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A 36 hour fast is a prolonged intermittent fasting common among experienced fasters who want to lose weight quickly. Athletes also use this fasting method to manage weight before matches. In addition to weight loss, it can improve insulin sensitivity, reduce inflammation, and improve mental function. However, it can also contain side effects such as dehydration, dizziness, fatigue, mood swings, and digestive issues. Let’s weigh its pros and cons and learn how to do it safely.
Highlights
What Is a 36 Hour Fast or Monk Fast?
A 36 hour fast, or a monk fast, is an extended intermittent fasting where you need to avoid calories for a continuous 36 hours [1].
Avoiding calories means you can’t eat solid foods, liquids with calories, or sugary drinks. The only things you can consume during fasting are non-caloric drinks like water, herbal tea, and black coffee. You can also use electrolytes to avoid dehydration.
You can decide when the fasting begins and ends. For example, you can fast from dinner on one day to breakfast on the following day.
You should choose the fasting period carefully and based on your body’s needs because avoiding calories for 36 hours activates various physiological changes in your body.
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What Are The Stages of The 36 Hour Fast?
Hour 0-6: The body begins to use the glucose (sugar) it has received from recent meals to provide energy. Insulin levels gradually decrease, so the stored glycogen (fat) is broken down and used to provide energy.
Hour 6-12: The body gradually uses all the glycogen stored, so it turns to stored fat for energy. Fat molecules are broken down into fatty acids. The liver tries to convert fatty acids into ketones as an alternative energy source.
Hour 12-18: Ketone production increases and fat becomes the main source of energy. Autophagy (cell-cleaning process) is started.
Hour 18-24: More ketone is produced, which improves mental clarity and insulin sensitivity as insulin levels decrease. Fat loss increases as the body relies only on fat for energy.
Hour 24-36: Autophagy increases. Metabolism tries to adapt to energy saving. Hunger hormones like ghrelin decrease, lowering the feelings of hunger.
Avoiding food for an extended period can have different pros and cons. Let’s start with its effect on weight management.
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Is a 36 Hour Fast Good for Weight Loss?
Yes, a 36 hour fast can boost weight loss because it causes various changes in the body, including:
Caloric Deficit: When you eat no calories, your body creates a caloric deficit that boosts weight loss [2].
Fat Burning: During fasting, your body burns carbohydrates to provide energy. After glycogen (sugar/ carb) stores are emptied, your body starts to burn stored fat to provide energy, which means fat burn and weight loss [3].
Better Insulin Sensitivity: Fasting can improve insulin sensitivity, meaning that the body can better control blood sugar levels and doesn't need to produce as much insulin to manage blood sugar, which helps prevent the storage of excess glucose as fat [4].
Reduced Fat Storage: When you have high insulin levels and your body can’t regulate it properly, you store fat and can’t break down fat for energy. By improving insulin sensitivity, your body can lower insulin levels, which facilitates the breakdown of stored fat (lipolysis) and reduces the creation of new fat cells [5].
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Decreased Hunger and Cravings: Improved insulin sensitivity can stabilize blood sugar levels, reducing sudden spikes and drops that lead to hunger and cravings, which can help you control your appetite and reduce overeating [6].
Lower Inflammation: Insulin resistance can cause chronic inflammation, which can interfere with weight loss. By improving insulin sensitivity, you can reduce inflammation, which can help your body respond better to weight loss efforts [7].
Appetite Regulation: Fasting can trigger hunger hormones like ghrelin and reduce appetite, which means less calorie intake during eating windows and better weight control [8].
When you consume fewer calories, especially from carbohydrates, your body can enter a fat-burning state called ketosis.
Dr. Jason Fung, Nephrologist and Author of "The Complete Guide to Fasting":
"Extended fasting, like the 36-hour fast, allows the body to enter deeper states of ketosis and autophagy, which can be incredibly beneficial for cellular repair and longevity."
Will a 36 Hour Fast Put Me in Ketosis?
Yes, a 36 hour fast can put your body into ketosis, where the body uses ketones (fat) as an energy source instead of glucose (sugar) from carbohydrates. But how much weight can you lose this way?
How Much Weight Can You Lose on a 36 Hour Fast?
