Blog Diets Fasting The Benefits of 40 Hours Fasting: Do They Outweigh The Potential Risks?

The Benefits of 40 Hours Fasting: Do They Outweigh The Potential Risks?

After you’ve conquered the 16-hour fast (famed for being a flexible and easier version for beginners), the 18-hour fast (at which point autophagy may start to increase) and even the revered 23-hour fast (that many take months to work toward), you’re on to the next challenge: a 40 plus hour fast.

This duration is often referred to as extended fasting, pushing the boundaries of endurance and willpower.

We should probably put this disclaimer right up front: prolonged fasting is not for everyone and should be embarked upon with caution, particularly for those with certain health conditions or pregnant or breastfeeding women.

But if you’re ready for it, you should consider whether the purported benefits such as amplified weight loss, improved brain health, and increased cellular repair, outweigh the potential risks like nutrient deficiencies or stress on the body.

Here’s what you need to know about balancing the benefits and risks of 40-hour fasting.

What Are The Stages of a 40-Hour Fast?

The stages of a 40-hour fast include the postprandial state, glycogen release, gluconeogenesis and mild ketosis, and ketosis with autophagy and growth hormone increase (13) (20). Each stage has its own set of symptoms, with hunger and fatigue being common in later stages.

Stage 1: 0-4 Hours – Postprandial State

In the first 4 hours after your last meal, your body is in the postprandial state, meaning it’s fully engaged in the process of digestion. The food you consumed is broken down into smaller molecules, such as glucose, which are released into the bloodstream. These molecules provide energy for your body’s cells, while the hormone insulin, secreted by the pancreas, ensures that the glucose enters the cells.

  • Symptoms: No noticeable symptoms as your body is operating as usual, fueled by the energy from your last meal.

Stage 2: 4-16 Hours – Glycogen Release

As you move beyond the 4-hour mark and continue up to 16 hours of fasting, your body enters a state called glycogenolysis. During this stage, glycogen, stored in your liver, is broken down into glucose to supply energy. This is your body’s way of maintaining blood sugar levels in the absence of new food intake.

  • Symptoms: Some people may begin to feel a little hungry or experience minor energy dips.
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Stage 3: 16-24 Hours – Gluconeogenesis and Mild Ketosis

Once glycogen reserves are depleted, the body initiates a process known as gluconeogenesis, which translates to ‘the creation of new sugar.’ 

The liver generates glucose from non-carbohydrate sources like lactate, amino acids, and glycerol to maintain blood sugar levels. Simultaneously, the body enters a mild state of ketosis, whereby it begins to burn fat for energy instead of glucose.

  • Symptoms: You may start to feel more serious hunger pangs, irritability, or fatigue. Some people experience what’s called the ‘keto flu‘ with symptoms like headaches, dizziness, or mental fatigue.

Stage 4: 24-48 Hours – Ketosis, Autophagy, and Growth Hormone Increase

After 24 hours of fasting, the body is primarily using fat for energy, and ketosis becomes more pronounced. This is when autophagy, a cellular cleanup process, is thought to really kick into gear. 

During this period, damaged cellular components are broken down and recycled. Growth hormone levels may also increase, promoting muscle growth and fat loss.

Symptoms: After an initial slump, many people report an increase in energy levels and mental clarity. The feeling of hunger might also start to decrease.

40 hours fasting  

What Happens If You Don’t Eat for 40 Hours, Consistently?

Not eating for 40 hours consistently is a form of prolonged fasting, which can have significant effects on your body. Some of the potential benefits and risks include:

See also
Intermittent Fasting 14/10 vs 16/8: Differences, Similarities and How To Make The Best Choice

More Efficient Digestion

When you embark on a 40-hour fast, your digestive system undergoes some beneficial changes that can improve your overall digestion. The break from constant food intake gives the digestive organs a chance to rest and repair, possibly promoting better functioning in the long run.

One of the key transformations involves the Migrating Motor Complex (MMC). The MMC is like the digestive system’s housekeeping service (14). 

When you’re not eating, this complex gets activated approximately every 90 minutes to two hours, sweeping through leftover undigested material in your gut. 

