A detailed science-based guide to water fasting for weight loss (with a calculator)

Welcome to my science based guide that will tell you everything you need to know about water fasting and weight loss.

By the time you finish reading this guide, you will know how to estimate your water fasting weight loss results pretty accurately.

I even created a handy water fasting weight loss calculator for you.

You will also see the 5 different types of weight you will be losing while water fasting and the science on how much of that lost weight will be your muscles.

You'll also learn some advanced water fasting tips that will help you better target your body fat and learn about muscle sparing fasting (a form of fasting that can help you fully prevent any loss of muscle mass).

But before we get started, here's a brief overview of everything you can expect to learn from this guide (use it for easier navigation):

How much weight can you lose?

Your weight loss during water fasting will depend mostly on your gender and your weight.

(And of course, on how long you plan on fasting.)

Luckily, science can already offer us with a pretty good estimate of how much weight you can expect to lose on a typical water fast.

According to research^[1] that compared the average weight loss results between men and women, men can expect to lose more weight than women during water fasting.

I've compiled the data from the study into this pretty long table:

And don't worry about having to do any manual calculations, I've prepared a handy calculator for you right below.

Water fasting weight loss calculator

Use this handy calculator to estimate the weight loss results you would get during a water fast.

I've made the calculator as simple as possible, so to use it, all you have to do is this:

1. Select your gender.
2. Enter the duration of the fast (1-30 days).
3. Enter your starting weight (in lbs or kg).
4. Press the Calculate button.
5. Check out the results below.

Ok, so, like I said, you don't need to do ANY manual calculations.

But just so you understand how the calculator works, here are two example calculations:

(Use the table or calculator above to follow along.)

Example 1 (for women):
You're a 150-lbs woman who plans on doing a 7-day water fast.

You can estimate your weight loss results by taking the value from the second column in the table for a 7-day water fast and multiply it by your current weight.

Your estimated 7-day water fasting results: 150 lbs * 4.5 / 100 = -6.8 lbs

Example 2 (for men):
You're a 180-lbs man who plans on doing a 14-day water fast.

You can estimate your weight loss results by taking the value from the third column in the table for a 14-day water fast and multiply it by your current weight.

Your estimated 14-day water fasting results: 180 lbs * 7.9 / 100 = -14.2 lbs

Easy, right?

Ok, now that you have a good idea about how much weight you can expect to lose, let's take a closer look at what really goes on under the hood.

Understanding your weight loss results

Sadly, when it comes to your water fasting weight loss results, you won't be losing "just" your body fat.

In fact, you'll end up losing FIVE different types of body weight.

So let's quickly go over all the different types of weight you'll be losing during a water fast, starting with water weight.

Water weight

If I had to name the single most widespread, yet totally irrational fear of things that cause weight gains, then drinking water would definitely be it.

So let me set the record straight once and for all:

Drinking plenty of water DOES NOT make you gain weight.

In fact, drinking plenty of water throughout your next water fast will only help keep your body properly hydrated and keep your metabolism properly revved up.

If you're interested in exactly how much water you need to be drinking at all times (not just during a water fast) to maximize the fat burning processes in your body, you can take a look at my thorough guide on drinking water and weight loss.

So let's take a closer look at another type of body weight, that you'll end up losing.

Glycogen

While your body stores away most of the energy in your body fat reserves, it also stores some energy in your muscles and your liver as glycogen.

And just like your body fat, your glycogen reserves (plus the water bound to them) can add quite a few extra pounds to your total body weight.

I can't cover all the details on how much extra weight you're carrying around because of glycogen or how to get rid of it in this guide.

(But you can read all about the fat burning benefits of getting rid of all the glycogen in your body in my science-based guide to glycogen and weight loss.)

But when it comes to how glycogen affects your water fasting weight loss results (and your energy leves during fasting), you need to understand what will happen during the first 3-5 days of a water fast:

• Your body will burn through most of your glycogen reserves, meaning that you can lose as much as 10 pounds (5 kilograms) of glycogen-bound water weight. Again, this will make your final water fasting weight loss results look pretty damn awesome, but this doesn't mean you're actually losing any extra body fat.
• After your body wipes out your glycogen reserves, you will start feeling a lot weaker and possibly even dizzy (find out how to deal with this in my general guide to water fasting). Like I said, glycogen is how your body stores energy, and with your glycogen reserves wiped out, you simply won't have as much spring in your step.
• You won't be able to reach your highest possible fat burn rate for as long as your body can still rely on glycogen for energy. So unless you follow the tips I'm going to reveal later on in the guide, you'll have to wait for 3-5 days before your body gets rid of glycogen at its own natural pace.

On to another type of weight you'll end up losing during water fasting.

Sodium

Sodium is a very common mineral in our diets (think table salt) and unless you're sticking to a specific, low-sodium diet, you're probably packing on quite a lot of extra pounds because of sodium (salt).

You see, just like glycogen, sodium too binds water to itself. So the more sodium you eat, the more sodium-bound water weight you might be carrying with you at all times.

Again, I can't cover all of the details on sodium and water weight in this guide to water fasting and weight loss.

