How a 72-Hour Fast Works For Weight Loss And Health Benefits

A 72-hour fast is an extended fasting method ensuring weight loss and health benefits. This comprehensive article will discuss the scientific data on 72-hour fasting’s weight loss and health benefits and provide tips on how to do it safely. 

Key Takeaways:

  • 72-hour fasting abstains any caloric intake for 3 consecutive days. Water, unsweetened tea or coffee, and other non-caloric beverages are allowed.
  • A 72-hour fast guarantees immediate weight loss due to water and fat loss from glycogen depletion, ketosis, and increased metabolic rate. 
  • 72-hour fasting improves brain and metabolic health, reduces inflammation, boosts immunity and healing, and lowers cancer risks. 
  • Gradual adjustments, proper hydration and refeeding, and mindful physical activities are crucial to maintaining a safe fasting period.

Table of Contents:

What Is a 72-Hour Fast?

During a 72-hour fast, you abstain from all caloric intake for 72 hours and only focus on water, tea, coffee, and other non-caloric beverages.

It is a prolonged intermittent fasting technique known as an extended water fast.

Unlike shorter intermittent fasting methods, a 72-hour fast extends the fasting window to three consecutive days.

This approach triggers various physiological responses, influencing metabolism, energy utilization, and cellular processes.

Who Should Not Do a 72-Hour Fast?

While the 72-hour fast has numerous health benefits, its extreme calorie restriction is not for everyone. The following population should avoid doing the 72-hour fast without medical supervision:

  • Children: Children are in a crucial phase of growth and development, and a 72-hour fast could disrupt their nutrient balance and hinder proper growth.
  • Pregnant and lactating women: These women have increased nutritional requirements to support their health and the growth and development of the fetus or infant.
  • People with a history of eating disorders: Fasting may trigger unhealthy behaviors or exacerbate existing psychological challenges related to food.
  • People with diabetes: Prolonged fasting can impact blood sugar levels and insulin requirements and may lead to diabetic ketoacidosis. 
  • People with maintenance medications: Medications often need to be taken with food to prevent gastric irritation, making fasting an impossible option.
  • Underweight people: The 72-hour fast may lead to excessive weight loss, which is not ideal for underweight people.

Does a 72-Hour Fast Offer Weight Loss Results?

Weight loss is one of the main benefits of 72-hour fasting. Studies show that a 3-day (72-hour) fast makes one lose more than 1.5% of the body weight immediately1 after the fasting duration. 

Weight loss in 72-hour fasting results from several physiological processes, including water loss and fat loss due to glycogen depletion, ketosis, and increased metabolic rate. Here’s how:

1. Glycogen Stores Depletion:

In the initial phase of a 72-hour fast, the body relies on stored glycogen for energy. Glycogen, the glucose in the liver and muscles, holds 3 grams of water per gram.

Studies show that glycogen stores start depleting after 24 hours of fasting.2 As glycogen depletes, water is also released, causing immediate weight loss due to water loss. This means that you will immediately see weight loss in the 72-hour duration.

2. Transition to Ketosis:

As glycogen stores diminish, the body transitions into a state of ketosis. A 2002 study confirms that the range of time it takes to induce ketosis is about 33 hours (range 17 to 48).3 This means that fasting for 72 hours results in good ketosis.  

During ketosis, the body utilizes stored fat as its primary energy source.4 Fatty acids are released from adipose tissue and converted into ketone bodies, providing energy for various bodily functions. 

This mechanism starts fat loss, which results in sustainable weight loss.

3. Caloric Deficit:

Fasting creates a caloric deficit as no external calories are consumed during the 72 hours. Studies have established that a negative energy balance is responsible for weight loss.5 This means you are using more energy than you consume. 

With extended fasting methods with no calorie intake, the body relies on fat for energy, leading to weight reduction. 

4. Increased Metabolic Rate:

Extended fasting can increase metabolic rate. The body may enter a state of energy conservation, slowing down non-essential functions. 

