How Long Should You Go Without Food When Fasting? A Guide to Safe Durations

December 15, 2025 •

4 min read

Research from Johns Hopkins neuroscientist Mark Mattson shows our bodies have evolved to thrive for long periods without food, a metabolic state that can offer health benefits. Knowing how long should you go without food when fasting depends on the method, your experience, and health goals.

Quick Summary

The safe duration for fasting depends on the specific method. Shorter intermittent fasts (12-24 hours) are suitable for beginners, while prolonged fasts (over 24 hours) should be approached cautiously. Medical supervision is recommended for extended periods and for individuals with underlying health conditions.

Key Points

Start Short: Beginners should begin with shorter fasts, such as the 12-hour method, which is the easiest and safest way to start intermittent fasting.
Progress Slowly: To achieve more profound metabolic benefits like deeper ketosis, you can gradually increase your fasting duration to 16 hours or longer.
Know the Timeline: Your body burns stored glucose for the first 8-12 hours, shifts to glycogen stores next, and then begins burning fat after approximately 24 hours.
Be Cautious with Prolonged Fasting: Fasts lasting 48 hours or more carry increased risks, such as dehydration and electrolyte imbalances, and should be approached with caution.
Consult a Doctor for Long Fasts: Any fast lasting longer than 72 hours, or if you have pre-existing medical conditions, requires medical supervision to ensure safety.
Prioritize Hydration: Drinking plenty of zero-calorie fluids and potentially supplementing with electrolytes is crucial, especially for fasts longer than 24 hours.
Listen to Your Body: Pay close attention to how you feel and stop fasting immediately if you experience dizziness, extreme weakness, or other signs of illness.

Understanding Fasting Durations

Fasting is the practice of abstaining from food for a set period, and its duration can vary significantly depending on the method and desired outcome. From purely metabolic changes that happen overnight to profound cellular shifts triggered by extended periods without food, the timeline is crucial for both safety and effectiveness. After around 8 hours, your body has used up its readily available glucose from your last meal. It then begins to tap into glycogen stores in the liver and muscles for energy. Once glycogen is depleted, typically after 12 to 24 hours, the body enters a state of ketosis, switching to stored fat as its primary fuel source. Longer fasts, extending to 48 or 72 hours, can trigger deeper cellular repair processes like autophagy. However, these prolonged durations also carry increased risks and require careful consideration.

Popular Intermittent Fasting Schedules

For those new to fasting, or seeking a sustainable routine, intermittent fasting (IF) offers manageable schedules. These methods cycle between periods of eating and fasting, making them less demanding than prolonged fasts.

12-Hour Fast: The simplest method, often including your overnight sleep. For example, fasting from 7 p.m. to 7 a.m. This is an excellent starting point for beginners to help the body get used to a routine.
16/8 Method: A widely practiced approach involving a 16-hour fasting window and an 8-hour eating window. This often means skipping breakfast and eating between noon and 8 p.m..
5:2 Method: This modified fast involves eating normally for five days of the week and restricting calories (500-600) on two non-consecutive days.
24-Hour Fast (Eat-Stop-Eat): This involves fasting completely for a full 24-hour period once or twice per week. This can be challenging and may cause temporary side effects like fatigue or headaches as your body adjusts.

The Difference Between Intermittent and Prolonged Fasting


Feature	Intermittent Fasting	Prolonged Fasting
Duration	Typically 8-24 hours	More than 24 hours, often 48-72 hours or longer
Frequency	Daily or a few days per week	Infrequently (e.g., quarterly) or only once
Metabolic Focus	Glycogen depletion and initial fat burning; easier entry into ketosis	Deeper ketosis, significant autophagy, and immune regeneration
Risk Level	Low to moderate for most healthy adults	Moderate to high, carries risk of dehydration and electrolyte imbalance
Supervision	Not typically required for healthy adults	Strongly recommended, especially for fasts over 72 hours

Navigating Longer Fasting Periods

Prolonged fasts, typically lasting 48 to 72 hours, push the body into deeper metabolic states, offering more pronounced benefits but also carrying greater risks. A 48-hour fast is one of the longest commonly practiced with intermittent fasting and can be done once or twice per month. It provides benefits like cellular repair and improved insulin sensitivity. A 72-hour fast can maximize cellular cleanup (autophagy) and immune system regeneration. However, fasts of this length or longer should only be undertaken with medical supervision.