(But if you want to see just how much extra weight we're talking about, you can take a look at my personal experiment with sodium and weight loss.)

The important thing to remember about sodium and water fasting is this:

Because water fasting means that you'll basically be getting in zero sodium, your body will finally get the chance to flush any extra sodium out of your body.

In other words, if you switch from a high-sodium diet (aka your standard modern diet) to water fasting, you'll quickly drop up to 4 pounds (2 kilograms) of sodium-bound water weight.

Again, this will make your final water fasting results look a lot better on paper, but it won't actually help you burn off any of that annoying body fat.

Time to take a look at another reason why your weight loss results after a water fast may be "inflated".

Digested food

If you're on just about any kind of a diet (where you're actually allowed to eat solid food), you'll always be carrying around a few pounds of partially digested food in your digestive tract.

But after you stop eating solid foods completely during a water fast, your colon will still keep emptying out for a few days into your fast.

In fact, some people even do a colon cleanse before or during a water fast - to help speed up this process and help the body get rid of "toxins".

(If you're one of those people who'd actually consider doing a colon cleanse for a water fast, then you should definitely read what science has to say about this in my science-based guide to colon cleansing and weight loss.)

Ultimately, with or without a colon cleanse, your weight may drop by a few pounds because at least some of the digested food will leave your body during a water fast.

In other words, this means that you can lose as much as 7 pounds (3 kilograms) of non-fat weight during your next water fast, just because your colon will empty out a bit more than usual.

So let's wrap up this part of the guide with what should be the ultimate target of any well-designed diet, your body fat.

Body fat

If you've read all about the FIVE types of weight loss results that have absolutely nothing to do with your body fat, then you might feel like you'll barely lose any body fat at all during a water fast.

But because you're not getting in any calories, you'll still be melting away that stubborn body fat at a highly accelerated pace.

Plus, the only way you could monitor your progress, would be with a body composition scale.

So until some more detailed research is done on this subject, I can only offer the water fasting weight loss calculator above.

Don't exercise for weight loss

It's widely known that exercise and weight loss go hand in hand, right?

So why not do some light exercise to boost your water fasting results, especially if you start feeling that "energy high" after a couple of days?

The problem is, research^[2] has shown that if you exercise during a water fast you will destroy your lean body mass at an alarmingly accelerated pace!

And this is true, even if you "only" do light/moderate exercise.

You can find more details in my guide on water fasting ketosis, or more precisely in the part that talks about the consequences of exercising in ketotis).

The bottom line is, you don't need to lie in bed all day while you're doing a water fast.

Just do your best to avoid any activities that raise your heart rate, make you start breathing heavily, or require a lot of energy to perform

So things like heavy work, weight lifting, or basically any kind of sports.

And while this wasn't exactly a tip that can boost your water fasting results, it's even more important you understand what you're NOT supposed to do.

Let's move on to taking a look at what science has to say about how much lean body mass you can lose during water fasting.

Loss of lean body mass

In one study^[3], scientists proved that it's possible to destroy as much as 13 pounds (5.9 kilograms) of lean body mass during a 21-day water fast.

(Your lean body mass is basically everything in your body that's not body fat...muscle mass, bone mass, water weight, glycogen, internal organs, etc.)

Now, maybe you're the kind of person who doesn't really care about how much muscle mass they lose during a water fast.

Maybe you just want to get rid of that annoying body fat as quickly as possible, and you're willing to sacrifice some lean body mass as you do that.

But even if that's you, you should know that the single most important muscle in your body that can suffer because of this (and that muscle just happens to be your most important vital organ as well) is your heart.

And while a short water fast isn't likely to cause any serious damage your heart, I'm telling you about this stuff to make sure you always stay on the safe side with fasting.

But still. If you ask me, losing even a single pound of muscle mass mass as you go after a quick drop of that number on your scale isn't really worth it.

Especially since science has already discovered a better and more effective way of fasting.

Prevent the loss of lean body mass

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 any loss of muscle mass.

You see, according to research^[4], 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.

This modern way of fasting has actually been proven to be a lot safer than water fasting in over 10,000 of medically-supervised cases^[5].

And the best part?

When done right, muscle sparing fasting won't slow down the incredibly fast fat burn water fasting can deliver.

When done right, muscle sparing fasting can actually offer higher fat burn rates than water fasting alone ever could.

(If you're interested, you can get more details in my science-backed guide to muscle sparing fasting.

Summary (actionable takeaways)

According to science, water fasting can be quite effective for weight loss.

But if you take a look under the hood, then your body fat isn't the only type of weight being lost.

The loss of muscle mass can get pretty high, especially in the early days, but fortunately, science already has an answer to this problem.

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 because of the scientifically proven fact that you hunger can literally disappear, water fasting has one major over most other diets and cleanses.

A complete science-based guide on how to break a water fast

A science-backed guide to water fasting and muscle loss

Written by Rok Sprogar, CNC, WLS

Rok Sprogar is a NASM Certified Nutrition Coach (NASM-CNC) and a NASM Certified Weight Loss Specialist (NASM-WLS) who has been contributing highly actionable and science-based content to Leanhigh.