Older studies state the resting metabolic rate started increasing6 after 36 hours of fasting. This is backed by a 2000 study stating that a 3-day fast increases metabolic rate by 14%.7

This decrease is often balanced by an increase in the release of norepinephrine and other catecholamines, promoting calorie expenditure.

5. Preservation of Lean Body Mass:

Contrary to some misconceptions, well-structured fasting periods, such as a 72-hour fast, may promote the preservation of lean body mass thanks to growth hormone, which increases during fasting.

Other Benefits of a 72-Hour Fast

1. Activates Autophagy

A 72-hour fast activates autophagy. This cellular cleansing process8 removes damaged or dysfunctional cellular components, promoting cellular renewal and overall cellular health.

Studies show that autophagy helps discard cancer cells while protecting normal cells,9 and dysregulation of autophagy results in chronic conditions such as diabetes, Crohn’s, and Parkinson’s disease.10

Moreover, newer studies state autophagy’s role in wound healing11 and its effect in lowering oxidative stress.12

2. Triggers Ketosis

Extended fasting induces ketosis, where the body utilizes ketone bodies as an alternative energy source.4 This metabolic shift enhances fat utilization, energy metabolism, and overall health.

Ketosis promotes fat oxidation,13 improves cognition,14 improves heart health,15 and has anti-cancer benefits.16

3. Regulates Blood Glucose Levels

A 72-hour fast stabilizes blood glucose levels by reducing reliance on glucose for energy. In a 2018 study, healthy women who did the 72-hour fast had significant decreases in glucose levels,17 body weight, and an increase in ketone bodies. 

Due to 72-hour fasting’s ability to regulate blood glucose levels, it has been the gold standard for diagnosing insulinoma,18 a benign tumor19 causing abnormally elevated insulin levels.

However, while 72-hour fasting increases lipid oxidation, prolonged fasting may result in insulin resistance20 among people with metabolic disorders like diabetes, making it ideal for healthy individuals. 

4. Improves Heart Health

Extended fasting offers cardiovascular benefits,21 including reduced blood pressure, improved cholesterol profiles, and enhanced heart function. These effects contribute to overall heart health and may reduce the risk of cardiovascular diseases.

It’s crucial to note that extended fasting causes lipolysis,22 a shift where the body breaks down fat, causing an elevation in plasma triglyceride and very low-density lipoprotein (VLDL).23 This only means that the body is actively burning fats for energy. 

Nevertheless, 72-hour fasting has been proven effective for people with elevated lipid levels. 

5. Boosts Brain Health

While the mechanism remains unclear, intermittent fasting has neuroprotective effects.24 Moreover, studies suggest intermittent fasting enhances cellular stress resistance, synaptic plasticity, and neurogenesis.25

Due to this reason, Intermittent fasting may delay or reverse Alzheimer’s disease26 and Parkinson’s disease.27

6. Reduces Inflammation

Fasting has been shown to reduce inflammatory markers, potentially mitigating the impact of inflammation on the body. 

Chronic inflammation28 results in various health conditions, including CVD, diabetes, malignancy, auto-immune disease, and chronic hepatic and renal disease. 

In a study of 8-week intermittent fasting,29 subjects showed reduced inflammatory markers such as CRP and calprotectin and were seizure-free throughout the intermittent fasting duration. Intermittent fasting also modulates immune cells and cytokines,30 preventing unnecessary inflammation. 

7. Boosts Immunity

Temporary stress on the immune system during fasting may stimulate the production of new immune cells, such as B and T cells,31 enhancing overall immune function. This immune-boosting effect could contribute to better resistance against infections.

In a 2021 study, 72-hour fasting increased leukocyte viability32 and number, especially the neutrophils, which act on immediate immune responses.

8. Detoxifies the Body

Fasting enhances the liver’s detoxification33 by keeping it in its best condition. Studies show that a 72-hour fast showed better liver test results,34 including serum alanine transaminase, aspartate aminotransferase, and hepatic steatosis and stiffness. 

These results mean the liver will be more efficient in detoxifying the body. 

9. Promotes Healing

Fasting heightens cellular repair processes, supports overall tissue repair and regeneration, accelerates injury healing, and contributes to improved recovery.

