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How long can a person fast safely?

Fasting is the voluntary abstinence from eating food and drinking liquids for a certain period of time. Fasting has become increasingly popular in recent years for health, religious, political or ethical reasons. But how long can you safely fast for? Here is an overview of the different types of fasting and how long they can be sustained.

Intermittent Fasting

Intermittent fasting (IF) cycles between periods of fasting and eating. It doesn’t specify which foods you should eat, but rather when you should eat them. The most popular IF protocols are:

  • 16/8 method: Fast for 16 hours per day and restrict eating to an 8-hour window.
  • 5:2 diet: Fast completely for 2 non-consecutive days per week and eat normally the other 5 days.
  • Eat-stop-eat: Fast completely for 24 hours 1–2 times per week and eat normally otherwise.

Typical intermittent fasts range from 14–36 hours at a time, once or twice per week. Anything under 24 hours is generally considered safe for most healthy adults.

Longer fasts of 48–72 hours can be done 1–2 times per month, but medical supervision is recommended to monitor any health complications.

Water Fasting

Water fasting restricts all food and allows only water, coffee, tea and other zero-calorie beverages during the fast. Most water fasts last 24–72 hours.

Fasts longer than 72 hours can cause muscle loss and other health complications. Medical supervision is necessary for any water fast lasting longer than 72 hours.

Juice Fasting

Juice fasts involve only consuming vegetable and fruit juices for a period of time. The duration can range from a few days to weeks at a time.

Here are general guidelines for juice fasts:

  • 3-5 days: Safe for most healthy adults with medical supervision.
  • 5-10 days: Increased risk of fatigue, diarrhea and cravings. Medical supervision highly recommended.
  • 10+ days: Higher risk of nutrient deficiencies, muscle loss and blood sugar dysregulation. Should always be monitored by a doctor.

Juice fasts lasting longer than 14 days can result in serious health complications and should only be done under medical supervision.

Intermittent Dry Fasting

Intermittent dry fasting cycles between short periods of consuming nothing at all (not even water) and eating/drinking normally.

Typical intermittent dry fasting protocols include:

  • 16/8 dry fast: 16 hours of fasting and 8 hour eating window.
  • 24 hour dry fast: 1 day eating normally, 1 day dry fasting.

Dry fasts longer than 24 hours are not recommended. Anything over 36 hours can be extremely dangerous without medical supervision.

Prolonged Fasting (7-21 Days)

Prolonged fasting of 7–21 days involves only consuming water and zero-calorie beverages for an extended period of time.

Potential benefits of prolonged fasting include: [1]

  • Weight loss
  • Lowered blood sugar and insulin
  • Lowered cholesterol and blood pressure
  • Increased growth hormone secretion
  • Increased cell repair (autophagy)

However, fasts longer than 72 hours require medical supervision to monitor for complications like:

  • Severe dehydration
  • Orthostatic hypotension (drop in blood pressure)
  • Electrolyte imbalances
  • Loss of lean muscle mass
  • Kidney stones
  • Menstruation loss in women

Prolonged fasting beyond 21 days can lead to severe malnutrition and should never be done without medical supervision.

Fasting Mimicking Diets

Fasting mimicking diets provide minimal calories (typically 500-600 per day) from plant-based foods for 5 days out of the month. This puts the body into a fasting state with benefits similar to water fasting.

Since some food is consumed, fasting mimicking diets of 5 days per month are generally safe for most healthy adults. But medical guidance is still recommended, especially for extended protocols of 7-14 days at a time.

Normal Calorie Diets with Time-Restricted Eating

Time-restricted eating limits food intake to certain hours of the day, like only eating within an 8-hour window. However, you can still consume your full calorie needs within that window.

Time-restricted eating patterns generally fall within the realm of safe fasting since full calories are consumed daily. However, eating in a smaller window can help regulate appetite hormones and blood sugar.


Here is a summary of the general guidelines for how long someone can safely fast for:

Fasting Type Safe Duration
Intermittent Fasting 16-36 hours
Water Fasting 24-72 hours
Juice Fasting 3-14 days
Intermittent Dry Fasting 24 hours or less
Prolonged Fasting 3-21 days
Fasting Mimicking Diet 5-14 days
Time-Restricted Eating No restriction

The longer the fast, the higher the risk of adverse effects. Fasts longer than 3 days should be medically supervised. Very low calorie diets for more than 2-3 weeks can lead to serious health risks.

Certain populations like children, pregnant women, diabetics and those with eating disorders should not fast without medical guidance. Overall, fasting durations should be tailored to each individual based on their health status and goals.

