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40-Hour Fast: Does It Work? [Benefits and Side Effects]

A 40-hour fast is a strict type of intermittent fasting, which is tried by veterans in fasting, athletes, and all those individuals pursuing certain health and fitness goals. But the question is if it really works and what its benefits and side effects are. Sometimes prolonged fasting can cause problems instead of solving them. Let’s see if a 40-hour fast’s benefits outweigh its side effects.

40-Hour Fast: Does It Work? [Benefits and Side Effects]

What Is a 40-Hour Fast?

A stricter version of intermittent fasting is a 40-hour fast where you need to avoid calories (food and caloric drinks) for a continuous period of 40 hours. Let’s see why individuals fast for 40 hours and what rules they need to follow.


The main purpose of a 40-hour fast is to help the digestive system rest with a prolonged break from food to gain health benefits and reach fitness goals.

This type of intermittent fasting can positively impact cellular repair, metabolic flexibility, fat burning, blood sugar regulation, insulin sensitivity, and weight loss.


  • Duration: The fast period is a continuous 40-hour period where you can’t consume any calories.
  • Beverages: During the fasting period, you can consume water, herbal teas, and non-caloric beverages. It’s better to drink low-calorie beverages during the eating period too.
  • No Caloric Intake: No calorie is allowed during the fasting period, i.e., you can’t consume solid foods, juices, sugary drinks, and caloric supplements.
  • Minimise Stimulants: Minimising or avoiding stimulants like caffeine helps increase the benefits of the fast.
  • Listen to Your Body: Monitor your body's reactions during fasting. In case of any severe discomfort, dizziness, or other adverse effects, it's advisable to break the fast, consume a healthy meal and seek medical care if needed.
  • Breaking the Fast: When the 40-hour fast is finished, break the fast with a nutritious meal.

Learn More: What Is Fat Fasting? Carnivore Method, Meal Plan, and Food List

Summary: A 40-hour fast includes avoiding calories for 40 hours, which helps cellular repair, fat burning, and blood sugar regulation. Its rules include staying hydrated, avoiding calories, minimising stimulants, and listening to the body. Consult a healthcare professional if needed.

A 40-hour Fast vs a 42- and 48-Hour Fasts

You may have heard about 42-hour and 48-hour fasting when searching for a 40-hour fast. Let’s compare and contrast them to see how they differ and which can be a better choice.


  • Duration: 40-hour fast lasts 40 hours; 42-hour fast lasts 42 hours; 48-hour fast lasts 48 hours.
  • Stimulants: 40-hour fast allows limited stimulants; 42- and 48-hour fasts have stricter stimulant limitations.
  • Meal Break: The timing of the meal breaks vary, which may affect nutrient reintroduction and metabolic shifts.


  • Intermittent Fasting: All three fasting methods include long periods of caloric restriction.
  • Hydration: All three fasts allow water, herbal tea, and non-caloric beverages during the fasting period.
  • Health Effects: All three positively impact cellular repair, fat burning, insulin sensitivity, blood sugar regulation, weight loss, and metabolic performance.
  • Caloric Intake: All the fasts prohibit calorie intake during fasting.
  • Consultation: It’s necessary to consult healthcare professionals before starting all three fasting methods.
  • Nutritious Break: All fasts focus on breaking the fast with nutritious, whole foods.
Summary: 40-hour, 42-hour, and 48-hour fasts differ in fasting duration, stimulants allowed, and breaking the fast. They are similar regarding caloric restriction, potential health benefits, hydration, and focusing on nutrient-dense breaks.

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Is Fasting for 40 Hours Healthy?

A 40-hour fast can be beneficial for our health (we will talk more about its health benefits in the following section) but deciding whether it’s healthy to fast a continuous 40 hours depends on many factors, such as your health status, medical conditions, and other personal parameters.

It means that we can’t say if prolonged fasting is healthy for everybody. To ensure that it works for you, you need to visit a healthcare professional and take some medical tests and decide if it’s healthy for you and how you can optimise it.

Generally speaking, a 40-hour fast can boost cellular repair, fat burning, insulin sensitivity, metabolic sensitivity, weight management, and other health benefits, which we’ll scrutinise in the section below.

