Welcome to my science-based guide that will explain everything you need to know about water fasting ketosis.

You will find out what ketosis is, how it works and how long it will take for your body to enter ketosis.

You'll also discover an amazing benefit of ketosis (extra calorie burn), how much muscle mass you can expect to lose, and how drinking water and exercise can be both positive and negative while in fasting ketosis.

Before we dive in, here's a quick overview of what I've covered in my science-backed guide (use this for quick navigation):

What is water fasting ketosis?

Simply put, water fasting ketosis is a state in which your body starts getting almost all of its energy from your body fat reserves.

You see, when you're on a normal diet, your body can draw energy from many different sources: the food you eat, the glycogen stored in your liver and muscles, your body fat stores, etc.

But during a water fast, since you're eating no food at all, most of these energy sources in your body will be completely wiped out after a few days.

That's a good thing, because that means your body is melting off that stubborn body fat at a very high rate.

And once that happens, your body enters ketosis, a state in which your body has no choice but to rely almost exclusively on your own body fat reserves.

Water fasting is one of the fastest ways to reach that state of super high fat burn called ketosis because you're literally getting in no calories at all.

Let's take a look at how quickly this can happen?

How long does it take to enter ketosis?

According to one study[1] of water fasting, it can take up to 3 days before your body can reach full water fasting ketosis.

Because, even if you stop eating food completely, your body will still have to exhaust all other sources of energy first.

For as long as your body can still rely on food in your digestive tract or the glycogen in your muscles and your liver, you won't be able to enter full ketosis.

In other words, if you do a water fast and decide not to do anything to speed things up, you can count on entering ketosis no sooner than day 3.

On to another amazing benefit of water fasting ketosis, higher calorie burn.

How many calories will you burn?

Most experts will tell you that weight loss basically comes down to calories eaten versus calories burned.

But water fasting ketosis can offer one huge advantage when it comes to melting away that stubborn body fat.

You see, according to science[2], water fasting ketosis can help you burn as much as 325 extra calories a day!

And that's on top of everything you'll burn because you're not eating food.

Why does this happen?

Well, your body has to "work" much harder to extract the needed energy from your own body fat reserves (as opposed to when it can get that energy from food).

In other words, your body will need to spend extra calories to extract, convert, and distribute the energy stored in your body fat reserves.

Which means that once you enter water fasting ketosis, you should be burning hundreds of extra calories per day.

How much muscle mass will you lose?

Even though our bodies are extremely efficient at surviving in water fasting ketosis, your body won't be able to extract 100% of needed energy from your body fat reserves alone.

And while you will be burning through those stored fat-calories at an extremely high rate, your body will still need to extract some energy from your lean body mass[3] too.

This means that even after you enter the efficient survival state of full ketosis, your body will still be forced to burn away some of your muscle mass for energy.

In one study[1], people did a 21-day water fast.

And the shocking thing that happened, was that those people lost as much as 13 pounds (5.9 kilograms) of their lean body mass during 3 weeks of water fasting.

So even though you will get to enjoy extremely high fat burn while in water fasting ketosis, you will still lose quite a lot of your muscle mass in the process.

Should you exercise while in ketosis?

One of the biggest mistakes I've seen people make is that they try to stay as active as possible during a water fast, sometimes long after they've already entered ketosis.

Some people even feel energized enough to go hiking or do other forms of more or less intense exercise.

And the worst part is, most of the people in the water fasting community are totally clueless about just how big of a mistake this can be.

In one study[4], people exercised at a moderate pace after they already reached full water fasting ketosis.

And the crazy thing that happened was that they destroyed over 100 grams of their lean body mass PER EVERY HOUR of exercise.

Just think about what this means for a minute.

You will lose some of your lean body mass (your muscles), even if you stay completely inactive during a water fast.

But if you force yourself into exercise while in ketosis, you WILL start destroying your lean body mass at an alarmingly accelerated rate.

The bottom line is, exercising while in ketosis won't help you burn a whole lot more body fat, but it WILL destroy your muscles at an extremely high rate.

So just DON'T exercise during water fasting ketosis.

The importance of drinking water

One more thing you need to understand is the importance of drinking enough water for as long as your body is in ketosis.

You see, science says that drinking water can give your metabolism (calorie burn rate) an immediate and pretty large boost[5].

But without enough fluids, the calorie burning process in your body called "thermogenesis", slows down.

You can learn more about this calorie burning process in my science-based guide to thermogenesis (the guide contains lots of different ways to speed up this calorie-burning process in your body).

And just to make things perfectly clear.

What you just read about drinking enough water isn't true just for water fasting ketosis.

You should keep your body properly hydrated at all times.

Not "just" to keep your metabolism revved up, but also for health reasons.

Summary (actionable takeaways)

In my science-based guide to water fasting ketosis, you've learned that:

  • ketosis is a state of extreme fat burn that your body enters once it wipes out all other energy sources,
  • it takes at least three days before you can enter a state of full ketosis,
  • ketosis can burn as much as +325 extra calories every day (in comparison to other diets),
  • exercising or staying too active can be a huge mistake during water fasting ketosis, because it can needlessly destroy A LOT of your lean body mass (muscle mass),
  • drinking enough water is not only important for health, but can actually help you burn more calories.

Written by

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.