Health Benefits of Fasting

Research shows that fasting can provide powerful health benefits for overweight individuals, people with type 2 diabetes, cancer patients and more. Here are some of the ways fasting may improve health:

  • Promotes weight and fat loss: By limiting calorie intake for extended periods, fasting can promote fat burning and weight loss. Multiple studies show that intermittent fasting leads to similar or greater weight loss than traditional calorie-restricted diets over 8-52 weeks.[2]
  • Reduces insulin and blood sugar: Fasting causes a drop in insulin levels, which allows the body to access and burn its stored sugar. This leads to reductions in blood glucose and insulin resistance.[3]
  • Lowers heart disease risk: Fasting can reduce LDL cholesterol, blood pressure, inflammation and other heart disease risk factors. However, these benefits seem most pronounced with longer fasts of 48+ hours.[4]
  • May help prevent cancer: Animal and test tube studies reveal that fasting can suppress the growth and spread of tumors and increase cancer cell death. The effects on human cancer patients are less certain.[5]
  • Boosts brain function: Fasting has been shown to increase growth hormone production and brain-derived neurotrophic factor (BDNF). This may delay brain aging and improve memory, learning and mental clarity.[6]

That being said, human studies on many of these effects are limited. More clinical research is needed to confirm the true long-term health benefits.

Safety and Side Effects

When done properly, fasting appears relatively safe for most healthy adults in the short-term. However, the following side effects are possible, especially when prolonging fasts:

  • Hunger: Feeling hungry during a fast is normal due to lower leptin levels, the satiety hormone. Many people find hunger goes away after the first 1-3 days.
  • Headaches: Temporary headaches can occur during the first day or two of a fast. Causes may include dehydration, hunger, caffeine withdrawal and changes in blood pressure.
  • Constipation: With no fiber intake, bowel movements may decrease on a fast. Be sure to stay hydrated to minimize constipation.
  • Reduced exercise performance: Low energy and dizziness may occur early in fasts, limiting your ability to exercise hard. Light walking can help improve circulation.
  • Heartburn: Fasting can increase stomach acidity, causing heartburn in some people. Antacids may provide symptom relief.
  • Muscle loss: Longer fasts of 3 or more days can result in a decrease in lean muscle mass as the body breaks down proteins for energy.

Overall, most people feel weak, tired and nauseous during the first 1-3 days of fasting as the body adapts to using fat and ketones for fuel. These effects typically resolve and energy levels normalize during extended fasts.

Who Should Not Fast?

While fasting can benefit many people, it may be hazardous for certain populations. The following individuals should not fast without medical supervision:

  • Children and teenagers
  • Pregnant and breastfeeding women
  • Malnourished people
  • Those with eating disorders
  • People taking certain medications
  • People with diabetes or blood sugar issues
  • Those with gout
  • Individuals with a history of fainting

People with the following conditions should consult a doctor before fasting since it may exacerbate symptoms:

  • Gastroesophageal reflux disease (GERD)
  • Peptic ulcers
  • Heart disease, arrhythmias or palpitations
  • Chronic obstructive pulmonary disease (COPD)
  • Gallstones
  • Adrenal fatigue
  • Anemia
  • Cognitive deficits or dementia

Tips for Fasting Safely

Here are some tips to fast more safely and comfortably:

  • Stay hydrated – Drink plenty of non-caloric fluids like water, herbal tea and broth.
  • Get electrolytes – Consume salt, potassium and magnesium to reduce headaches, muscle cramps and dizziness.
  • Rest and relax – Avoid strenuous physical activity, especially when beginning a fast.
  • Consider caffeine – 1-2 cups of coffee or tea may help suppress hunger and give energy.
  • Try apple cider vinegar – 1-2 tablespoons diluted in water may aid digestion and reduce cravings.
  • Chew sugar-free gum – This may help satisfy the urge to eat.
  • Distract yourself – Keep busy with work, hobbies, socializing and other activities.
  • Break your fast slowly – Gradually transition back to normal eating with fruits, broth and boiled vegetables.

Bottom Line

In general, intermittent fasting and fasts under 24-36 hours appear safe for most healthy adults without supervision. Longer fasts of up to 72 hours can be tolerated by some individuals but medical guidance is recommended.

Prolonged fasts of up to 21 days should only be done under medical supervision to monitor for complications. Fasting is not recommended for children, pregnant women, people with certain chronic diseases or anyone underweight.

Overall, fasting durations should be tailored to each individual based on their health status and goals. People who choose to fast should stay well-hydrated, get plenty of rest and break their fast slowly. Consulting with a doctor first can help ensure fasting is done safely.