Blog
Ramon Siljade
5-6 min
Water fasting for 5 days: what happens and how is it different from the 5 Day Fast?
Summary
Water fasting means you temporarily stop eating and only drink water. During 5 days of water fasting, your body can use more fats for fuel and produce ketones. 5 Day Fast works differently: it's a structured programme, building on research into fasting mimicking diets, where you do actually carry on eating.
Five days of water fasting can make the scales drop quickly. But that initial result can be misleading: part of the weight loss comes from changes in glycogen, water, and gut contents, not just fat loss. In this article, you'll read what happens during 5 days of water fasting, the risks involved, and how this approach differs from a structured fasting mimicking programme like 5 Day Fast.
What is water fasting?
During strict water fasting, you temporarily eat nothing and only drink water. You don't consume any calories or macronutrients.
That's different from intermittent fasting, where you still eat within a specific eating window. It also differs from a fasting mimicking diet, where you carry on eating according to an energy-restricted menu with specific amounts of fats, carbohydrates, and proteins.
Water fasting can be part of a medically supervised protocol. Whether there are guidelines on preparation, monitoring, and food reintroduction depends on the protocol followed.
Water fasting can be physically and mentally tough, especially when it lasts for several days. That's why it's important to assess beforehand whether this approach suits your situation.
What happens during 5 days of water fasting?
During a multi-day fast, the balance between the fuels your body uses changes.
At first, it uses available glucose and stored glycogen, among other things. Glycogen is the stored form of carbohydrates and is mainly found in the muscles and liver.
Glycogen is stored together with water. Research in humans has estimated that each gram of glycogen can be stored with roughly 3 to 4 grams of water (1).
When glycogen stores drop, the amount of water linked to them changes too. That helps explain why your body weight can drop quickly in the first phase of fasting. So part of that drop is water loss, not fat loss.
As fewer carbohydrates are available, the body uses more fat as an energy source. The liver also produces ketones. These act as an alternative energy source for organs like the brain and become more important when less glucose is available.
With longer fasts, ketosis usually becomes more noticeable after a few days. In a study looking at five days of water fasting, for instance, ketones were clearly elevated on day three and day five (2). How this progresses varies from person to person and depends, among other things, on your starting point and activity level.
A higher ketone reading doesn't in itself mean an approach is better or more suitable.
During multi-day water fasting, you might experience issues like hunger, weakness, dizziness, irritability, poor concentration, disrupted sleep, or feeling cold. The period after fasting needs attention too: how you start eating again forms part of the preparation.
How much weight do you lose with water fasting?
There is no set number of kilos that everyone loses during five days of water fasting. What's more, weight loss doesn't automatically mean you've lost the same amount of body fat.
A drop on the scales can happen for several reasons:
lower glycogen stores
less water stored with glycogen
less content in the digestive system
loss of fat mass
changes in other components of fat-free mass
Because of this, you can't tell exactly how much fat you've lost just from the number on the scales.
Part of the weight can return once you start eating normally again, as glycogen and water stores are topped back up. That doesn't automatically mean you've regained fat in a short space of time.
What does this mean for losing weight?
When it comes to weight loss, it's not just the change during fasting that matters, but also what happens afterwards.
The outcome depends, among other things, on your body composition, energy expenditure, how long the fasting period lasts, and your diet afterwards. So a temporary drop on the scales says very little about keeping the weight off in the longer term.
For weight loss, a more gradual approach with a moderate calorie deficit, plenty of protein, and exercise can be an alternative. Strength training can help preserve muscle mass here.
Water fasting on its own doesn't provide a plan for exercise or nutrition afterwards; such arrangements might, however, be part of a supervised protocol. It shouldn't simply be used as a repeated quick fix for weight loss.
Is 5 days of water fasting safe?
Going five days without food is a drastic change. Whether this is sensible depends, among other things, on your health, medication use, and supervision.
Don't start multi-day water fasting on your own initiative if you:
take medication or have a medical condition, including diabetes
are pregnant or breastfeeding
are under 18
are underweight
have or have had an eating disorder
These examples aren't an exhaustive list. Check with a doctor beforehand whether multi-day fasting is suitable for your situation.
A strict fasting period can also affect your relationship with food. Extra caution is particularly important if you already struggle with restriction, binge eating, food guilt, or body image.
During multi-day fasting, the body can also break down its own protein for energy and glucose production. However, a measured drop in fat-free mass isn't purely muscle tissue: changes in water and glycogen play a part too.
When should you stop?
