Your step-by-step, science based guide to water fasting

Written by , Last updated: October 10, 2018

Welcome to my step-by-step, science based guide to water fasting that will show you how to do a water fast both safely and effectively.

My guide will take you by the hand and show you exactly how to prepare for a water fast, how much water you should drink while fasting, and how to end a water fast.

Your step-by-step, science based guide to water fasting

On top of this, I'll throw in a few advanced water fasting tips that'll help you maximize the health benefits of your next water fast.

Before dive in, here's a quick overview of everything you can expect to learn from my water fasting guide:

What is water fasting?

Dr Alan Goldhamer, the founder of TrueNorth Health Center in Santa Rosa, California (where thousands of patients have already experienced the health benefits of water fasting) says:

Fasting, by definition, is the complete absence of all substances except water in an environment of complete rest.

In other words, water fasting is a "diet" where you literally can't drink (or eat) anything else but pure water.

And of course, with your body getting in exactly zero calories, you can't expect to be able to do a lot of heavy work or intense exercise.

Now that you know what you're getting yourself into, let's take a look at what you need to do before you can start a water fast.

How to prepare for a water fast?

How important is it to properly prepare yourself for a water fast?

Well, it depends.

If you're one of those people who can eat to their heart's content one day, and then easily stay away from food the next day, then you can simply do a water fast without any preparations.

But if you could end up stuffing your face with just about any fattening food you can get your hands on just a few days (or even hours) into your water fast, then you'd probably do yourself a favor by planning a smart transition into your next water fast.

If you take a little time to properly prepare for a water fast, this will literally skyrocket your chances of actually staying on the wagon.

So let's take a look at a couple of options on how to properly transition into your next water fast.

Get used to the idea of fasting before you even begin

A proper transition into a fully blown water fast can make the whole fasting experience way easier on you. Plus, it will greatly improve your odds of actually sticking to the fast for as long as you decide to.

So, as one of the first things you can do to prepare yourself for a water fast, is to start reducing the size of your meals before you even begin your fast.

You can take your time and gradually decrease your portion sizes in the weeks before you begin a water fast, but I suggest you do it for at least a couple of days before you start.

If you do this, both your mind and your stomach will already be better prepared to go completely without food once your fast begins.

Transition into a water fast

Another thing you can do, is use intermittent fasting to make the transition into a full water fast easier on you.

How does that work?

It's simple. You could, for example, kick things off by not eating until 12:00 every day. As you get closer to your fast, you could start going without food till 16:00. After a few more days, you could fast until 19:00, and so on...

If you've never done a water fast before, this will give you an excellent insight into how things will go down once you start your water fast.

If, for example, you're having a really tough time staying away from food until 19:00, then you might want to give yourself some more adjustment time before diving into a full water fast.

I already have a couple of fasts under my belt, and I can tell you that a few weeks of intermittent fasting before the real thing can be a real game changer.

Once you get fully comfortable with the idea of not eating until 19:00, your mind and your stomach will be almost fully prepared to go completely without food.

Now, let's take a look at a pretty stubborn myth in the water fasting community.

Do you need a colon cleanse before a water fast?

One of the first things people learn in the water fasting community, is that you need to do a colon cleanse before starting your water fast. Or at least, that you need to do a colon cleanse after you've already fasted for a couple of days.

This idea is based on a very old theory of autointoxication. Physicians in ancient Egypt believed that stools should not stay stuck in your digestive system for too long, or the bacteria in your colon will start producing toxins from them.

During a water fast, those unknown toxins could be absorbed into your bloodstream, which could supposedly poison or "disease" your body.

Modern science, on the other hand, has found no evidence that supports this outdated theory of autointoxication[1].

Water fasting and colon cleansing

The only thing that's actually been proven, are the health complications that were brought on by many different colon cleansing procedures on the market today.

Some of those risks include electrolyte imbalances[2] and water intoxication[3].

Other side effects include nausea, diarrhea, and nervous disturbances[4].

Since no evidence supports the idea of having to do a colon cleanse before or during a water fast (while plenty of evidence exists that colon cleansing procedures can be dangerous) I would definitely advise against cleansing your colon before or during a water fast.

On top of that, I personally read through dozens of scientific studies on water fasting, and none of them used colon cleansing before or during the fasts. Not a single person in those studies suffered from autointoxication, self-poisoning, or any other disease.

If the medical professionals who conducted those studies felt a colon cleanse is unnecessary for water fasting, then that's good enough for me.

In other words, as far as modern science is concerned, you can start a water fast without any special preparations or invasive colon cleansing procedures.

