To understand how to fast for 40 days, it is essential to first understand that a 40-day complete fast (with only water) carries extreme physiological risk and is not recommended for most people. This article covers why extended fasting can be dangerous, what happens to the body during the process, safer alternatives through intermittent fasting, and what to eat if you are undertaking a modified extended fast.
The idea of fasting is closely connected to several historical and cultural traditions around the world. Fasting generally involves abstaining from food completely or partially for a certain period of time, often lasting anywhere from 12 to 24 hours. However, some individuals attempt an extended 40-day fast, which is incredibly challenging for physical, emotional, and mental health. It can be difficult to navigate daily life without the meals that provide essential nutrients and energy, or the beverages that keep you refreshed throughout the day. Some people only allow themselves water and non-sugary beverages during their 40-day fasting period, which places an immense demand on the body’s reserves.
When considering an extended fasting period, it is vital to approach the topic with caution and a focus on long-term wellness. Should you do it? There are significant physiological downsides to extended fasting, and exploring more sustainable, modified approaches is often the best path forward. Let’s look closer at the realities of the 40-day fast.
A 40-day fast or any extended fast beyond 24-72 hours carries significant health risks and should only be considered with comprehensive medical supervision. The information in this article is educational and does not constitute medical advice. Speak with your healthcare provider before attempting any extended fasting protocol. If you experience severe symptoms during any fast, stop immediately and seek medical attention.
Can You Fast for 40 Days Without Food?
Fasting for 40 days without food may be a personal choice for some individuals. However, no matter what your reason is—whether cultural, personal, or for wellness goals—such a fast carries significant risk for most people.
There are many complex factors that impact the length for which a person can survive without food. Fasting for 40 days may physically happen in rare, supervised instances without food, but the physiological toll is immense (1). Scientists have not studied human starvation extensively for ethical reasons, but some reports have noted that death can occur between 43 and 70 days of fasting (2).
Shorter-period intermittent fasting has attracted research interest for its potential associations with weight management (3), but an extended 40 day water fast is a completely different scenario.
Fasting for 40 days with only water can cause extreme changes to your body. When you go without food, your body starts to obtain its energy and fuel from its own internal stores (4). Your body naturally requires a steady intake of nutrients from food to support everyday functions. It uses carbohydrates, proteins, and fats, in addition to essential minerals and vitamins, to renew cells and fuel vital bodily processes (5).
Therefore, fasting for 40 days without food is generally discouraged, even for healthy people.
To better understand how the body adapts when deprived of external food sources, consider this breakdown of the body’s primary fuel sources over time:
| Fasting Phase | Time Without Food | Primary Fuel Source |
|---|---|---|
| Phase 1 | 0 to 12 hours | Glycogen (stored carbohydrates in the liver and muscles) |
| Phase 2 | 12 hours to several days | Fat stores (the body enters ketosis to use fat for energy) |
| Phase 3 | Extended starvation | Muscle protein (the body begins breaking down muscle tissue) |
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Is a 40-Day Fast Appropriate for Most People?
A 40 day fast is not considered appropriate for most people due to significant physiological risks.
Some individuals are highly goal-oriented and may consider extreme lifestyle shifts to reach their personal wellness milestones. But if you wonder whether you should attempt this, you should know what to expect.
The following groups of people should avoid extended fasting completely (6):
- pregnant and breastfeeding women
- people with a history of eating disorders
- elderly people, children, and teenagers
- people with diabetes who are taking insulin or other medications.
Although there are historical accounts of individuals undertaking long-term fasts, the physiological strain is immense. Instead of attempting complete abstinence from food, the most appropriate fasting approach for your individual needs might be a modified routine that allows for nutrient intake, or a much shorter intermittent fasting protocol.