Stop fasting and contact a doctor straight away if you get severe dizziness, confusion, fainting, or heart palpitations accompanied by feeling seriously unwell. Call 999 (or 112) in case of unconsciousness, chest pain, or severe shortness of breath.
Water fasting is no substitute for medical care. Nor should it be presented as a proven way to detox the body, cure diseases, or guarantee fat loss.
What is the difference between water fasting and 5 Day Fast?
The main difference is straightforward: with strict water fasting, you eat nothing. With 5 Day Fast, you carry on eating according to a structured fasting mimicking menu.
Water fasting
During water fasting, you only drink water and consume no food.
Water fasting can be part of a supervised protocol with guidelines on preparation, monitoring, and food reintroduction. Without those guidelines, you have to decide for yourself what to do if you feel unwell, how much you exercise, and how to start eating again afterwards. So the level of guidance depends on the protocol followed.
5 Day Fast
5 Day Fast is a five-day fasting mimicking programme. You follow an adaptable, energy-restricted menu with specific foods and amounts.
The programme includes a menu, a shopping list, exercise advice, and all supplements in one pack.
5 Day Fast builds on research into fasting mimicking diets, including the work of Prof. Valter Longo. Research into specific fasting mimicking diets has investigated changes in body weight and metabolic and cardiometabolic markers, among other things (3,4).
5 Day Fast isn't suitable for everyone either. Check the suitability test on the website beforehand and speak to your doctor if you have an illness or take medication.
What does research into fasting mimicking diets say about muscles?
Alongside changes in body weight, potential loss of muscle mass is also a point of concern with multi-day fasting. That's why research into specific fasting mimicking diets has also looked at muscle strength and muscle volume.
In a randomised study involving 24 physically active men, no significant changes in muscle strength or muscle volume were found after three cycles of a fasting mimicking diet (5).
This study looked at a specific fasting mimicking protocol, not the commercial 5 Day Fast programme.
Why do some people choose 5 Day Fast instead of water fasting?
The practical difference mainly comes down to the preparation and daily routine. With 5 Day Fast, you get a menu, a shopping list, and instructions for five days. This means you don't have to work out the food choices and portions yourself.
That can be practical for people looking for a fasting programme with a menu and instructions, without completely stopping eating. Whether repeating it periodically is appropriate depends on your personal situation and suitability.
Conclusion: water fasting or a fasting mimicking approach?
With five days of water fasting, you eat nothing and drink only water. Body weight can drop, but that change isn't purely fat loss. Glycogen, water, and gut contents play a part too.
5 Day Fast takes a different approach: you carry on eating according to an energy-restricted menu, with a shopping list, supplements, and instructions for five days.
So the comparison is mostly about food intake and how the approach is set up.
Which approach is suitable depends on your personal situation and suitability. Results vary from person to person.
After five days of water fasting, the scales often show a lower number. That initial figure can be misleading: part of the weight is often down to glycogen, water, and gut contents, not just fat. In this article, you'll read what happens during 5 days of water fasting, what risks are involved, and how a fasting mimicking programme like 5 Day Fast differs.
After five days of water fasting, the scales often show a lower number. But that initial result can be misleading: part of the weight loss comes from changes in glycogen, water, and gut contents, not just fat loss. In this article, you'll read what happens during 5 days of water fasting, what risks are involved, and how this approach differs from a structured fasting mimicking programme like 5 Day Fast.
What is water fasting?
What is water fasting?
During strict water fasting, you temporarily eat nothing and only drink water. You don't consume any calories or macronutrients.
During strict water fasting, you temporarily eat nothing and only drink water. You don't consume any calories or macronutrients.
That's different from intermittent fasting, where you still eat within a specific eating window. It also differs from a fasting mimicking diet: where you carry on eating according to an energy-restricted menu with specific amounts of fats, carbohydrates, and proteins.
That's different from intermittent fasting, where you still eat within a specific eating window. It also differs from a fasting mimicking diet: where you carry on eating according to an energy-restricted menu with specific amounts of fats, carbohydrates, and proteins.
Water fasting can be part of a medically supervised protocol. Whether there are guidelines on preparation, monitoring, and food reintroduction depends on the protocol followed.
Water fasting can be physically and mentally tough, especially when it lasts for several days. That's why it's important to assess beforehand whether this approach suits your situation.
What happens during 5 days of water fasting?
Water fasting is sometimes used within a medical protocol. Whether there are guidelines on preparation, monitoring, and food reintroduction depends on that protocol.
Water fasting can be physically and mentally tough, especially when it lasts for several days. That's why it's important to assess beforehand whether this approach suits your situation.