Time to take a look at how to actually do a water fast.

How to do a water fast safely and effectively

In this part of my water fasting guide, I'll tell you all about how to do a water fast correctly.

Water fasting isn't exactly rocket science, because there's literally just one major rule you need to remember:

Don't eat or drink anything else but pure water.

While you could get away with some other stuff without breaking the fast, your fast couldn't be called a pure water fast anymore.

Let's kick things off with how much water you need to drink every day during a water fast.

How much water should you drink on a water fast?

The officially recommended water intake for adult men stands at about 3 liters (13 cups) of water per day. Women, on the other hand, should drink about 2.5 liters (11 cups) of water every day[5].

Since you won't be eating or drinking anything else during a fast, you can't go wrong if you just stick with the recommended daily water intake.

How much water do you need to drink during a water fast

The easiest way for you to tackle this is to pre-fill the bottles with the daily amount of water every morning.

If you fill three 1-liter (33.8 fl oz) bottles and place them on your kitchen counter, they will serve as a nice visual reminder of how much water you have to get through by the end of each day.

Oh, and don't drink too much water at once. I know this is common sense to 99% of people, but you'd be surprised...

Just try to drink down all the water slowly and evenly throughout the day.

Personally, I don't think we really need to drink that much water during a water fast, mostly because we won't be eating any dry/dehydrating foods during the fast.

In words of one of my precious reader, drinking the recommended daily amount of water will make you "piss like a racehorse". But who am I to go against the official recommendations (oh, my).

Now, let's take a look at another unpleasant thing that can catch you off guard during a water fast (but it won't surprise you, because you have this guide).

Avoid feeling lightheaded/dizzy during a water fast

Water fasting has been proven to cause something called "orthostatic hypotension"[6], which is a temporary drop of your blood pressure that can happen if you stand up too quickly.

This won't start happening before you reach full ketosis (so at least 2-3 days into a fast), but even then, a lot of people never run into this issue.

Handling the lightheadedness and dizziness during water fasting

Like I said, you can completely avoid this problem if you're simply careful about getting up slowly. But just in case you do get caught by surprise, here's what you need to do:

If you just stood up too fast from a sitting or lying position, and suddenly felt lightheaded or even dizzy, you need to bring your head back down as quickly as possible.

Either immediately sit or lie back down, or just bring your head down by bending forward. With your head down, hold onto your knees for a second or two, and everything should return to normal. You're ready to (slowly) stand up again.

What's the absolute worst thing that could happen? In a highly unlikely scenario, if you really sprung up on your feet from a lying position with full force, you could lose consciousness for a few seconds, and possibly hurt yourself as you fell to the ground.

The bottom line is, just take it easy when you're getting up and you should be alright.

How long can you safely water fast for?

If you're completely new to water fasting, I'd say your first water fast shouldn't be longer than 3 days (or 72 hours).

But even with some fasting experience under your belt, people in the water fasting community agree that if you're going to do a water fast longer than three days, it's best to check in to a fasting retreat and do the fast under medical supervision.

And I couldn't agree more. A fasting retreat can make the whole experience of fasting indescribably easier on you, because 1.) countless distractions and temptations of your everyday life simply don't exist, 2.) you're surrounded by likeminded people who are also interested in fasting, and 3.) you have a medical professional that can answer any of your questions handy at all times.

The only downside to a fasting retreat is, not everyone can afford the price and/or to leave their own lives behind for a week or two.

How long can you safely water fast for

No matter how long you ultimately decide to fast for, always be mindful of any warning signs. It's normal to feel a bit weaker than usual, to feel lightheaded when getting up, or even run into some temporary heart palpitations during a water fast.

But as soon as something starts feeling too extreme, do yourself a favor and end the fast by following the instructions below.

How to properly end a water fast?

Because you won't be eating any solid food during your water fast, your digestion might not be ready to handle your normal diet right off the bat.

Eating too much, especially of the hard to digest foods right after you end a water fast could cause some unpleasant issues.

Here's a list of foods you can easily follow to properly end a water fast and avoid any potential complications along the way.

A list of foods to break your water fast with

  1. fruit juices
  2. vegetable juices
  3. raw fruits
  4. yogurt
  5. raw leafy green vegetables (lettuce, spinach...)
  6. vegetable soups
  7. cooked vegetables (potatoes, rice...)
  8. raw vegetables (celery, carrots...)
  9. cooked grains and beans
  10. milk, dairy, eggs
  11. fish, meat
  12. everything else
List of foods to break a water fast with

The foods on this list are already sorted according to how tough they will be to digest. The easily digestible foods are on top of the list, while the hard-to-digest ones are at the bottom.