The first important point to note about weight loss on a 36 hour fast is that the first weight you lose on intermittent fasting is probably water weight, not fat.
It will take some time for the body to enter ketosis and start burning fat. Although water weight loss is also weight loss, it can be restored by eating food or drinking.
The amount of weight you can lose on a 36 hour fast depends on various personal factors like your initial weight and metabolic function, and there is no one-size-fits-all answer.
However, most people lose 1-3 pounds of water weight, depending on how much glycogen (fat) their body has stored.
Actual fat loss during a 36 hour fast can be around 0.25-0.5 pounds since burning a pound of fat requires a deficit of about 3,500 calories, which is more than most people burn in 36 hours.
If you do not notice weight loss after a 36 hour fast, it may be because of one or some of the following scenarios.
Dr. Valter Longo, Director of the USC Longevity Institute:
"Periodic fasting, including 36-hour fasts, has shown to significantly improve metabolic health and reduce risk factors for diseases by promoting cellular regeneration."
Why Am I Not Losing Weight on My 36 Hour Fast?
You may not lose fat on a 36 hour fast as much as you like because of:
Water Retention
Your body may hold the water inside or retain water due to various factors, such as eating too much salt, hormonal changes, or high levels of stress, which can mask fat loss on the scale.
Metabolic Adaptation
Your metabolism might slow down in response to fasting, reducing the number of calories you burn, which is a natural survival mechanism that conserves energy during periods of low food intake.
Overeating After Fasting
If you eat significantly more than usual before or after your fast, you might offset the caloric deficit created during a 36 hour fast. Overeating in response to hunger or a sense of reward can negatively affect the benefits of fasting.
Muscle Loss
Extended fasting can lead to muscle loss, especially if you don’t consume enough protein during the eating window, which can lower your metabolic rate. Less muscle mass means fewer calories burned at rest, making weight loss more challenging.
Hormones like cortisol (the stress hormone) and insulin can affect weight loss. High cortisol levels can lead to weight retention, especially around the belly.
Insulin sensitivity can also impact how your body stores and uses fat. High insulin sensitivity makes your body hold on to excess fat and makes fat loss difficult.
Imbalance Caloric Intake and Output
You need to carefully decide the number of calories you should consume before and after fasting. Misjudging these numbers or overestimating the number of calories burned through exercise can negatively impact weight loss.
Body Composition Changes
Sometimes, the scale does not show fat loss accurately. You might be losing fat but gaining muscle, which can result in no significant change in weight.
The good news, however, is that weight loss is not the only benefit you can get from extended intermittent fasting. A 36 hour fast can also lead to different health benefits.
Intermittent fasting, including a 36 hour fast, can increase insulin sensitivity and lower the risk of type 2 diabetes. A 2005 study reported that intermittent fasting had a positive effect on insulin sensitivity and reduced insulin levels in participants with insulin resistance [9].
Autophagy
Autophagy, this cell-cleaning process, can lower the risk of many diseases, including neurodegenerative disorders, cancer, and metabolic conditions. It can also improve metabolic health, immune function, and cellular resilience [10] [11].
Autophagy is a gradual process that doesn’t happen all at once, but it takes time to peak and provides health benefits. It usually takes 12-24 hours for the autophagy results to appear.
A 2007 study showed that fasting activates autophagy, a cleaning process in the body to remove damaged cells and improve cellular health and longevity [12].
Brain Health and Mental Clarity
During extended fasting, when the body is not processing food, it focuses on repairing and optimizing cellular function, which can result in mental clarity.
A 2021 study analyzing various clinical studies indicated that intermittent fasting can lead to anatomical and functional changes by producing a special protein for neuron growth and cognitive function.
These changes can help improve epilepsy, Alzheimer’s disease, multiple sclerosis, Parkinson’s disease, ischemic stroke, autism spectrum disorder, mood changes, and anxiety disorders [13] [14].
Chronic inflammation can lead to various diseases, including cardiovascular, neurodegenerative, autoimmune, respiratory, and diseases, metabolic problems, digestive disorders, cancer, and skin conditions.