A practical alternative may be intermittent fasting, which involves shorter periods of complete or partial abstinence from eating. Here is a comparison of common intermittent fasting methods that are widely practiced:
| Type of Fast | Fasting Period | Difficulty | Good For Beginners? |
|---|---|---|---|
| 12-Hour Fast | 12 hours fasting, 12 hours eating | Low | Yes, excellent starting point |
| 16:8 Method | 16 hours fasting, 8 hours eating | Moderate | Yes, once adjusted to 12 hours |
| 5:2 Diet | Normal eating for 5 days, modified intake for 2 | Moderate | Yes, allows for flexibility |
| Alternate Day | Fasting or heavily modified intake every other day | High | No, requires adaptation |
| 24-Hour Fast | 24 hours of fasting once or twice a week | High | No, better for experienced individuals |
| Warrior Diet | 20 hours fasting, 4 hours eating | Very High | No, highly restrictive |
The last three types of intermittent fasting can be particularly challenging. They may occasionally cause fatigue, irritability, or headaches for some people. It’s often recommended to start with gentler versions, like the 12-hour fast, before you consider exploring more involved routines. It’s important to begin any new eating routine mindfully, ideally under the guidance of a healthcare provider.
Read more: 6 Dirty Fasting Mistakes You’re Making That May Get In The Way of Weight Loss
How Much Weight Can You Expect to Lose on an Extended Fast?
Many people explore dietary changes because they are looking for ways to support their wellness goals. However, relatively little research has been conducted on the outcomes of extended fasting compared to daily intermittent fasting. There are some studies that have demonstrated the effects of a 40 day water fast on body mass, but these are highly specific clinical scenarios.
In one literature review of medically supervised water fasts—where people consume only water or a tiny amount of prescribed clear liquids each day—researchers reported that participants lost the following body weight percentages. These were medically supervised fasts and outcomes will vary considerably for any individual, and this level of supervision is essential (7):
| Fasting Duration | Body Weight Change (Research Range) | Notes |
|---|---|---|
| 5 Days | 4% to 6% decrease | Closely monitored by clinical staff |
| 7 to 10 Days | 2% to 10% decrease | Requires significant medical oversight |
| 15 to 20 Days | 7% to 10% decrease | High risk; strictly supervised clinical setting |
Only a few studies followed up to see if the body mass changes were maintained over time. In one study, the mass was regained after three months (8). In two others, participants were encouraged to mindfully manage their intake after the fasts ended and experienced different variations of stability (9, 10).
Another case report discussed the starvation of a 34-year-old male who voluntarily undertook a 50-day fast. During this time, the individual consumed only coffee, water, tea, and daily multivitamins. His initial weight was 96.8 kg and after the 50-day fasting period, he weighed 75.4 kg (2). This was a single case report of one individual and cannot be generalised to the wider public.
Even if someone were to undergo an extended fast in a supervised clinical setting, research suggests they are highly likely to experience a rebound in body mass once they start consuming a normal diet again.
So, should you use a 40-day fast for weight loss? The correct answer is no. Shorter-period IF has attracted research interest for potential associations with weight management (11). A balanced approach to eating, sleeping well, and leading an active lifestyle carries far fewer of the significant risks associated with extended fasting.
What Happens to Your Body During Extended Fasting?
Long-term fasting over an extended period is intensely demanding on the body. Your systems will gradually go through profound physiological shifts. Understanding the stages of fasting can help clarify why extended protocols require medical supervision.
Here are some potential experiences and side effects associated with extended fasting (12):
- Dehydration: You don’t only hydrate yourself by drinking water; you naturally absorb a significant amount of water from the foods you eat. With food avoidance, you may be more likely to experience dehydration, as you’re missing out on the fluids and electrolytes normally provided by meals.
- Increased stress levels and sleep deprivation: Many people find it difficult to relax and fall asleep when they are experiencing hunger. Additionally, individuals might experience headaches or a lack of focus during prolonged fasting periods, which can further impact overall rest and productivity.
- Heartburn: Without regular food intake, the stomach’s normal digestive rhythm changes. Smelling food or even thinking about it during extended fasting periods can prompt the stomach to produce acid, potentially resulting in heartburn.
- Sickness: Longer periods without food can lead to feelings of general sickness, including headaches, constipation, and irritability. Choosing periodic or time-restricted fasting can help minimize these experiences (13).
- More weight upon refeeding: It is a common human experience to seek comfort or reward after a period of intense restriction. This is why many people find themselves eating more than usual on non-fasting days. In the long term, this can lead to an overall increase in caloric intake compared to a steady, balanced routine.
