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What Really Happens When We Fast

author avatar Dr. Eric Berg 08/31/2023

There are a lot of incredible things that happen in our bodies when we fast. When we don't eat quite so often, there are many different processes that get turned on that help to heal, rebuild, and support our bodies. Read on to learn more about what happens when we fast and why you might want to try fasting for yourself.

In this article, I'll go over what happens within your body at the different stages of fasting so that you can better understand the health benefits and why they occur.

I'll cover:

Let's look at what happens at different time points when you go longer and longer without eating.

Fork and knife on wood circle making a clock out of silverware, intermittent fasting concept.


After 12 hours of fasting...

You get a spike in growth hormone. The growth hormone is a hormone that has many different functions in the body that are beneficial to your health.

For example, growth hormone is:

  • An anti-aging hormone.
  • The main fat-burning hormone in the body.
  • Involved in healing your joints.
  • Important for making new proteins.

People will often take growth hormones to help heal certain parts of their bodies. But one of the incredible things about fasting is that it will trigger growth hormone naturally.

After about 12 hours of fasting, your growth hormone levels will increase. And they just keep going up the longer you go without consuming calories.


After 18 hours of fasting...

A process called autophagy beings. Autophagy is a process in the body that recycles old and damaged proteins. Starting at about 18 hours into fasting, your body will start to take any of the old, damaged proteins lying around and turn them into amino acids that your body can use to rebuild new tissues.

Autophagy word cloud with words like fasting, ketosis, protein, restriction, and more.

For example, the process of autophagy can help to get rid of advanced glycation end products (AGEs). These are proteins that are made when glucose and protein combine and form a very sticky, harmful substance. Autophagy will break them down and turn them into something useful instead.

Autophagy will also help to clean the body of amyloid plaques, which are a certain type of protein that can wreak havoc in the brain.

Autophagy also kills off things like microbes, yeast, and other pathogens so that you can stay healthy.


After 24 hours of fasting...

At this point in your fast, many different healing processes kick into action.

One of the most important things to happen after you fast for 24 hours is that the glycogen reserves stored in your liver become depleted.

The more glycogen (stored glucose) from your liver is depleted, the more your body will begin to run on ketones as an alternative source of fuel. Additionally, your body will start using energy stored in fat and turn it into ketones. Using fat and ketones instead of glucose as fuel does a lot of good in the body.

Ketones are a preferred fuel source and come along with a lot of health benefits. They are an appetite suppressant that helps with fat and weight loss, they have antioxidant qualities, they provide oxygen to the body, they are highly efficient, and so much more.

Here are some of the other great things that start to happen at the 24-hour mark of a fast:

  1. Inflammation decreases. After 24 hours, inflammation levels can drop significantly. So if you are dealing with any inflammatory conditions, you may notice they improve at this point.
  2. Your gut starts to heal. Because your digestive system has a chance to take a break and chill out, your gut can start to heal. You may notice that conditions like SIBO improve, for example.
  3. Heart function improves. Your heart loves using ketones as a fuel source. So your heart health can really benefit from fasting this long.
  4. Brain function improves. Your brain also thrives on fasting. Something called BDNF is created, which is a factor that helps you to make new brain cells. At about this 24-hour mark, your brain can actually start to repair.

Old fashioned alarm clock with woman in background about to eat. Fasting concept, eating window.


Between 24 and 48 hours of fasting...

The health benefits continue to add up at this point.

Some of the additional benefits that happen during this time period include:

  1. Antioxidant activity increases. Your body starts to produce more antioxidants during this healing process of fasting. These are important for preventing long-term damage and letting the body start to heal.
  2. Your body is using fuel much more efficiently. You may be surprised to learn that when you don't eat and fast instead, your body actually requires fewer nutrients. This is because your body is using fuel more efficiently. Using fat and ketones as fuel is more efficient. Additionally, the autophagy process is providing you with recycled material to create new proteins and structures. This all adds up to your body having to rely less on nutrients to run smoothly.

After 48 hours of fasting...

You'll start to experience additional healing effects. For example, here are some of the things that can start to happen:

Medical drawing showing stem cell differentiation into muscle cell, blood cell, nerve cell, cardiac cell.

  1. Stem cells get stimulated. Stem cells are undifferentiated cells that don't yet have a purpose in the body. They can turn into different types of tissues depending on what the body needs. This is one of the incredible benefits of a fast – your body can start to use stem cells, which results in healing, repair, and anti-aging effects. Click here to learn more about using fasting to enhance stem cells.
  2. The risk of certain cancers goes down, and certain tumors shrink. As if the other benefits of a fast weren't enough, not eating as often can also support specific types of cancer and tumors. Learn more here.
  3. Your body starts making more mitochondria. These are the energy factories of the cell, and they increase in number when you do fasting.

After 72 hours of fasting...

After a few days of not eating, you can experience even greater health benefits. Prolonged fasting that lasts 72 hours can be really powerful every once in a while.

Here are some of the things that happen after this long:

  • Even more stimulation of stem cells. Stem cell stimulation starts at about 48 hours but just increases from there. So the longer you go without a meal, the more your stem cells can be used for healing and regeneration in the body.
  • Better immune system function. Your immune system activity improves the longer you fast, too. And a happy immune system means a happy body.

While there are benefits of this prolonged type of fast, I only recommend doing a 72-hour fast periodically. It shouldn't be done on a regular basis. You also need to work up to this long of a fast gradually so that you avoid problems that can occur with such a long fast like getting the "keto flu", weakness, or feeling cold while fasting.

Cartoon illustration of woman practicing intermittent fasting, looking at clock waiting to eat.


How to fast

Wanting to try a fast and see if it can help with brain function, immune activity, inflammation, weight loss, and so much more?

The good news is that fasting doesn't have to be super complicated or extreme. In fact, it can be an easy daily habit to get into. Here are my tips for doing fasting:

  • I recommend doing intermittent fasting on a regular basis. A great intermittent fasting pattern to shoot for is 18 hours of fasting and a 6-hour eating window each day. That means you'd eat a meal, wait 6 hours, eat another meal, and then fast the other 18 hours of the day. All your food and calories should come within the 6-hour eating window.
  • If you are already used to intermittent fasting, try adding on periodic prolonged fasting to achieve additional health benefits. This might mean fasting for 24 or 48 hours periodically. 
  • I think it is best to drink water (i.e. water fasting) and take your nutrients during a fast. If you are deficient in nutrients, you can have problems while fasting. So drink water and take your nutrients – especially minerals, B vitamins, and salt.

Intermittent fasting concept, cartoon illustration of clock, glass of water, mug of tea.


The bottom line

Whether you are looking for weight loss or anti-aging effects or better heart function, your body can benefit so much from fasting.

As you can see from the many health benefits outlined above, the body can do some pretty magical things when you don't eat quite as often and restrict your calories to shorter time windows.

So try intermittent fasting. Intermittent fasting is a great tool for weight loss and for supporting many common health conditions. And you might even try a few prolonged fasts now and then.

Have you ever tried fasting? What benefits did you notice? Share in the comments section below.

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Disclaimer: Our educational content is not meant or intended for medical advice or treatment.

Editor’s Note: This post has been updated for quality and relevancy.

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