting regimen, especially if you have an underlying health condition like diabetes or heart disease. A physician can provide personalized advice based on your health history.
Hydration is key. Drinking plenty of water, and potentially zero-calorie drinks like black coffee and herbal tea, is essential during any fast to prevent dehydration. For fasts over 24 hours, electrolyte supplementation may be necessary to balance sodium, potassium, and magnesium.
Listen to your body. Hunger pangs are normal, but if you experience severe symptoms such as extreme fatigue, confusion, or persistent nausea, you should stop your fast immediately and eat.
Break your fast properly. Following a prolonged fast, a gradual reintroduction of food is necessary to prevent digestive upset. Start with small, easily digestible meals and avoid overeating.
Maintain a healthy diet. On eating days or within your eating window, it's vital to consume a balanced, nutritious diet rich in whole foods, protein, and fiber. Fasting is not an excuse to binge on unhealthy foods.

Conclusion: Finding the Right Fasting Duration for You

Determining how long are you allowed to fast is not a one-size-fits-all answer but depends on individual health, experience, and goals. For most people, shorter intermittent fasting protocols (12–24 hours) are the safest and most sustainable methods, offering numerous potential health benefits. Extended fasts beyond 24 hours carry higher risks and require medical supervision to be safe. By starting with shorter durations, staying hydrated, and listening to your body's signals, you can find a fasting regimen that works for you. Remember, consulting with a healthcare provider is the most responsible first step before undertaking any new dietary practice, especially one involving prolonged food abstinence.

For more detailed, scientific information on the physiological effects of fasting, you can refer to authoritative sources such as those found on the National Institutes of Health website.

Frequently Asked Questions

For most healthy adults, a 24-hour fast, like the 'Eat-Stop-Eat' method, is generally considered safe when done infrequently (1-2 times per week). Beginners should start with shorter fasts to assess their body's reaction.

The longest recorded fast in medical history was by a man named Angus Barbieri, who, under strict medical supervision, went without solid food for 382 days in the 1960s. This was an extreme, medically monitored case and is not a safe practice for the general public.

During a fast, it is important to stay hydrated by drinking water. Zero-calorie beverages such as plain black coffee and unsweetened herbal tea are also typically allowed and can help manage hunger.

Certain individuals should not fast without a doctor's clearance, including those who are pregnant or breastfeeding, children and teenagers, individuals with diabetes, people with a history of eating disorders, and those who are underweight.

You should break your fast and seek medical advice if you experience severe symptoms like extreme dizziness, confusion, persistent nausea, or significant fatigue that prevents you from carrying out daily tasks.

Fasting, particularly intermittent fasting, can lead to weight loss by creating a caloric deficit. Studies show it can be effective, but results are often comparable to other calorie-restricted diets, and maintaining weight loss long-term requires healthy eating habits during non-fasting periods.

Breaking a prolonged fast requires caution to avoid refeeding syndrome. Start with small portions of easily digestible foods like broth or steamed vegetables. Avoid processed foods, sugar, and heavy meals, and gradually increase your food intake over several days.