During a 40-hour fast, your MMC gets extra opportunities to clean house, which may help keep your gut in tip-top shape.

Speedy Weight Loss

If you know the calories burned while fasting for 24 hours, then you know exactly how effective a 40-hour fast can be as far as weight loss is concerned. In fact, it’s touted as a quicker way to achieve the weight loss results commonly seen with intermittent fasting.

By the 40-hour mark, multiple processes are happening in your body, related to weight loss:

  • Fat-burning lipolysis – Ketosis is now in full force, making fat stores the primary energy source (4).
  • Metabolic flexibility – The longer you fast, the more adapted your body becomes to using fat for energy. This is thought to increase your metabolic flexibility and make it easier for you to transition between periods of eating and fasting (18).
  • Increased growth hormone – As mentioned earlier, growth hormone levels rise during a 40-hour fast. Growth hormones may help improve muscle mass and metabolism, which can contribute to weight loss (9).

Notably, weight loss results from 40-hour fasting may vary depending on individual factors such as body composition, activity levels, and overall dietary habits.

See also
Endomorph Intermittent Fasting: How It Works

Better Cognitive Function

The brain is said to be one of the beneficiaries of a 40-hour fast.  At the surface level, some people say fasting makes them feel more alert and focused. 

On a deeper level, fasting which involves ketone production may have neuroprotective effects, meaning it may help protect brain cells from damage and improve overall cognitive function (10). 

BDNF, or brain-derived neurotrophic factor, is another key player in cognitive function. During a fast, levels of BDNF increase and may help improve neuronal health and promote the growth of new neurons (22).

However, some may experience brain fog during extended fasts. This is typically temporary and usually resolves after the transition phase into ketosis, but if it persists, consider breaking your fast.

40-Hour Fast Autophagy

Autophagy deserves its own section here, as it’s one of the most significant biological processes that is thought to occur during a prolonged fast such as a 40-hour fast (21). 

Autophagy is a natural process in which cells break down and recycle damaged or unnecessary components to create new ones. This process can help maintain cellular health and function, improve immune system function, and may even extend lifespan (2)

During a 40-hour fast, autophagy is believed to be increased due to the absence of incoming nutrients. 

Additionally, studies have shown that autophagy may also play a role in fighting against certain diseases such as cancer and neurodegeneration (1).

To promote autophagy during a 40-hour fast you must hydrate and consume enough electrolytes. These nutrients help maintain cellular function and support the recycling process of autophagy.

Improved Insulin Sensitivity

Fasting has been suggested to improve insulin sensitivity, which is crucial for maintaining healthy blood sugar levels (3). This could be especially beneficial for those with conditions like type 2 diabetes or metabolic syndrome.

See also
How To Break A Fast The Right Way: A Comprehensive Guide

During a 40-hour fast, your body uses up stored glucose and starts using ketones for energy instead. This can help lower blood sugar levels and reduce insulin resistance.

In addition, fasting has been shown to increase the expression of genes that improve insulin sensitivity and decrease inflammation  (19).

However, anyone with diabetes or insulin resistance should consult with their healthcare provider before attempting any length of fasting. 

Read more: Radish Recipes: Fun Ways To Enjoy This Root Vegetable

Immune System Support

Last, but certainly not least, a 40-hour fast can have positive effects on your immune system. Autophagy plays a key role here as well, by eliminating damaged or infected cells and promoting the production of new ones.

Fasting has also been shown to decrease inflammation, which is crucial for maintaining a strong immune system (15). Chronic inflammation is linked to various health conditions such as heart disease, cancer, and autoimmune disorders (5).

40 hours fasting  

Risks Associated With a 40-Hour Fast

Despite the numerous benefits, there are risks associated with a 40-hour fast that should be considered.

Some people may experience symptoms such as dizziness, fatigue, or headaches during extended fasts (17). This is typically temporary and can be resolved by staying hydrated and ensuring proper electrolyte intake.

Fasting for longer periods of time may also lead to nutrient deficiencies if not properly managed (11). This is especially true if you’re doing 40-hour fasts twice a week or more; you simply don’t have enough time to replenish essential nutrients.