Learn More: 16-Hour Fast: Benefits and Results [The Best Foods to Eat]

The Benefits of a 40-Hour Fast

While various studies have been conducted on fasting, including a 40-hour fast, further studies need to confirm the effectiveness of intermittent fasting on humans.

Here are the following science-backed benefits of a 40-hour fast.

Cellular Autophagy and Repair

Fasting can lead to autophagy, which removes and recycles damaged components to promote cellular health and longevity (source).

Insulin Sensitivity and Blood Sugar Regulation

Intermittent fasting can increase insulin sensitivity and improve blood sugar levels, which positively affects diabetes management and decreases the risk of type 2 diabetes (source).

Learn More: Can You Drink Diet Soda While Fasting?

Weight Loss and Fat Metabolism

Long periods of fasting periods can increase fat oxidation and boost weight loss by burning fat for energy (source).

Cardiovascular Health

A 40-hour fast can benefit heart health by decreasing risk factors like blood pressure, cholesterol levels, and inflammation (source).

Neurological Health and Cognitive Function

A 40-hour fast can have neuroprotective effects, improving brain activity and decreasing the risk of neurodegenerative diseases (source).

Summary: Research shows a 40-hour fast can have various benefits, including autophagy, insulin sensitivity, weight loss, and improved cardiovascular and neurological function.

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The Side Effects of a 40-Hour Fast

Despite the many benefits a 40-hour fast can have, it may induce some side effects in some individuals. It doesn’t mean that they can definitely happen for everyone, but knowing the side effects help us make more informed decisions.

Here are some of the most common side effects of a 40-hour fast.

  • Hunger and Irritability: Restricting caloric intake can cause hunger and irritability in prolonged fasting.

Source: Johnstone, A. (2015). Fasting for weight loss: an effective strategy or latest dieting trend? International Journal of Obesity, 39(5), 727-733.

  • Reduced Energy and Fatigue: A 40-hour fast can decrease energy levels and cause fatigue since the body tries to use stored energy.

Source: Gabel, K., Hoddy, K. K., Haggerty, N., Song, J., Kroeger, C. M., Trepanowski, J. F., ... & Varady, K. A. (2018). Effects of 8-hour time restricted feeding on body weight and metabolic disease risk factors in obese adults: A pilot study. Nutrition and Healthy Aging, 4(4), 345-353.

  • Dehydration and Electrolyte Imbalance: Long hours of fasting without consuming enough water and beverages can cause dehydration and electrolyte imbalances.

Source: Hartman, M. L., Veldhuis, J. D., Johnson, M. L., Lee, M. M., Alberti, K. G., Samojlik, E., & Thorner, M. O. (1992). Augmented growth hormone (GH) secretory burst frequency and amplitude mediate enhanced GH secretion during a two-day fast in normal men. The Journal of Clinical Endocrinology & Metabolism, 74(4), 757-765.

  • Blood Sugar Fluctuations: Long hours of fasting can cause blood sugar fluctuations, especially hypoglycemia (low blood sugar) in some dieters.

Source: Antoni, R., Robertson, T. M., Robertson, M. D., & Johnston, J. D. (2016). A pilot feasibility study exploring the effects of a moderate time-restricted feeding intervention on energy intake, adiposity and metabolic physiology in free-living human subjects. Journal of Nutritional Science, 5, e2.

  • Gastrointestinal Distress: Fasting can cause bloating, constipation, diarrhea or other digestive issues.

Source: Chung, H., Chou, W. C., Sears, D. D., & Patterson, R. E. (2016). Webster's new world public speaking handbook (4th ed.). New York, NY: Wiley.

  • Dizziness and Weakness: A 40-hour fast can cause dizziness, weakness, or lightheadedness.

Source: Heilbronn, L. K., Smith, S. R., Martin, C. K., Anton, S. D., & Ravussin, E. (2005). Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism. The American Journal of Clinical Nutrition, 81(1), 69-73.

  • Mood Changes and Mental Health: Consuming no calories can cause mood swings or anxiety or worsen mental health conditions in some individuals.

Source: Hajek, P., Myers, K., Dhanji, A. R., & West, O. (2018). Mood and neuropsychological effects of different doses of nicotine gum in habitual smokers and never-smokers. Psychopharmacology, 235(3), 907-914.