This is mainly due to the activation of immune cells32 that respond to injury immediately, kicking off the healing process. Moreover, autophagy in fasting also helps with faster wound healing. 

10. Increases Growth Hormone Production

Intermittent fasting’s ability to increase growth hormone (GH) release has been established since the 19th century. Older studies have proven intermittent fasting’s ability to increase the frequency and amplitude of GH.35

GH helps regulate muscle mass,36 which indicates its protective effects during fasting periods. 

11. Lowers Risk for Cancer

Research suggests that 72-hour fasting reduces the risk of certain types of cancer thanks to its ability to induce autophagy.37 Fasting may influence cellular processes that inhibit the development and progression of cancer cells.

Moreover, studies suggest that combining fasting with chemotherapy, immunotherapy, or other treatments38 increases treatment efficacy and reduces side effects. 

How to Do a Safe 72-Hour Fast

72-hour fast (3 days)

1. Prepare

Before initiating a 72-hour fast, please familiarize yourself with the fasting process and its potential effects. Consult with healthcare professionals, especially if you have underlying health conditions or are on medications. 

2. Introduce Fasting Gradually

If you’re new to extended fasting, consider a gradual approach. Start with shorter intermittent fasting periods and gradually extend the fasting window. This acclimation helps your body adapt and minimizes potential discomfort.

You can safely do the 72-hour fasting once a month and slowly adjust the frequency if your body tolerates it well. 

3. Hydrate Well

Water intake is crucial to prevent dehydration, and adding electrolytes can help maintain proper balance and minimize potential side effects like headaches or dizziness. 

Remember that you can only drink non-caloric beverages. If you opt for sports drinks, check the nutrition facts. 

4. Be Active

While light physical activities like walking are generally safe during a 72-hour fast, avoid intense exercises to prevent overexertion. If you feel weak or dizzy, consider opting for rest or gentler activities.

5. Break the Fast Gradually

After a 72-hour fast, prioritize nutrient-rich and easily digestible foods during refeeding. Incorporate fruits, vegetables, and lean proteins to provide essential nutrients without overwhelming your digestive system.

6. Monitore and Adjust

Regularly monitor your body’s responses during and after the fast. If incorporating 72-hour fasts regularly, schedule check-ins with healthcare professionals. 

Should You Try It?

The 72-hour fast is a safe and effective way to lose weight and improve overall health by promoting cellular regeneration, detoxification, and reduced inflammation. 

If you have tried 24-hour and 48-hour fasting in the past, you can try doing the 72-hour fast once a month. However, those without prior fasting experience must start gradually to prevent side effects. 

If you have an underlying medical condition, consult your healthcare provider for safety recommendations. 

Frequently Asked Questions

What does a 72-hour fast do to your body?

72-hour fasting decreases water weight and kickstarts fat loss, resulting in immediate weight loss. It also promotes cognitive and metabolic health, reduces inflammation, and detoxifies the body. 

How much weight can you lose on a 72-hour fast?

Everyone loses weight differently, but studies show a minimum of 1.5% of immediate weight loss after fasting. 

What breaks a 72-hour fast?

Any caloric intake breaks a 72-hour fast. This includes solid and liquid foods with calories. Otherwise, you can have as many non-caloric drinks as you like while fasting. 

What is allowed during a 72-hour fast?

Non-caloric drinks, such as water, unsweetened tea and coffee, non-caloric sodas, and energy drinks, are safe to consume during a 72-hour fast. Light exercises are also allowed. 


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Mag. Stephan Lederer, MSc.

Mag. Stephan Lederer, MSc. is an author and blogger from Austria who writes in-depth content about health and nutrition. His book series on Interval Fasting landed #1 on the bestseller list in the German Amazon marketplace in 15 categories.

Stephan is a true man of science, having earned multiple diplomas and master's degrees in various fields. He has made it his mission to bridge the gap between conventional wisdom and scientific knowledge. He precisely reviews the content and sources of this blog for currency and accuracy.

Click on the links above to visit his author and about me pages.

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