A lot of people ask about the “stages” of fasting.
In other words, what happens inside your body when you fast? And how long does it take?
Today I’ll share with you a fasting timeline, and some key health benefits that happen along the way.
I’ll also share some tips from my own experience that can help you smooth the transition between stages, and make things easier.
By understanding the basic physiology of fasting, I think you’ll not only be more motivated to get started, you’ll also be better prepared to succeed.
Let’s dive in.
The Stages of Fasting: An Overview
In this section, you’ll find a short summary of each fasting stage, along with some basic definitions that will help you understand the rest of the article.
I’ve divided fasting into five stages (and one for eating).
- Stage 0: Eating
- Stage 1: Fed
- Stage 2: Transition
- Stage 3: Ketosis (i.e. Major Fat Burning)
- Stage 4: Rejuvenation
- Stage 5: Rebuilding
Stage 1 starts as soon as you finish eating.
First, it takes a few hours to digest your food. Then it takes another 12-24 hours to use up the glucose (a type of sugar) stored in your liver. Stored glucose is known as “glycogen”.
If you’re not familiar with how glycogen works, this short video explains the basics:
Stage 2 (the “transition”) starts when you run out of liver glycogen, and your body starts switching over to use fat as your primary energy source.
How does your body know to start burning fat?
Because the hormone called insulin goes down.
Insulin goes up whenever you eat, and it’s main job is to store energy, including body fat. But when insulin goes down, that process reverses and you burn more body fat.
Here’s a summary image that we’ll revisit later:
In Stage 3 (ketosis), you’ve transitioned to full fat-burning mode, and your liver is turning some of the fat into ketones.
Ketones are the alternative energy source your brain uses when you fast, or when you eat a very low carb (ketogenic) diet.
(By the way, if you’re “in ketosis” that just means there are ketones in your blood).
After you’ve been fasting for a few days or so, some special “rejuvenating” processes start happening in your body. That’s stage 4, and we’ll get into the specifics later on.
When you end your fast and start eating again, your body “rebuilds” structures that have been broken down, while making some improvements. That’s stage 5.
If any of this doesn’t completely make sense yet, don’t worry! That was just a quick overview.
For the full story, read on.
Stages 1-3 of Extended Water Fasting
In this section, I’ll cover more details about fasting stages 1-3, and the associated health benefits.
I’ll also give you some tips for how to make the transition between each stage smoother and easier.
By the way, “water fasting” just means you keep drinking water while you fast. And that’s the approach I recommend.
That’s why in this particular article we’re mainly talking about extended, multi-day fasting.
How Fasting Maximizes Fat Burning
Let’s say you fast for a few days.
Once you finish digesting your food, you’ll use the glycogen in your liver to maintain your blood sugar.
When you run out of glycogen* (which takes about 12-24 hours, depending on your activity level and what you ate beforehand), your body knows it’s time to switch into fat-burning mode.
As insulin drops, your body fat gets released, thanks to an enzyme called “hormone-sensitive lipase”.
In other words, low insulin is like the “key” that unlocks your fat storage.
As you release more body fat, ketones and fatty acids become your dominant energy source, instead of sugar.
Why does this matter?
First, it means there is actually plenty of energy available when you fast! It just takes a little time to fully access it.
Second, it means that fasting is literally the best way to burn body fat.
There’s nothing else you can do to lower insulin as much as fasting. And the lower insulin goes, the more fat you burn.
(*Note about blood sugar: Even when you run out of glycogen, your blood sugar never drops to zero. That’s because your liver can make more sugar from little bits of fat and protein, through a process called gluconeogenesis. You rely on that somewhat during the transition stage, before ketones mostly take over as your main energy source.)
Understanding The “Switch Point” Makes Fasting Easier
Do you see that intersection in stage 2, where your blood sugar & insulin are coming down, and your ketones are going up?
Let’s call that the Switch Point (SP).
I’ve noticed that whenever I reach this SP, my energy usually wanes a bit. And sometimes it gets harder to focus.
You may have a similar experience.
That’s probably because at that moment in time, both your blood sugar AND your ketone levels are kind of low.
How can understanding this help you?
First, when you reach the SP, you may be tempted to give up. But, if you know better times are just around the corner, it’ll be easier to keep going.
