An actionable and science-backed guide to water fasting and muscle loss

Hey, and welcome to my science-backed guide that will explain everything you need to know about water fasting and muscle loss.

I'll show you the science on how much muscle mass you can expect to lose during a water fast (with a daily breakdown).

You'll also see how water fasting compares to other weight loss diets (mixed and ketogenic) and discover a scientifically proven way to completely prevent any loss of muscle during water fasting.

But before we jump in, here's a short overview of everything that's covered in the guide (use it for quick navigation).

Comparison to other weight loss diets

Let's take a look at a study^[1] that took a closer look at muscle loss during water fasting, a ketogenic, and a mixed weight loss diet.

Here's how the study went down:

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

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

Here are the results of the study:

At first glance, muscle loss seems to be the worst (by far) during water fasting.

Because looking only from the muscle loss results of this study, you could say that:

During water fasting you lose around 50 grams of muscle mass a day, which is 5 times faster than on a mixed 800-calorie weight loss diet (9.5 grams of muscle mass a day) and about 3 times worse than on a 800-calorie ketogenic weight loss diet (17.9 grams of muscle mass a day).

But there's more to it.

Total weight loss versus muscle loss

While it's true that muscle loss during water fasting will be pretty high, we also have to consider how quickly your total weight will be coming off.

Let's say you wanted to lose 44 pounds / 20 kilograms of total weight, which 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:

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.

So because your total weight will be dropping a lot faster, you'll also lose a lot less total muscle weight.

But there's one more ting you need to understand about daily muscle loss during a water fast.

How much muscle will you lose a day?

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.

But take a look at what another study^[2] on water fasting and muscle loss discovered:

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

This happens because of a survival mechanism, a protein-sparing effect that sadly only kicks in quite late into the fast.

And now for the most important question when it comes to losing water fasting and muscle loss:

How to prevent muscle loss?

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 during fasting.

How can you do it?

According to scientific research^[3], 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".

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

Technically, if you do this, you will no longer be doing a 100% pure water fasting, but a protein-sparing modified fasting.

Or as I prefer to call it, muscle sparing fasting.

If you agree with me that taking a bit of protein during a fast to fully prevent muscle loss, you can find more information in my science-based guide to muscle sparing fasting.

Summary (actionable takeaways)

Here's a summary of all the actionable tips you've just learned on water fasting and muscle loss:

• the rate of muscle loss during water fasting is around 50 grams a day,
• you will lose muscle mass 5 times faster during water fasting than during a 800-calorie mixed diet (9.5 grams of muscle mass a day),
• you will lose muscle mass 3 times faster during water fasting than during a 800-calorie ketogenic weight loss diet (17.9 grams of muscle mass a day),
• but with water fasting, you will also reach your weight loss goals sooner, meaning that you won't lose 3-5 times more muscle mass in total (see explanation above),
• you lose a lot more muscle at the beginning of a water fast than in the later stages of fasting (because of a survival mechanism called the protein sparing effect),
• a safer an smarter alternative to water fasting is protein-sparing modified fasting (also known as muscle sparing fasting).

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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.