At the end of your water fast, you can simply start with the foods at the top of the list and slowly make your way down. With every meal, you can include more and more foods from the list.

EXAMPLE: Break the water fast with some orange juice (food group #1). Eat a banana (food group #3) a few hours later. If everything feels ok, you can make a small salad with yoghurt topping (food groups #4 and #5) as your next meal. And so on...

Just make sure to end your fast with small meals, spaced at least two hours apart. As you include more and more foods from the list, you can also start slowly increasing the size of your meals.

What can happen if you ignore the food list?

In one recorded case, a "professional faster" ended a 30-day water fast with fruit, one of the most easily digestible foods on the planet. But because he ate a little bit too much, he had to deal with some pretty severe stomach pains and cramping for a while[7].

Now, 30 days is a LONG water fast, and I strongly advise against ever fasting for 30 days in one go (unless you have a ton of fasting experience and you're doing a supervised fast).

If you fasted for "just" a couple of days, it's highly unlikely you'll run into any trouble as you end your fast. In fact, you'll probably be able to return to your old diet right off the bat. But if you fasted for a week or more, you'll need to slowly transition back into your normal diet.

The general rule is, the longer you fasted, the more careful you need to be about how you break the fast. But no matter how short you water fast was, you simply can't go wrong if you take some time to ease back into your normal diet.

Depending on how long you fasted, it could take anywhere from 1-4 days for your digestion to get up to 100% again.

The bottom line is, most people won't need to follow the food list to the letter. It's ok to skip a step or two as you go through it, but it can never hurt to keep it in mind as a general guideline.

Now that you know how to actually do a water fast, we can take a look at some of the most important benefits of water fasting.

The science-proven benefits of water fasting

If you're like most people, then the single most important benefit of water fasting to you is probably weight loss.

But what many people don't know is that modern science has discovered water fasting can literally "kill" your hunger and cravings (you'll see the science on this in a bit).

And the third, by far the most amazing benefit of water fasting would be its potential to treat cancer, something that science is only starting to take a closer look at.

Without further ado, let's look at the single most popular benefit of water fasting, weight loss.

How much weight can you actually lose with water fasting?

Yes, weight loss is easily the most common reason why people look into water fasting in the first place.

Because it's true, water fasting IS one of the fastest weight loss methods in existence. I, for one, have personally proven it's possible to lose almost 3 pounds per day (or almost 15 pounds in 5 days) with water fasting.

The benefits of water fasting #1

So, if you don't really care about much about how much weight you will lose during your next water fast (or how to boost your weight loss results even further), then all you need to do is follow the instructions in this guide.

But if you are interested in the science that will skyrocket your fat burn rate, protect your lean body mass, and show you the exact steps I took to lose almost 3 pounds a day, then you'll love to read my advanced guide to water fasting for weight loss.

And while it's true that my own water fasting experiment was a bit extreme (I really did a lot of stuff to boost my water fasting results even further), you can definitely count on losing at least a pound or two per day.

On to the next amazing benefit of water fasting, a severe reduction or even complete disappearance of hunger and cravings.

Disappearance of hunger and cravings

If you're looking into water fasting to get rid of a couple of pounds, but you're thinking there's no way you could go completely without food, here's some surprising science for you:

In a study[8], where a group of people fasted for two weeks, they proved that hunger and cravings almost completely disappear during the water fast.

The benefits of water fasting #2

In another water fasting study[6], where people fasted for as long as 117! days, scientists also confirmed that "hunger was virtually absent".

From my personal experience with water fasting, I can tell you that as long as you can get through that first awkward phase (the first day or so), cravings and hunger DO become way easier to handle. During one of my fasts, I literally felt like I no longer care about food at all.

And while water fasting definitely isn't a walk in the park, it can actually be easier to handle than some of the less restrictive diets. While any diet (that actually allows you to eat food) might seem easier in comparison to water fasting, the hunger and cravings might actually be tougher to handle with a little food than with no food at all[8].

On to the next amazing benefit of water fasting.

Can water fasting really heal cancer?

If you've been a part of the water fasting community for a while, you've probably heard of a few miraculous cancer recovery stories. Personally, I've read about "terminal" cases (as diagnosed by modern medicine), who made a full recovery just because they lived off nothing but water for a few weeks.

First of all, I have no way of knowing whether or not any of those stories are true. Second, I'm not a qualified medical professional. So, if you've been diagnosed with cancer, make sure to discuss any water fasting options with your own doctor.