To illustrate the physiological progression, here is a general overview of the stages of fasting:
| Phase | Time Without Food | Key Physiological Effect |
|---|---|---|
| Early Fasting | 1 to 3 days | Blood sugar drops; the body transitions to burning stored fat (ketosis). |
| Prolonged Fasting | 4 to 7 days | Ketosis deepens; hunger may fluctuate; potential for fatigue and dehydration increases. |
| Extended Fasting | 1 to 2 weeks | The body focuses heavily on conserving energy; metabolic rate may slow down. |
| Severe Deprivation | 2 to 4 weeks | Muscle breakdown accelerates to provide essential amino acids for vital organs. |
| Critical Starvation | Up to 40 days | High risk of severe organ strain, extreme weakness, and life-threatening complications. |
Potential Benefits of Intermittent Fasting (Not Extended Fasting)
While an extended 40-day complete fast is not recommended due to significant risks, intermittent fasting protocols have been studied for their potential wellness applications. If you are exploring intermittent fasting, there may be some associations worth discussing with your healthcare provider. Let’s take a look at five researched areas related to intermittent fasting.
- It may support metabolic health. In one study published in 2023, an intermittent fasting regimen demonstrated associations with improved glucose tolerance compared to traditional continuous restriction over a six-month period, although these differences were less pronounced after 18 months (14).
- It may support heart health. One review found that alternate-day fasting protocols may be associated with decreased levels of total cholesterol for some individuals, which is an important metric for cardiovascular wellness (15).
- It may help with weight loss. One review found that intermittent fasting may be an effective approach for weight management for some individuals (16), offering a practical alternative to continuous daily calorie tracking.
- It may relate to longevity research. Some research has explored potential longevity-associated mechanisms with IF, looking at how periodic fasting might influence cellular repair processes over time (17).
- It may support the body’s response to inflammation. A review in 2022 found that both intermittent fasting and mindful energy-restricted routines may be associated with reduced levels of C-reactive protein (18), which is a common marker used to measure inflammation in the body (19).
It is important to remember that these potential fasting for 40 days benefits or water fasting benefits discussed in popular culture are generally extrapolated from studies on short-term intermittent fasting—not an extended 40-day complete fast.
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How to Fast More Sustainably: 6 Practical Tips
Do you want to feel balanced and energized during your wellness routine? If you do, you should not fully abstain from food for 40 days. Exploring sustainable habits and learning how to do a 40 day fast safely means modifying your approach drastically. Here are six practical rules of fasting to help you approach intermittent or modified fasting sustainably.
| Tip # | Tip | Key Action |
|---|---|---|
| 1 | Have short fasting periods | Start with a 12-hour window and gradually adjust based on your comfort level. |
| 2 | Stay hydrated | Drink plenty of water and unsweetened herbal teas throughout the day. |
| 3 | Relax and rest | Incorporate gentle movements like walking or stretching, and prioritize sleep. |
| 4 | No feasts after fasts | Break your fast gently with a modest, easily digestible meal. |
| 5 | Distract yourself | Keep your mind engaged with hobbies, walks, or socializing. |
| 6 | Eat nutrient-dense food | Focus on whole foods, fiber, and protein during your eating windows. |
- Have short fasting periods. You already know about the six popular intermittent fasting regimens. Start with the gentlest one. Attempting to drink only water for weeks on end will likely make you feel constantly hungry, irritated, and discouraged, and it poses significant medical risks. Sticking to shorter, manageable fasting windows is a much more supportive approach.
- Stay hydrated. Dehydration can cause fatigue, dry mouth, headaches, and thirst (20), so it is incredibly important to consume enough water while you are fasting.
- Relax and rest, but don’t get bored. Although strenuous activities may feel too demanding during a fasting window, mild movements such as those in yoga, gentle stretching, or a casual walk may be beneficial for your physical and mental wellbeing.
- No feasts after fasts. When it’s time to eat after a fasting window, it can be tempting to consume a large amount of food very quickly. Breaking your fast with a lavish, heavy meal can lead to uncomfortable bloating. It is much gentler on your digestive system to eat a moderate amount of food slowly.
- No thoughts about food. Try to gently distract yourself with engaging activities if you find yourself preoccupied. Take a scenic walk around the city, try a new creative hobby, or spend quality time with friends.