Learn More: [Expert Guide] 20-Hour Fast Results, Side Effects, and How to

Summary: A 40-hour fasting can have side effects like hunger, fatigue, dehydration, blood sugar changes, and mood swings in some individuals.

How to Do the 40-Hour Fast?

To make drastic changes to your eating habits, like fasting for 40 hours, you first need to consult a healthcare professional to make sure it’s safe. After that, you can follow the steps below to start a healthy 40-hour fast.

To prepare for a 40-hour fast:

  • Set a Specific Time: Pick the best period you can fast for 40 hours without significant physical or mental stress.
  • Hydration: Drink enough water before and during the fast.

Then, to do the actual fast:

  • Start with an Early Dinner: Have a balanced and nutritious dinner containing lean proteins, vegetables, and whole grains, early in the evening, around 6-8 hours before your usual bedtime.
  • Stay Hydrated: Drink enough water, herbal tea, or other non-caloric beverages during the fast.
  • Plan Distractions: Stay busy to keep your mind off the diet and can handle the long hours more easily.
  • Focus on Electrolytes: Add some salt to your water to maintain electrolyte balance.
  • Listen to Your Body: Monitor your body’s changes and break the fast if you feel too much hunger, dizziness or discomfort.
  • Sleep Well: Sleeping enough and well can help manage hunger and support your body during the fasting period.

To Break the Fast:

  • Reintroduce Nutrients Gradually: Avoid overeating and break the fast with a small but nutritious meal containing protein, healthy fats, and carbohydrates.
  • Include Whole Foods: Avoid processed foods and opt for lean proteins, vegetables, fruits, and whole grains.
  • Stay Hydrated: Keep drinking water during the eating period to stay hydrated.
  • Listen to Your Body: Monitor your body’s reactions to food after the fast.

After the fast try to

  • Reflect and Learn: Keep a record of your body’s reactions and changes to make necessary changes and let your body adapt better.
  • Space Out Fasts: Consult a healthcare professional to determine enough time you need to wait before starting the next round of fasting to avoid negative effects on metabolism or energy levels.

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What Happens to Your Body on a 40-Hour Fast?

Various physiological changes occur in the body during a 40-hour fast including the following.

  • 0-6 Hours: Your body keeps digesting and absorbing nutrients from your last meal. Blood sugar level start to decrease as the body burn glucose to produce energy.
  • 6-12 Hours: The body starts to burn glycogen stored in the liver and muscles for energy. Insulin levels drop so that the stored fat can be broken down into fatty acids.
  • 12-18 Hours: Fat breakdown leads to ketosis and the body begins to produce ketones. Hunger hormones are suppressed, which may reduce appetite.
  • 18-24 Hours: Ketosis continues and ketones become the main energy source, sparing muscle protein. More growth hormone is produced to support fat metabolism and muscle maintenance.
  • 24-30 Hours: Cellular autophagy increases as the body breaks down and recycles damaged or dysfunctional cellular components to produce energy. Insulin sensitivity increases, which helps regulate glucose.
  • 30-40 Hours: Ketosis continues and more ketones are produced to provide energy for various tissues, including the brain. The body relies mainly on fat for energy, which boosts fat oxidation. Also, the body tries to conserve energy, which may slightly decrease the metabolic rate.
  • After 40 Hours: Hunger may reappear, signalling the body's need for nourishment as the body keeps using fat for energy.

Learn More: 24-Hour Fast: How to Do It? Is It Good for Weight Loss? [Expert Guides]

How Often Should I Do a 40-Hour Fast?

The frequency of fasting for 40 hours depends on various personal factors and only two people can work together to find it out, i.e., you and your physician!

The factors involved in deciding the frequency of a 40-hour fast are your individual health status, goals, and lifestyle. You can use the following tips before deciding on the frequency of fasting for 40 hours.

  • Consult a healthcare professional.
  • Listen to your body.
  • Start slowly with shorter fasting periods to see how your body reacts.
  • Review your goals as they influence the fasting frequency. Some goals include weight loss, improved metabolic health, improved general health, etc.
  • Find a balanced approach to health and combine fasting with a good diet and enough daily exercise to see the best results.
  • Give your body enough time between fasting periods to recover and get adequate nutrients.
  • Don’t just do what others do as you need to consider your daily routines, work commitments, social events, and lifestyle when planning fasting periods.