During a multi-day fast, the balance between the fuels your body uses changes.
What happens during 5 days of water fasting?
At first, it uses available glucose and stored glycogen, among other things. Glycogen is the stored form of carbohydrates and is mainly found in the muscles and liver.
During a multi-day fast, the balance between the fuels your body uses changes.
Glycogen is stored together with water. Research in humans has estimated that each gram of glycogen can be stored with roughly 3 to 4 grams of water (1).
At first, it uses available glucose and stored glycogen, among other things. Glycogen is the stored form of carbohydrates and is mainly found in the muscles and liver.
When glycogen stores drop, the amount of water linked to them changes too. That helps explain why body weight can change in the first phase of fasting. Part of that change is water loss, not fat loss.
As fewer carbohydrates are available, the body uses more fat as an energy source. The liver also produces ketones. These act as an alternative energy source for organs like the brain and become more important when less glucose is available.
Glycogen is stored together with water. Research in humans has estimated that each gram of glycogen can be stored with roughly 3 to 4 grams of water (1).
With longer fasts, ketosis usually becomes more noticeable after a few days. In a study looking at five days of water fasting, for instance, ketones were clearly elevated on day three and day five (2). How this progresses varies from person to person and depends, among other things, on your starting point and activity level.
A higher ketone reading doesn't in itself mean an approach is better or more suitable.
During multi-day water fasting, you might experience issues like hunger, weakness, dizziness, irritability, poor concentration, disrupted sleep, or feeling cold. The period after fasting needs attention too: how you start eating again forms part of the preparation.
What do the scales say after water fasting?
When glycogen stores drop, the amount of water linked to them changes too. That helps explain why body weight can change in the first phase of fasting. Part of that change is water loss, not fat loss.
During water fasting, body weight can change. That doesn't mean all the weight lost is fat.
As fewer carbohydrates are available, the body uses more fat as an energy source. The liver also produces ketones. These act as an alternative energy source for organs like the brain and become more important when less glucose is available.
A change on the scales can happen for several reasons:
lower glycogen stores
less water stored with glycogen
less content in the digestive system
loss of fat mass
changes in other components of fat-free mass
With longer fasts, ketosis usually becomes more noticeable after a few days. In a study looking at five days of water fasting, for instance, ketones were clearly elevated on day three and day five (2). How this progresses varies from person to person and depends, among other things, on your starting point and activity level.
Because of this, you can't tell exactly how much fat you've lost just from the number on the scales.
Part of the weight can return once you start eating normally again, as glycogen and water stores are topped back up. That doesn't automatically mean you've regained fat in a short space of time.
A higher ketone reading doesn't in itself mean an approach is better or more suitable.
What does this mean for your weight?
When it comes to your weight, it's not just the change during fasting that matters, but also what happens afterwards.
The outcome depends, among other things, on your body composition, energy expenditure, how long the fasting period lasts, and your diet afterwards. So a temporary change on the scales says very little about keeping the weight off in the longer term.
During multi-day water fasting, you might experience issues like hunger, weakness, dizziness, irritability, poor concentration, disrupted sleep, or feeling cold. The period after fasting needs attention too: how you start eating again forms part of the preparation.
Water fasting on its own doesn't provide a plan for exercise or nutrition afterwards. It's not an approach to simply repeat.
What do the scales say after water fasting?
There is no set amount of weight that everyone loses during five days of water fasting. What's more, weight loss doesn't automatically mean you've lost the same amount of body fat.
Is 5 days of water fasting safe?
A drop on the scales can happen for several reasons:
Going five days without food is a drastic change. Whether this is sensible depends, among other things, on your health, medication use, and supervision.
lower glycogen stores
less water stored with glycogen
less content in the digestive system
loss of fat mass
changes in other components of fat-free mass
Don't start multi-day water fasting on your own initiative if you:
Because of this, you can't tell exactly how much fat you've lost just from the number on the scales.
take medication or have a medical condition, including diabetes
are pregnant or breastfeeding
are under 18
are underweight
have or have had an eating disorder
Part of the weight can return once you start eating normally again, as glycogen and water stores are topped back up. That doesn't automatically mean you've regained fat in a short space of time.
What does this mean for your weight?
When it comes to your weight, it's not just the change during fasting that matters, but also what happens afterwards.
The outcome depends, among other things, on your body composition, energy expenditure, how long the fasting period lasts, and your diet afterwards. So a temporary drop on the scales says very little about keeping the weight off in the longer term.
Water fasting on its own doesn't provide a plan for exercise or nutrition afterwards. It's not an approach to simply repeat.