Second, there are some “hacks” you can use to smooth the transition, and make things easier for yourself.
A Ketogenic Diet Can Give You a Head Start on Fasting
So the Switch Point can be a rough spot, especially if it’s your first time starting an extended fast.
But what if you can get your ketones up before you start fasting?
Well you can. And it’s as simple as eating a ketogenic (low carb, high fat) diet.
Notice how the SP is higher now. That means your energy level won’t dip as much, because you already have lots of fat and ketones available.
Even if you just ate keto for a week or so beforehand, your body could produce enough ketones (and release enough fatty acids) to make that transition significantly easier.
And the longer you eat keto, the easier it will be. That’s because your body will steadily get better at using fat for energy (aka becoming “fat-adapted”).
“Training Wheels” Are Another Way to Smooth the Transition
Even if you don’t eat a ketogenic diet before your extended fast, there’s still something you can do to make the transition stage easier.
If you’re ok with doing a sort of “dirty”, or “modified” fasting (where you’re not being super strict), you could simply eat something at the SP, to tide you over.
In other words, use “training wheels”, in the form of a carefully-selected food, drink, or supplement.
The key is to pick something that’s low in carbs, and low in total energy (i.e. calories), but still gives you a little boost.
One of my favorites-
Half of an avocado. It won’t really raise your blood sugar, and it won’t prevent your body from ramping up ketone production.
But it will give you a little bit of extra energy during that potentially challenging transition.
Here are some other options I’ve tried that work well
- bone broth
- chia seeds
- dill pickles
- pumpkin seeds
These can all tide you over at a critical moment, without disrupting your fast enough to matter.
But feel free to be creative and try something different.
Note: If you want even more tips to make fasting as painless as possible, head over and get my Beginner’s Guide to Easy Fasting!
Your Hunger Will Mostly Go Away After Stage 2
You may be worried you’ll get really hungry on a multi-day fast.
Sure, you’ll probably get hungry early on, especially around the Switch Point. But now you know some tricks to deal with that, so it shouldn’t derail you.
And once you get into stage 3, your hunger pretty much goes away.
There are a few reasons for that:
First, remember your insulin level will be low. And with lower insulin, you won’t have as many cravings.
Second, remember your ketones will be high, and having high ketones suppresses your appetite.
There’s also this other “hunger hormone” called ghrelin, which rises and falls every day.
But when you fast for multiple days, ghrelin gets lower and lower each day. So you don’t feel as hungry.
I’ve certainly noticed this. While I often have some ups and downs during the first couple days of fasting, after that I don’t really get hungry anymore.
Sure, sometimes food crosses my mind, or I think, “heh, food, that’s a thing.” Other times I think it would be fun to eat something.
But I’m not really hungry.
Various hormones help you control hunger when you fast. And the longer you fast, the less hungry you’ll be.
Stage 4: Rejuvenation
Fasting has been a normal part of human life for millennia.
And lucky for us, our bodies figured out a way to put it to good use.
That’s why after a few days of fasting, some really cool things start happening in your body.
Collectively, it’s a sort of renewal, or “rejuvenation”.
Autophagy Recycles Worn Out Proteins
The best-known of these rejuvenating processes is autophagy.
Autophagy is a funny-sounding word that refers to something like “spring cleaning” inside your cells.
Basically, it’s when you break down old or worn-out proteins all over your body, so you can recycle and reuse them.
Autophagy could be viewed as a sort of “detoxification”, because it eliminates lots of junk inside your cells.
It can also lower your risk of various chronic illnesses.
For example, in Alzheimer’s disease (the most common type of dementia), you get a buildup of misshapen proteins in your brain. And autophagy helps clear those proteins away.
Autophagy Fasting Timeline
So, how long do you need to fast to stimulate autophagy?
Well, autophagy starts ramping up in stage 2 (the “transition”), but gets much stronger in stage 3 and beyond.
Other things (like exercise) can stimulate autophagy as well, but nothing does so more powerfully than fasting.
That’s one reason why fasting reduces your risk of just about every chronic disease.
Prolonged Fasting Rejuvenates Your Immune System
In addition to clearing out old proteins inside your cells (autophagy), prolonged fasting also helps you recycle old cells.
This happens through another funny-sounding process, called “apoptosis”. That’s when your body identifies cells that should probably be eliminated, and programs them to self-destruct.