Learn More: 48-Hour Fast: Benefits and Results: Is It Safe?

40-Hour Fast and Autophagy

A 40-hour fast can initiate autophagy in the body, which is a cellular cleansing process activated when the body is under stress like during fasting, helping the body get rid of damaged or dysfunctional components.

During fasting, when glucose and glycogen are used up, the body starts to use fatty acids and ketones to produce the necessary energy for the body, which triggers autophagy to optimise energy use and ensure cellular survival.

Autophagy means breaking down and recycling cellular components like damaged proteins, organelles, and other cellular debris, i.e., cleaning cellular waste and helping the body reproduce new, healthy cells.

Autophagy leads to various health benefits, including cellular repair, cellular longevity, disease prevention, immune system support, and metabolic health.

Learn More: The 96-Hour Fast Benefits and Results: Is It Safe?

40-Hour Fast and Weight Loss

One of the main benefits of a 40-hour fast is weight loss, but the amount of it depends on various individual factors. You can’t lose weight healthily just by avoiding food for long hours. You need to do it step by step as explained earlier in this article.

Anyhow, here's how a 40-hour fast can impact weight loss:

  • Caloric Deficit:

Fasting for 40 hours restricts calorie intake, which leads the body to use stored fat for energy, resulting in fat melt and weight loss over time.

  • Fat Oxidation:

The process of shifting from depleted glycogen to fat for energy is called fat oxidation, which can lead to increased fat loss, boosting weight loss.

  • Insulin Sensitivity:

Fasting helps insulin sensitivity, which means better regulating blood sugar levels. Improved insulin sensitivity prevents overeating and promotes fat loss.

  • Hormonal Changes:

Fasting impacts appetite and metabolism hormones related to appetite (ghrelin and leptin), which reduces hunger and decreases food intake, boosting weight loss.

  • Increased Metabolic Rate:

Intermittent fasting can increase metabolic rate, which involves burning more calories and losing more weight.

  • Appetite Regulation:

Fasting can suppress appetite, making you less prone to overeating.

Learn More: 18-Hour Fast Diet: Benefits and Weight Loss Results

What Should I Break My 40-Hour Fast With?

Start with small meals and don’t put too much pressure on your body, especially the digestive system, by eating a heavy meal. Let your body adapt to the long fasting situation. Here are the foods you need to eat and avoid for a healthy fast.

Foods to Eat

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Foods to Avoid

  • Processed Foods: Including fast food, sugary snacks, packaged and processed foods
  • Highly Sugary Foods: Including candies, pastries, soda and sugary beverages
  • Greasy and Fried Foods: including fried chicken, french fries, deep-fried foods
  • Excessive Dairy: Including heavy cheeses, cream-based dishes
  • Large Portions
  • Alcohol

Learn More: The 60-Hour Fast Benefits and Results: Is It Safe?

What Can I Drink on a 40-Hour Fast?


A 40-hour fast can have positive impacts on health and fitness if done correctly. Simply put, it can be beneficial for cell repair, weight loss, and blood sugar levels, but the side effects include hunger, dizziness, and fatigue.

You need to stay hydrated during the fasting period and eat nutrient-dense foods out of the fasting period to provide the necessary vitamins and minerals and let your body adapt to the strict fasting situation.

Also, if you’re new to fasting, it’s better to start with less restrictive versions of intermittent fasting, like 12-hour fasting, to see how your body reacts.

It’s no secret that consulting a healthcare professional helps you avoid unwanted health issues and ensure you make the most of fasting while achieving your health and fitness goals.

Remember that health always comes first!


Here are the answers to the most frequently asked questions about a 40-hour fast.

What Are The Stages of a 40-Hour Fast?

  1. Fed state (0-3 hours)
  2. Early fasting state (3-18 hours)
  3. Fasting state (18-40 hours)

What Happens If You Don't Eat for 40 Hours?

In addition to the side effects mentioned above, a 40-hour fast can cause dehydration if you don't drink enough fluids.

Can Fasting Reduce Belly Fat?

Yes, fasting can help reduce belly fat and, generally, body fat.

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