- Eat nutrient-dense food after the fasting period. Foods rich in fibre, protein, vitamins, and minerals support nutritional needs during and after a fasting period (21). Focusing on these whole foods can help you maintain steady energy and overall wellness. (22)
If your routine constantly impacts your productivity, your sleep, or your mood, you should consider adjusting or pausing the process. Always listen to your body and discuss your plans with a professional.
Frequently Asked Questions
Is fasting for 40 days bad for you?
Yes, a complete 40-day fast without food is bad for your health and carries extreme physiological risks. Your body requires a steady intake of nutrients to function properly and maintain energy (22).
A lack of these essential nutrients over an extended period can lead to severe health issues, fainting, falls, lack of focus, as well as reductions in heart rate and blood pressure (23). Instead of an extended fast, it is often more practical to explore milder intermittent fasting methods that allow for regular, daily food consumption.
What should I eat during 40 days of fasting?
If you are undertaking a modified fasting routine over 40 days, you should focus on consuming small amounts of easily digestible, nutrient-dense foods.
If you are considering a strict water-only fast, you should consume nothing and must consult a doctor immediately due to the high risks involved. For a modified approach, consider incorporating vegetable broths, soft cooked vegetables, and diluted juices.
When looking for foods to break a prolonged fast, it is crucial to start very slowly. Begin with light liquids like broth or diluted juice, slowly progress to soft foods like bananas, cooked vegetables, or scrambled eggs, and then gradually reintroduce light proteins and normal solid foods over several days to avoid digestive distress.
What happens if you don’t eat food for 40 days?
Not eating food for 40 days places immense, life-threatening stress on the body’s vital systems.
Extended starvation can lead to numerous severe physiological complications, including bone loss (where bones become weak and thin), muscle weakness, muscle wasting, severe constipation, dry skin, loss of menstruation (in females), extreme fatigue, and an inability to maintain normal sleep patterns. Ultimately, abstaining from food for this duration could even pose the risk of death (24), which is why medical professionals strongly advise against it.
What is the longest healthy fast?
The longest fasting duration appropriate for an individual depends entirely on their personal health profile and the type of fasting being practiced. Medical professionals generally advise that a complete water fast should not exceed 24 to 72 hours, and even then, only under medical supervision.
Intermittent fasting protocols can be followed over several weeks or longer, as they allow for daily nutrient intake. Because everyone’s body responds differently to dietary changes, you should always discuss your individual situation and goals with your healthcare provider before beginning any fasting routine.
The Bottom Line
Understanding how to fast for 40 days primarily involves recognizing the profound risks associated with prolonged starvation and identifying safer, more sustainable alternatives. While there are historical accounts of individuals fasting for 40 days without food, it is an extreme undertaking that is not appropriate or recommended for the vast majority of people. Due to its dangerous nature, extensive clinical research on long-term human starvation is limited.
Furthermore, certain populations—such as breastfeeding and pregnant individuals, children, teenagers, older adults, people with a history of eating disorders, and those taking specific medications—should avoid fasting altogether.
A more sustainable approach for those interested in exploring dietary windows is intermittent fasting, which offers several methods that can be adapted to fit different lifestyles and experience levels. While short-term intermittent fasting has attracted research interest for its potential wellness associations, attempting an extended 40-day water fast carries far fewer benefits and significantly higher risks, including dehydration, sleep deprivation, heartburn, and severe clinical consequences.
If you are interested in modifying your eating habits, always consult with your healthcare provider to find the most appropriate and supportive routine for your individual needs.
A 40-day fast or any extended fast beyond 24-72 hours carries significant health risks and should only be considered with comprehensive medical supervision. The information in this article is educational and does not constitute medical advice. Speak with your healthcare provider before attempting any extended fasting protocol. If you experience severe symptoms during any fast, stop immediately and seek medical attention.
DISCLAIMER:
This article is intended for general informational purposes only and does not serve to address individual circumstances. It is not a substitute for professional advice or help and should not be relied on for making any kind of decision-making. Any action taken as a direct or indirect result of the information in this article is entirely at your own risk and is your sole responsibility.
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- Complete and Voluntary Starvation of 50 Days (2016, journals.sagepub.com)
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