With that out of the way, here's some science that proves water fasting could actually be effective for cancer treatment:

The benefits of water fasting #3

In one study[9], they discovered that water fasting can be as effective as chemotherapy drugs in delaying tumor growth and that it can increase the effectiveness of chemotherapy drugs in certain types of cancers.

Another piece of research[10] looked at cancer patients who fasted for different amounts of time before and after chemotherapy. Fasting was able to reduce or in some cases completely eliminate the negative side effects of chemotherapy (fatigue, weakness, hair loss, headaches, nausea, vomiting, diarrhea, numbness, tingling...)

While a lot more research needs to be done before the final verdict on treating cancer with water fasting can be reached, the current evidence certainly suggests water fasting could potentially replace, or at least boost the effectiveness of certain chemotherapy drugs in cancer treatment.

On to taking a closer look at what happens to your muscles during a water fast.

Muscle loss

In this part of the guide I'll explain everything you need to know about water fasting and muscle loss.

Here, you'll see the science on how much muscle mass you can expect to lose every day during water fasting, a ketodenic diet and a mixed diet.

(This will help you understand that the loss of muscle mass isn't just a problem with water fasting, but with standard weight loss diets as well.)

I'll also talk about what you can do to fully prevent that unnecessary loss of muscle mass.

Let's dive right in.

Muscle loss during water fasting, a mixed diet, and a ketogenic diet

I found a study[11] that took a really close look at muscle loss during water fasting, a ketogenic diet, and a mixed diet.

Here's how the study went down:

The people on the mixed diet and on the mixed diets both ate 800 calories a day.

And of course, the people doing a water fast, ate 0 calories a day.

Here are the results of the study:

Weight loss Fat loss Muscle loss Water loss
Mixed diet 277.9 g/day 166.7 g/day 9.5 g/day 101.7 g/day
Ketogenic diet 446.6 g/day 163.4 g/day 17.9 g/day 265.3 g/day
Water fasting 750.7 g/day 243.1 g/day 50.4 g/day 457.2 g/day

On the surface, this table tells you that you'll lose muscle mass 5 times faster during a water fast than on some standard, 800-calorie weight loss diet (50 grams a day versus 10 grams a day).

But there's a catch.

The speed of muscle loss isn't the only thing that matters

While it's true that water fasting will destroy a lot of your muscle mass per day, you also have to consider how quickly your total weight (or better yet, your body fat) will be coming off.

So let's say you wanted to lose 20 kilograms (44 pounds) of total weight (this is how most people set their weight loss goals anyway).

Here's how long that would take on a water fast, a mixed diet, and a ketogenic diet:

Days Fat loss Muscle loss Water loss
Mixed diet 72 days 12.0 kg 0.7 kg 7.3 kg
Ketogenic diet 45 days 7.3 kg 0.8 kg 11.9 kg
Water fasting 27 days 6.5 kg 1.3 kg 12.2 kg

As you can see from the second table, you won't actually lose 5 times as much muscle mass on a water fast as on some standard diet.

(In fact, you won't even lose twice as much muscle mass.)

Because, even though you'll be losing a lot more muscle per day on a water fast, you'll also reach your final weight loss goal almost 3 times faster.

And that's exactly why you'll lose a lot less muscle mass in total.

But let's take an even closer look at the loss of muscle mass during a water fast.

A daily breakdown of muscle loss during a 21-day water fast

The table above, the one that says you'll be losing about 50 grams of muscle mass per day, may fool you into thinking that you'll be losing 50 grams of muscle every single day.

Here's what one study[12] on muscle loss during water fasting found out:

DayMuscle loss
169 grams
261 grams
362 grams
458 grams
553 grams
649 grams
737 grams
842 grams
935 grams
1033 grams
1127 grams
1226 grams
1325 grams
1424 grams
1523 grams
1622 grams
1717 grams
1816 grams
1918 grams
2016 grams
2115 grams

So the truth is, you'll be losing a lot more muscle mass in the early days of your fast than in the later stages of your fast.

This happens because of the so-called protein-sparing effect, but sadly, that doesn't kick in until quite late in the fast.

And now for easily the most important question when it comes to losing muscle during a water fast:

How to prevent muscle loss during water fasting

First of all, there's no way to do a 100% "pure" water fast without losing muscle.

But if you're willing to eat (or drink down) a tiny amount of protein during your water fast, then it's actually possible to completely prevent nay loss of muscle during fasting.

How can you do it?

According to research[13], all you need is around 0.6 grams of protein per pound (or 1.3 grams of protein per kilogram) of your "ideal body weight".