These examples aren't an exhaustive list. Check with a doctor beforehand whether multi-day fasting is suitable for your situation.
Is 5 days of water fasting safe?
Going five days without food is a drastic change. Whether this is sensible depends, among other things, on your health, medication use, and medical supervision.
A strict fasting period can also affect your relationship with food. Extra caution is particularly important if you already struggle with restriction, binge eating, food guilt, or body image.
When should you stop?
Stop fasting and contact a doctor straight away if you get severe dizziness, confusion, fainting, or heart palpitations accompanied by feeling seriously unwell. Call 999 (or 112) in case of unconsciousness, chest pain, or severe shortness of breath.
Don't start multi-day water fasting on your own initiative if you:
take medication or have a medical condition, including diabetes
are pregnant or breastfeeding
are under 18
are underweight
have or have had an eating disorder
Water fasting is no substitute for medical care and does not guarantee any specific result.
These examples aren't an exhaustive list. Check with a doctor beforehand whether multi-day fasting is suitable for your situation.
A strict fasting period can also affect your relationship with food. Extra caution is particularly important if you already struggle with restriction, binge eating, food guilt, or body image.
During multi-day fasting, the body can also break down its own protein for energy and glucose production (3). However, a measured drop in fat-free mass isn't purely muscle tissue: changes in water and glycogen play a part too.
What is the difference between water fasting and 5 Day Fast?
The main difference is straightforward: with strict water fasting, you eat nothing. With 5 Day Fast, you carry on eating according to a structured fasting mimicking menu.
When should you stop?
Water fasting
Stop fasting and contact a doctor straight away if you get severe dizziness, confusion, fainting, or heart palpitations accompanied by feeling seriously unwell. Call 999 (or 112) in case of unconsciousness, chest pain, or severe shortness of breath.
During water fasting, you only drink water and consume no food.
Water fasting can be part of a supervised protocol with guidelines on preparation, monitoring, and food reintroduction. Without those guidelines, you have to decide for yourself what to do if you feel unwell, how much you exercise, and how to start eating again afterwards. So the level of guidance depends on the protocol followed.
5 Day Fast
Water fasting is no substitute for medical care and does not guarantee any specific result.
5 Day Fast is a five-day fasting mimicking programme. You follow an adaptable, energy-restricted menu with specific foods and amounts.
The programme includes a menu, a shopping list, an exercise plan, and all supplements in one pack.
What is the difference between water fasting and 5 Day Fast?
The main difference is straightforward: with strict water fasting, you eat nothing. With 5 Day Fast, you carry on eating according to a structured fasting mimicking menu.
5 Day Fast builds on the fasting mimicking research by Prof. Valter Longo. (3)
5 Day Fast isn't suitable for every situation either.
Water fasting
During water fasting, you only drink water and consume no food.
Within a medical protocol, there may be guidelines on preparation, monitoring, and food reintroduction. Without those guidelines, you have to decide for yourself what to do if you feel unwell, how much you exercise, and how to start eating again afterwards.
Don't start if you are under 18, pregnant or breastfeeding, have or have had an eating disorder, or are acutely ill (with a fever, for example).
5 Day Fast
Check with your doctor first if you are over 70, have a BMI below 18.5, have diabetes or another metabolic condition, have chronic digestive problems for which you take medication, or are receiving treatment for a chronic condition (such as heart, kidneys, or liver). Taking medication and in doubt? Speak to your doctor first as well.
5 Day Fast is a five-day fasting mimicking programme. You follow an adaptable, energy-restricted menu with specific foods and amounts.
Take the suitability test on the website beforehand. Got questions? Email us at support@5dayfast.nl.
Why do some people choose 5 Day Fast instead of water fasting?
The programme includes a menu, a shopping list, an exercise plan, and all supplements in one pack.
The practical difference mainly comes down to the preparation and daily routine. With 5 Day Fast, you get a menu, a shopping list, and instructions for five days. This means you don't have to work out the food choices and portions yourself.
That can be practical for people looking for a fasting programme with a menu and instructions, without completely stopping eating. Whether repeating it periodically is appropriate depends on your personal situation and suitability.
5 Day Fast builds on the fasting mimicking research by Prof. Valter Longo (4).
5 Day Fast isn't suitable for every situation either. Don't start if you are under 18, pregnant or breastfeeding, have or have had an eating disorder, or are acutely ill (with a fever, for example).
Conclusion: water fasting or a fasting mimicking approach?
With five days of water fasting, you eat nothing and drink only water. Body weight can drop, but that change isn't purely fat loss. Glycogen, water, and gut contents play a part too.