A 2014 study on the effects of intermittent fasting on neuroinflammation and memory impairment found that intermittent fasting can decrease oxidative stress and inflammation [15].
Metabolic Benefits for Type 2 Diabetes
Various studies indicate a positive relation between intermittent fasting and treating type 2 diabetes. Fasting can improve insulin sensitivity, which helps reduce the risk of type 2 diabetes. It can also promote cellular repair and improve metabolism [16].
Despite these positive health effects, a 36 hour fast can come with some side effects, especially in those new to extended fasting and individuals with underlying health conditions.
Dr. Satchin Panda, Researcher at the Salk Institute for Biological Studies:
"Fasting for extended periods, such as 36 hours, can help reset your circadian rhythms and improve overall metabolic health."
The Side Effects of a 36 Hour Fast
Dehydration
Headache and Dizziness
Fatigue and Weakness
Hunger and Cravings
Mood Changes
Digestive Issues
Dehydration
Prolonged fasting decreases satiety, which may stop you from drinking enough water and cause dehydration.
A 1982 study published in the Journal of Clinical Endocrinology and Metabolism reported that fasting for 36 hours without fluid intake resulted in significant water loss and dehydration.
To avoid dehydration, you can drink enough water, electrolyte drinks, and other non-caloric beverages during fasting hours [17].
Headache and Dizziness
A 2023 study on intermittent fasting over 1 to 3 months on male and female participants ranging from 18-35 years old found that avoiding food for an extended period, like in a 36 hour fast, can lead to headaches and dizziness [18].
Various studies have indicated that not receiving enough calories daily, like during extended intermittent fasting, can cause lethargy, fatigue, and weakness [19].
It is necessary to monitor your symptoms and break the fast if you feel serious fatigue, headache, or similar side effects.
Hunger and Cravings
Hunger and cravings are among the most common side effects of intermittent fasting, which are actually normal since you avoid food!
Various studies have confirmed these side effects. For example, a 2018 study on 112 men and women found that although intermittent fasting can lead to weight loss, it can cause hunger and cravings [20].
Another 2019 observational study on 1422 participants over 12 months found that although adverse effects were observed in less than 1% of the population, the participants showed hunger symptoms during the early days of fasting [21].
A 2016 study on the psychological effects of intermittent fasting on women showed that some individuals may experience irritability and mood swings during fasting as a result of low blood sugar levels [22].
Digestive Issues
Avoiding food for 36 hours can cause indigestion, bloating, diarrhea, and nausea in some individuals because of consuming low levels of calories [23].
Anyhow, if you feel the benefits outweigh the side effects and have decided to try a 36 hour fast, you need to learn how to do it safely and healthily.
How to Do the 36 Hour Fast in a Healthy Way?
Here are the steps you need to take to try a 36 hour fast healthily:
Before You Start
Consult a healthcare professional to ensure a 36 hour fast is suitable for you, especially if you have pre-existing medical issues.
Choose your fasting period based on your schedule and preferences. For example, you can start after dinner and break the fast with breakfast the day after next.
Drink plenty of water in the hours before starting the fast to stay hydrated.
Dr. Mark Mattson, Professor of Neuroscience at Johns Hopkins University:
"Extended fasting periods, like the 36-hour fast, can enhance brain health by stimulating the production of brain-derived neurotrophic factor (BDNF), which supports neuron growth and protection."
During the Fast
Don’t consume food or beverages during the 36 hour fast. You can only drink non-caloric beverages.
Drink enough water, herbal tea, and black coffee to stay hydrated and curb hunger.
Pay attention to your body's reactions and break the fast if you feel severe discomfort, dizziness, or other adverse effects.
Breaking the Fast
Break the 36 hour fast with whole, nutritious foods. Eat meals containing lean proteins, healthy fats, and complex carbohydrates.
Start with small portions and let your body adapt to the new eating habits.
After the Fast
Return to your regular eating habits and avoid overeating.
Keep drinking water throughout the day to stay hydrated.
Some individuals assume they can regularly fast to gain more and more benefits. But there is a limit, and you should try a 36 hour fast based on the following important considerations.
No one can decide the frequency of 36 hour fasting better than you, as you know your goals, health status, and comfort level. Anyhow, you can check the general benefits, considerations, and cautions of different frequencies below.