If you're willing to eat that small amount of protein (or even drink them down in a form of protein isolate), you can fully prevent any loss your muscle mass WITHOUT slowing down fat loss.

Technically, this can no longer be called water fasting, but muscle sparing fasting (you can find more details on muscle sparing fasting here).

Before I wrap things up, let me give you just a couple more handy water fasting tips.

A few more beneficial water fasting tips

If you aren't exactly dead set on doing a 100% pure water fast, you can use any one of these water fasting tips to enjoy some extra benefits during your next water fast:

Instead of drinking only water, you can drink a zero-calorie drink like oolong tea instead. Not only will this speed up fat loss, but will also make you less tired throughout the fast. If you're interested, you can find more details in my guide to drinking oolong tea for weight loss.

If you're doing a water fast for weight loss, then you might also be interested in how to trigger the calorie burning process in your body called thermogenesis. Again, you can read all about this in my science based guide to boosting your weight loss results with thermogenesis.


All in all, there really isn't all that much you need to know about how to do a water fast.

With just one major rule (drinking enough water throughout your fast) and a few small water fasting tips (not standing up too quickly) to follow, water fasting is one of the simplest "diets" on the planet.

But just because water fasting is simple, that doesn't mean it's easy.

But with the scientifically proven fact that you hunger can literally disappear, water fasting has one major thing going for it (that most other diets and cleanses don't).

Also, water fasting can destroy about 5 times as much muscle mass per day, than your standard, 800 calories a day, mixed diet.

But the muscle loss during a water fast isn't as critical, because you'll also reach your weight loss as much as 3 times faster than on a standard diet (meaning you won't lose nearly as much muscle mass in total).

But no matter how you spin it, the loss of muscle mass is pretty much inevitable during water fasting (that's IF you insist on doing a pure water fast).

But if you can eat (or drink down) 0.6 grams of protein per pound (or 1.3 grams of protein per kilogram) of your "ideal body weight", you can fully reverse the loss of muscle mass.

(And you get to do it without slowing down those sky-high fat burn rates water fasting CAN deliver.)


1. Chen TSN, Chen PSY. Intestinal autointoxication: a medical leitmotif. J Clin Gastroenterol 1989;11:434-441.

2. Dunning MF, Plum F. Potassium depletion by enemas. Am J Med 1956;20:789.

3. Ziskind A, Gelis SS. Water intoxication following tap water enemas. J Dis Child 1958;96:699-704.

4. Kelvinson RC. Colonic hydrotherapy: a review of the available literature. Compl Ther Med 1995;3:88-92.

5. Institute of Medicine. Dietary Reference Intakes: Water, Potassium, Sodium, Chloride, and Sulfate. Washington, DC: National Academies Press; 2004.

6. Drenick EJ, Swendseid ME, Bland WH, Tuttle SG. Prolonged starvation as treatment for severe obesity. JAMA 1964;87:100-5.

7. Benedict, FG. A study of prolonged fasting. Carnegie Institute of Washington, publication 203, 1915.

8. Lappalainen R, Sjoden PO, Hursti T, Vesa V. Hunger/craving responses and reactivity to food stimuli during fasting and dieting. Int J Obes 1990;14:679-688.

9. Lee C, et al. Fasting cycles retard growth of tumors and sensitize a range of cancer cell types to chemotherapy. Sci Transl Med 2012;4:124-127.

10. Safdie FM, Dorff T, Quinn D, Fontana L, et al. Fasting and cancer treatment in humans: a case series report Aging 2009;1:988-1007.

11. Yang M-U, Van Itallie TB. Composition of weight loss during short-term weight reduction. Metabolic responses of obese subjects to starvation and low-calorie ketogenic and nonketogenic diets. J Clin Invest. 1976; 58: 722-30.

12. Owen OE , Smalley KJ , D'Alessio DA , Mozzoli MA , Dawson EK. Protein, fat, and carbohydrate requirements during starvation: anaplerosis and cataplerosis. Am J Clin Nutr. 1998; 68: 12-34.

13. Bistrian, B. R., Blackburn, G. L, Flatt, J. P., Sizer, J., Scrimshaw, N. S., and Sherman, M. Nitrogen metabolism and insulin requirements in obese diabetic adults on a protein-sparing modified fast. Diabetes. 1976; 25: 494-504.

David Brown

Hi, I'm David Brown.

I'm the guy who singlehandedly turned into a widely popular weight loss resource that gets visited by tens of thousands of people every month.

I reached this level of popularity because I invested thousands of hours into translating complicated science into highly actionable weight loss tips that ANYONE can start using in their lives right away.