Another concern is the potential for disordered eating habits or an unhealthy relationship with food. It’s important to approach fasting in a balanced and sustainable way, rather than using it as a quick fix for weight loss.

Lastly, there are some groups of people who should never attempt a 40-hour fast, such as pregnant or breastfeeding women, children, and individuals with certain medical conditions or who are taking chronic medications. These groups should consult with a healthcare professional before attempting any type of fasting.

See also
7 Zero Calorie Drinks for Intermittent Fasting (And a Few More to Avoid)

Is a 40-Hour Fast Worth It?

Yes, and no.

A 40-hour fast may be worth it for you if:

  1. You’re healthy, not pregnant or breastfeeding, and have consulted with a healthcare professional.
  2. You’re looking for the potential 40-hour fast health benefits such as improved cognitive function, reduced inflammation, and improved insulin sensitivity.
  3. You have a balanced approach to fasting and are not using it solely for weight loss purposes.
  4. You started with shorter fasts and have gradually worked your way up to a 40-hour fast.

On the other hand, a 40-hour fast may not be worth it for you if:

  1. You have a history of disordered eating or an unhealthy relationship with food.
  2. You’re pregnant or breastfeeding.
  3. You have a medical condition or are taking medication that could be negatively affected by fasting.
  4. You’re solely using it as a quick fix for weight loss rather than incorporating it into a balanced and sustainable lifestyle.
  5. You’re a beginner to fasting and have not tried shorter fasts first.

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The Bottom Line

Fasting for 40 hours has gained popularity due to its purported benefits such as weight loss, improved brain health, and increased cellular repair. However, it is not suitable for everyone and should be approached with caution. 

Despite the potential risks such as nutrient deficiencies, many people report increased energy levels and mental clarity, and you may want to try this extended fast if it’s safe for you to do so.


This article is intended for general informational purposes only and does not address individual circumstances. It is not a substitute for professional advice or help and should not be relied on to make decisions of any kind. Any action you take upon the information presented in this article is strictly at your own risk and responsibility!


  1. Autophagy in inflammation, infection, neurodegeneration and cancer (2013, ncbi
  2. Autophagy: cellular and molecular mechanisms (2010,
  3. Beneficial effects of intermittent fasting: a narrative review (2023,
  4. Biochemistry, Lipolysis (2023,
  5. Chronic inflammation in the etiology of disease across the life span (2020,
  6. Consumer Reports of “Keto Flu” Associated With the Ketogenic Diet (2020,
  7. Cyclic AMP (2008,
  8. Effects of Commercially Available Dietary Supplements on Resting Energy Expenditure: A Brief Report (2014,
  9. Effects of Growth Hormone on Glucose, Lipid, and Protein Metabolism in Human Subjects  (2009,
  10. Effects of Ketone Bodies on Brain Metabolism and Function in Neurodegenerative Diseases (2020,
  11. Effects of Periodic Religious Fasting for Decades on Nutrient Intakes and the Blood Biochemical Profile (2021,
  12. Fasting Drives Nrf2-Related Antioxidant Response in Skeletal Muscle (2020,
  13. Fasting: Molecular Mechanisms and Clinical Applications (2014,
  14. Interdigestive migrating motor complex -its mechanism and clinical importance (2013,
  15. Intermittent fasting and immunomodulatory effects: A systematic review (2023,
  16. Intermittent fasting and weight loss (2020,
  17. Intermittent Fasting: Benefits, Side Effects, Quality of Life, and Knowledge of the Saudi Population (2023,
  18. Metabolic Flexibility as an Adaptation to Energy Resources and Requirements in Health and Disease (2018,
  19. Molecular Mechanisms of Intermittent Fasting-induced Ischemic Tolerance (2020,
  20. Physiology, Fasting (2023,
  21. The effect of prolonged intermittent fasting on autophagy, inflammasome and senescence genes expressions: An exploratory study in healthy young males (2023,
  22. The Effects of Intermittent Fasting on Brain and Cognitive Function (2021,
  23. The Role of Intermittent Fasting and Dieting on Cognition in Adult Population: A Systematic Review of the Randomized Controlled Trials (2023,


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