When attempting a prolonged fast, specific precautions are essential for safety.

Prioritize Hydration and Electrolytes: It is vital to drink plenty of zero-calorie fluids, such as water, black coffee, or tea, to prevent dehydration. Longer fasts may require electrolyte supplements to replace lost sodium, potassium, and magnesium.
Avoid Intense Exercise: Minimize intense physical activity, especially during fasts longer than 24 hours, to reduce the risk of dizziness and fatigue.
Plan a Gradual Refeed: Reintroducing food too quickly after an extended fast can be dangerous, causing refeeding syndrome. Start with small, easily digestible foods like broth and cooked vegetables before moving back to larger meals.
Monitor Your Body: Listen to your body and stop fasting immediately if you feel unwell. Signs to watch for include extreme weakness, unexpected sickness, or discomfort.

Who Should Not Attempt Fasting?

While fasting can be a safe practice for many, it is not suitable for everyone. Certain individuals and health conditions require that you avoid fasting or only do so under close medical supervision.

Pregnant or Breastfeeding Women: Increased nutritional needs make fasting unsafe.
Individuals with Eating Disorders: Those with a history of anorexia or bulimia should avoid fasting, as it can worsen their condition.
People with Diabetes: Fasting can lead to dangerously low blood sugar levels (hypoglycemia) and should only be performed with a doctor's guidance.
Underweight Individuals: Fasting can exacerbate low body weight and nutrient deficiencies.
Children and Adolescents: Growing bodies have unique nutritional requirements, making fasting inappropriate.
Those on Certain Medications: Some medications need to be taken with food, and fasting can interfere with their effectiveness or safety.

Conclusion

Ultimately, there is no single answer to how long should you go without food when fasting, as the ideal duration is a highly personal decision based on individual health, goals, and experience. For beginners, starting with shorter intermittent fasting periods, like the 12- or 16-hour methods, is the safest and most sustainable approach. As your body adapts, you can gradually experiment with longer fasts, but always exercise caution and listen to your body's signals. For any fast lasting more than 72 hours, or if you have pre-existing health concerns, consulting a healthcare professional is not only a recommendation but a necessity. The most effective eating pattern is one you can maintain long-term while prioritizing overall health and well-being.

Frequently Asked Questions

While both 12-hour and 16-hour fasting can be effective for weight loss, the 16-hour fast may be more beneficial. The longer duration allows for a greater depletion of glycogen stores, which can trigger higher levels of fat burning.

When fasting for 24 hours, your body first uses up its glucose and glycogen stores. After about eight hours, it begins converting stored fat and protein into energy. A 24-hour fast can lead to fatigue or irritability, but these effects often lessen with time.

A 48-hour fast is generally safe for healthy individuals but should only be attempted after you are comfortable with shorter fasts. It is associated with side effects like hunger, dizziness, and fatigue. Ensure you stay well-hydrated with electrolytes and refeed gradually.

Fasting is not recommended for pregnant or breastfeeding women, individuals with a history of eating disorders, those who are underweight, or children and adolescents. People with diabetes, low blood pressure, or those on certain medications should only fast under medical supervision.

After 72 hours of fasting, your body enters a state of deeper ketosis and maximizes the cellular repair process known as autophagy. Extended fasts like this can also trigger immune system regeneration, but medical supervision is crucial to manage risks like electrolyte imbalance.

Yes, drinking plenty of calorie-free fluids like water, black coffee, or unsweetened tea is essential during fasting to prevent dehydration. For prolonged fasts, supplementing with electrolytes can also be important.

Break a prolonged fast slowly to avoid complications like refeeding syndrome. Start with small, easily digestible meals, such as broth and cooked vegetables. Gradually increase food intake over several days before returning to normal eating patterns.