36 Hour Fast Once a Week
Benefits: A 36 hour fast once a week can have metabolic benefits, promote weight loss, and boost autophagy.
Considerations: Doing a 36 hour fast once a week can be suitable for many individuals, especially those with previous experience in fasting.
Cautions: In case of extreme hunger, fatigue, or other negative effects, adjust the fasting schedule or consult a healthcare specialist.
36 Hour Fast 2 Times a Week
Benefits: Doing a 36 hour fast two times a week can increase insulin sensitivity and metabolic flexibility and speed up weight loss.
Considerations: Fasting for 36 hours twice a week is more challenging and requires gradual adaptation. Avoid it if you are new to fasting.
Cautions: If you experience nutrient deficiency or overexertion, you must seek medical help.
Benefits: Doing 36 hour fasting more than twice a week can improve weight loss and metabolic results.
Considerations: You must be a veteran of fasting to try this frequency, as you need to gain enough nutrients.
Cautions: Overdoing fasting can lead to nutrient deficiencies, fatigue, sleep disorders, and disruptions to daily life. It is safer to be patient and keep it slow to get the best results healthily, as good things take time!
You can take a look at the tables below that compare different fasting plans. If you are new to fasting, you can start with simpler versions to avoid health problems.
Dr. Rhonda Patrick, Biomedical Scientist:
"Intermittent fasting, including longer fasts like the 36-hour fast, has profound effects on mitochondrial function and biogenesis, contributing to better energy efficiency and reduced oxidative stress."
The 24-Hour vs. 36 Hour Fast
The following table summarizes the main features of a 24-hour vs. a 36 hour fast.
Aspect
24-Hour Fast
36-Hour Fast
Fasting Period
Avoiding calories for a full day, e.g., from dinner to dinner or from lunch to lunch
One whole day plus an additional 12 hours, e.g., from dinner to breakfast on the day after tomorrow
Benefits
Good insulin sensitivity - Weight loss - Metabolic improvements
Benefits of a 24-hour fast - Better cellular repair due to autophagy
Feasibility
Suitable for those who have done 8-, 12-, and 16-hour fasts - Good for those seeking more challenging fasts
Better for experienced fasters - Very challenging for beginners
Frequency
Depends on personal goals - Can be done once or twice a week or more
Usually once a week
If you aim to lose weight and gain metabolic benefits, you can use both fasting methods. If you seek autophagy benefits, a 36 hour fast is more suitable.
A 36 hour fast is not so comfortable for beginners, and a 24-hour fast or shorter periods can be better.
Your daily routine and lifestyle determine which fasting method works better for you.
If you consider these extended fasting periods, you need to consult a healthcare specialist first to ensure they are safe for you.
Break the fast with a small but nutritious meal. The first meal must be small and easily digestible so the body and digestive system can adjust. Also, drink water or herbal tea gradually to rehydrate your body and avoid digestive issues.
Also, the meals after the fast must contain various macronutrients, including protein, healthy fats, and complex carbohydrates, to provide sustained energy.
Here are examples of the foods best to eat and avoid after a 36 hour fast. But you can always adjust based on the specific diet you are on.
Foods to Eat
Proteins: Lean protein sources like chicken, turkey, fish, tofu, or legumes
Healthy Fats: Avocados, nuts, seeds, and olive oil
Complex Carbohydrates: Whole grains like brown rice, quinoa, sweet potatoes, or whole wheat bread
Vegetables: Various colorful vegetables
Fruits: Fresh fruits like berries, apples, or oranges
Highly Processed Foods: Sugary snacks, fast food, and heavily processed foods
Excessive Sugar: Sugary drinks, candies, and desserts
Fried and Greasy Foods
Large Portions
We generally assume that after 36 hours of not eating, we will be super hungry. But this may not be the case for everyone, and some may not have an appetite for food quickly after the fast. Let’s learn why and what to do with it.
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Why Am I Not Hungry after Fasting for 36 Hours?
Not feeling hungry after fasting for 36 hours is likely due to a combination of decreased ghrelin levels, ketosis, autophagy, and psychological adaptations. These factors work together to suppress appetite and make longer fasting periods more manageable. This is why you may not feel hungry even after fasting.