Among the cells that your body breaks down during a long fast are a lot of your white blood cells. In other words, your immune system.
And here’s the really cool part:
While you fast, you make more stem cells. And when you start eating again, your body uses those stem cells to rebuild a new-and-improved immune system.
As a result, prolonged fasting has been shown to reduce autoimmune symptoms (from conditions like multiple sclerosis, or Crohn’s disease, for example).
Pretty cool, eh?
Extended Fasting Renews Other Organs, Too
Turns out, it’s not just your immune system that gets broken down and rebuilt during a prolonged fast. Other organs follow suit.
(So far, this has mainly been studied in animals. But there’s no reason to believe it doesn’t apply at least somewhat to humans as well.)
Here’s Valter Longo (head of the Longevity Institute at USC) discussing this process a little bit:
Why does this matter?
Because it means extended fasting can probably help heal various organs throughout your body.
This includes your liver, your pancreas (which makes insulin), and many others.
And that’s good news if you have type 2 diabetes, or prediabetes. When your liver and pancreas start working better, those conditions tend to quickly improve.
Fasting Can Also Rejuvenate Your Mind
Beyond all this scientific mumbo-jumbo, fasting has some dramatic subjective benefits as well.
For example, when I fast for a few days or more, the mental clarity I experience is unrivaled. So it kind of mentally rejuvenates me as well.
I also tend to feel a kind of “euphoria” that just doesn’t happen when I’m eating.
These amazing mental benefits are probably mainly due to having super-high ketones. Ketones are good for your brain, and they can also help you focus better.
Now, I’m not saying you won’t ever feel fatigued, irritable, or just kinda cruddy. It’s normal to have some ups and downs, particularly when your body’s first adapting.
But if you give yourself some time, and keep working at it, you can have some pretty amazing experiences as well.
Why Don’t Your Muscles Waste Away When You Fast?
Have you ever been told that if you fast, your muscles will shrivel up?
That’s a common fear. But don’t worry, it’s mostly nonsense. 😉
I wrote about this topic in a lot more detail in my post about whether fasting burns muscle, but here’s the super-short version:
First, your body isn’t dumb. It doesn’t break down your muscles when it can mostly just use body fat for energy instead.
Second, when you do need protein, muscle isn’t the only source. There’s also a bunch of connective tissue and other structures, which have lots of protein your body can use.
(And remember autophagy? That’s a way to find worn out or defective proteins throughout your body, which can then be recycled.)
Third, ketones help preserve your muscles, by sending a signal directly to your muscle protein, telling it not to break down. So you preferentially use other proteins instead.
Fourth, exercise promotes muscle growth even when you don’t eat!
The bottom line?
As long as you keep using your muscles while you fast (through moderate exercise), they won’t get significantly smaller.
Lastly, there’s another thing that helps maintain your muscles….
Stage 5: Rebuilding
We talked a lot about how your body breaks things down when you fast.
It burns a lot of fat, clears out a lot of old junky proteins, and even recycles old cells.
But what happens when you STOP fasting?
Well, then it’s time to rebuild.
Growth Hormone Helps Rebuild Your Muscles and Other Structures
After a couple days of fasting (and beyond), your body does something to prepare for the end of your fast.
It makes more growth hormone.
Growth hormone not only helps maintain your muscles while you fast, it also helps build them up again when you break your fast.
Growth hormone also helps you build up the other organs that were broken down while you fasted (like your immune cells, or your liver).
It’s all part of a cycle of renewal.
It’s almost as if our ancestors did a lot of fasting and our bodies evolved to know how to handle it. 😉
There are various stages your body goes through to get into a full-blown “fasted state”.
Short-term “intermittent” fasting gets you into the very early stages of fasting. And that can be really beneficial.
But multi-day “extended” fasts take you much further along the fasting timeline. And that’s when some really cool things start happening.
This includes a more profound transition from sugar-burning to fat-burning, as well as rejuvenation and renewal throughout your body.
By understanding what’s happening inside the body when you fast, you’ll be more motivated to fast, and better prepared to deal with any rough patches.
That way you can get the full benefits of fasting, which will help you take control of your health. 🙂
I hope you found this article useful!
And if you did, please share it with someone else who can benefit.