5 Day Fast takes a different approach: you carry on eating according to an energy-restricted menu, with a shopping list, supplements, and instructions for five days.
Check with your doctor first if you are over 70, have a BMI below 18.5, have diabetes or another metabolic condition, have chronic digestive problems for which you take medication, or are receiving treatment for a chronic condition (such as heart, kidneys, or liver). Taking medication and in doubt? Speak to your doctor first as well.
Pick your timing carefully if you are dealing with prolonged high stress or burnout symptoms, if you are recovering from an injury, or if you have a competition or major event within ten days. In doubt? Take the suitability test first.
Why do some people choose 5 Day Fast instead of water fasting?
So the comparison is mostly about food intake and how the approach is set up.
Which approach is suitable depends on your personal situation and suitability. Results vary from person to person. 5 Day Fast does not guarantee fat loss, better sleep, or a specific number on the scales.
The practical difference mainly comes down to the preparation and daily routine. With 5 Day Fast, you get a menu, a shopping list, and instructions for five days. This means you don't have to work out the food choices and portions yourself.
That can be practical for people looking for a fasting programme with a menu and instructions, without completely stopping eating. Whether repeating it periodically is appropriate depends on your personal situation and suitability.
Conclusion: water fasting or a fasting mimicking approach?
With five days of water fasting, you eat nothing and drink only water. Body weight can drop, but that change isn't purely fat loss. Glycogen, water, and gut contents play a part too.
5 Day Fast takes a different approach: you carry on eating according to an energy-restricted menu, with a shopping list, supplements, and instructions for five days.
So the comparison is mostly about food intake and how the approach is set up.
Which approach is suitable depends on your personal situation and suitability. Results vary from person to person. 5 Day Fast does not guarantee fat loss, better sleep, or a specific number on the scales.
Sources
Olsson, K. E., & Saltin, B. (1970). Variation in total body water with muscle glycogen changes in man. Acta Physiologica Scandinavica, 80(1), 11–18. https://doi.org/10.1111/j.1748-1716.1970.tb04764.x
Jiang, Y., et al. (2021). Five-day water-only fasting decreased metabolic-syndrome risk factors and increased anti-aging biomarkers without toxicity in a clinical trial of normal-weight individuals. Clinical and Translational Medicine, 11(8), e502. https://doi.org/10.1002/ctm2.502
Brandhorst, S., Choi, I. Y., Wei, M., et al. (2015). A periodic diet that mimics fasting promotes multi-system regeneration, enhanced cognitive performance, and healthspan. Cell Metabolism, 22(1), 86–99. https://doi.org/10.1016/j.cmet.2015.05.012
Mishra, A., Fanti, M., Ge, X., et al. (2023). Fasting mimicking diet cycles versus a Mediterranean diet and cardiometabolic risk in overweight and obese hypertensive subjects: a randomized clinical trial. npj Metabolic Health and Disease, 1, Article 1. https://doi.org/10.1038/s44324-023-00002-1
Nardon, M., Venturelli, M., Ruzzante, F., Longo, V. D., & Bertucco, M. (2022). Fasting-Mimicking-Diet does not reduce skeletal muscle function in healthy young adults: a randomized control trial. European Journal of Applied Physiology, 122, 651–661. https://doi.org/10.1007/s00421-021-04867-2
Olsson, K. E., & Saltin, B. (1970). Acta Physiologica Scandinavica, 80(1), 11–18. https://doi.org/10.1111/j.1748-1716.1970.tb04764.x
Jiang, Y., et al. (2021). Clinical and Translational Medicine, 11(8), e502. https://doi.org/10.1002/ctm2.502
Cahill, G. F., Jr. (2006). Annual Review of Nutrition, 26, 1–22. https://doi.org/10.1146/annurev.nutr.26.061505.111258
Brandhorst, S., Choi, I. Y., Wei, M., et al. (2015). Cell Metabolism, 22(1), 86–99. https://doi.org/10.1016/j.cmet.2015.05.012
Olsson, K. E., & Saltin, B. (1970). Acta Physiologica Scandinavica, 80(1), 11–18. https://doi.org/10.1111/j.1748-1716.1970.tb04764.x
Jiang, Y., et al. (2021). Clinical and Translational Medicine, 11(8), e502. https://doi.org/10.1002/ctm2.502
Brandhorst, S., Choi, I. Y., Wei, M., et al. (2015). Cell Metabolism, 22(1), 86–99. https://doi.org/10.1016/j.cmet.2015.05.012
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