However, if you experience unusual hunger patterns or any discomfort, it is always a good idea to consult a healthcare professional.
Also, if you are an athlete or are used to doing regular exercise, you need to consider the following points if you want to do a 36 hour fast.
Yes, you can work out during a 36 hour fast, but your exercise routine should match your energy levels and body needs. Drink enough water, monitor your body’s changes, and adjust the intensity of your workouts.
Activities like walking, light jogging, yoga, or stretching are generally safe. High-intensity workouts, such as heavy lifting or intense cardio, are more challenging and can lead to dizziness or fatigue.
Pay attention to how you feel. If you experience dizziness, fatigue, or any discomfort, it is best to stop and rest.
A common concern for athletes and individuals with regular exercise is losing muscle mass during fasting. Let’s see if a 36 hour fast can result in muscle mass loss.
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Will I Lose Muscle During a 36 Hour Fast?
While some muscle loss might occur during a 36 hour fast, it is usually not much if you manage it properly. Eating enough protein, staying hydrated, doing resistance training, and using amino acid supplements can help preserve muscle mass.
Increased growth hormone and autophagy that occur during fasting can also support muscle preservation. So, it is generally safe to work out during fasting without much to worry about.
However, some individuals need special care regarding fasting and should NOT try it without the direct supervision of healthcare professionals. Let’s see if you are among these individuals.
Who Should Not Consider Doing the 36 Hour Fast at All?
Those with underlying medical conditions
Pregnant and breastfeeding women
Children and adolescents
Individuals with eating disorders
Those who have had recent surgery or illness
Those under specific medication
Highly active individuals or athletes)
Individuals with low body weight
Sensitive individuals
If you have consulted physicians and nutritionists and found that a 36 hour fast is safe for you, consider the following tips that can help you get better fasting results.
Plan activities and keep busy to take your mind off food and fasting.
Avoid intense exercise while fasting and go walking or do yoga instead.
Break the fast with small, nutritious meals, and avoid overeating and consuming sugary or processed foods after the fast.
Monitor your body’s reactions and break the fast if you notice dizziness, extreme fatigue, mood swings, or excessive thirst.
Rest and sleep adequately during fasting.
Summary
The 36 hour fast is a prolonged intermittent fasting and is mostly tried by those with more experience in fasting. This kind of fasting can boost metabolic health, help with weight loss, and renew cells.
However, it is not the best type for those who are new to fasting. Consulting a healthcare professional is necessary before doing prolonged fasting.
Plan carefully based on your goals and needs to avoid nutrient deficiencies and health issues. Also, remember to drink lots of water during and off the fasting period to avoid dehydration.
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FAQs
Here are answers to the most frequently asked questions about a 36 hour fast.
How Do I Prepare for a 36 Hour Fast?
Eat a meal containing protein, healthy fats, and complex carbohydrates before starting, and drink plenty of water.
Can I Drink Coffee or Tea During the 36 Hour Fast?
Yes, black coffee and unsweetened herbal teas are fine, but avoid any caloric drinks.
Why Am I Not Hungry after Fasting for 36 Hours?
Ketosis, reduced ghrelin, and autophagy that happen during fasting can suppress appetite, making you feel less hungry.
Can I Work Out While Fasting?
Yes, but do low- or moderate-intensity workouts and drink enough water and electrolytes.
How Do I Break the 36 Hour Fast Safely?
Start with a small, easily digestible meal. Eat gradually and avoid sugary, fried, or processed foods.
Can I Lose Muscle During a 36 Hour Fast?
Yes, but if you stay hydrated, maintain protein intake, and do light resistance exercise, it can be minimal.
How Long Does Autophagy Take to Start?
Autophagy usually begins 12-24 hours into the fast. Benefits increase gradually with fasting duration.
How Often Should I Do a 36 Hour Fast?
Once a week is safe for most people. Twice a week is possible for experienced fasters.
Can Fasting Affect Mood or Energy?
Some may feel irritability, fatigue, or lightheadedness. Rest and hydration help reduce these symptoms.
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