What Happens to your Body While Fasting

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Are you wondering what will happen to your body when you fast?  If you are, you should know that fasting will provide benefits that extend far further than just burning fat and making your diet more sustainable and enjoyable.

When your body goes without food for a certain period of time, you’ll experience changes that affect your brain, your digestive system, immune system, hormones and even the function of all the microscopic cells found within your body. 


When you first begin fasting a few things happen at the cellular level that may cause hunger and fatigue if you’re a beginner. This is because your body will normally get it’s energy from glucose or carbohydrates.

So, depending on how much sugar you have in your blood and how much glycogen or carbohydrates you have stored in your muscles and your liver, it can take anywhere from 8 to 48 hours without food for your body to switch from mostly using glucose or carbs for energy to breaking down fat and using ketone bodies for energy in a process known as gluconeogenesis (1)

Now, to most people, the concept of going anywhere from 16 hours to up to a few days without food may sound incredibly foolish and unhealthy. However, the exact opposite is actually true.

It hasn’t always been so mainstream to eat at least 3 times a day everyday. Human beings have evolved to be perfectly capable of going long periods without food.

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This is supported by research on modern hunters and gatherers that shows that they go between intermittent periods of feeding and fasting depending on the availability of food (2). For example, modern bushmen living in Africa go through frequent periods of food deprivation ranging from 1 to 10 days when they can’t catch any game, when they have seasonal droughts, or especially when animals migrate. 

The truth is that most healthy adults have evolved to be perfectly capable of fasting, with the good news being that you don’t have to fast for 10 days to experience the benefits for your body. Just 16 hours will provide some really unique beneficial effects. 

Effects on the Body

When going through your first 16 hours without food, the first thing you should expect is to start feeling really hungry right around the regular meal times that you’re skipping. This is because of a hunger hormone in your body known as Ghrelin, which stimulates your appetite when it’s released. Studies show that ghrelin levels will spike at the times of the day that you normally eat, which for most people is breakfast, lunch, and dinner (4).

This will happen to your body automatically, but if you give in and eat before your fast is over, the same thing will happen again the next day. On the other hand, if you just ride out that hungry feeling, within about 2 hours after the initial spike in ghrelin, it should drop back down to normal levels and your appetite should drop along with it (4).

As your body adapts more and more to skipping a meal, or multiple meals throughout the day, you’ll feel less of those automatic cravings to eat during the key times of the day that you normally find yourself hungry.

If you allow yourself to adapt to consistently fasting for just 16 hours a day, while eating a healthy diet during the other 8 hours you’re body will start to become much more sensitive to insulin.

Having higher insulin sensitivity will not only help enhance fat loss, it’ll also help you feel much better when your body doesn’t have carbs available for energy, and it’ll help reverse or reduce the risk of a number of serious diseases like diabetes, high blood pressure, heart disease, and even thyroid conditions.


Now, during each of your 16 hour fasts, about 6 hours into the fast your body will begin going through another incredible process known as autophagy.

This is a process where your body begins destroying old or damaged parts of your cells. When you’re always eating, your body doesn’t get a chance to do this because it’s constantly rebuilding rather than breaking down. Breaking down unnecessary and dysfunctional components of your cells allows them to be recycled so they can perform optimally.

This process also helps reduce inflammation throughout the body which helps fight a number of diseases (5).

The benefits of autophagy will also extend to your skin cells, potentially slowing down wrinkles, age spots, and acne. Your immune system will also improve thanks to autophagy. Your body will repair and produce brand new white blood cells which will be stronger at fighting diseases.

On top of that, it may even help your body slow down the aging process and extend our lives.

Now, this claim something that still requires more research, but there have been some very interesting studies on animals that support this theory.

For example, rats were divided into two groups at birth. One group was fed every daywhile the other group was fed every other day. This is also known as an alternate day fast.  

Not only did the rats in the fasting group age much slower, they also lived 83% longer than the rats who were fed daily (6). There are other animal studies that support these results and show that fasting can help with longevity even when calories aren’t controlled or restricted (7).


Now, as your body cleanses and repairs itself with autophagy, there will also be a rise in the rates of neurogenesis in the brain.

Neurogenesis is the growth and development of new brain cells and nerve tissues.

Higher rates of neurogenesis have been linked to increased brain performance, and improved memory, mood, and focus. 

According to the head of the neuroscience lab at the National Institute on Aging, fasting only twice a week could significantly lower the risk of developing Alzheimer’s disease or Parkinson’s and it can help you develop protective measures against damage from a stroke (8). This is because the shock of fasting stresses the brain in a good way, just like stressing your muscles when you workout. This stress leads your brain to create new cells.

The stress from both exercising and fasting increases the production of protein in the brain, which promotes the growth of neurons allowing them to send signals and communicate better with each other. This may explain why many people feel a lot more focused and productive while fasting.

Fasting also makes a lot of sense from an evolutionary survival perspective. This is because we would want the brain to work optimally whenever food wasn’t available because that would help us with the problem solving process that’s required to actually find some food and survive.

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Now, one of the most important of the proteins that gets a boost from fasting is called BDNF.

BDNF has been referred to as “Miracle Grow for your brain.” It makes your brain more resilient to stress and more adaptable to change while helping with the production of new brain cells.

It also help your brain with protecting your current brain cells while improving memory, and learning. Having enough BDNF is especially important as we age because low levels of BDNF are linked to dementia, Alzheimer’s, memory loss and other brain processing problems.

Some good news is that studies show that intermittent fasting anywhere from 16 to 18 hours can boost BDNF levels by fifty to a hundred percent.

In similar studies, fasting up to 36 hours has been shown to boost BDNF levels by up to four hundred percent. If you were to fast for even longer lengths of time, like 36 hours, 48 hours, or 72 hours, your body would continue going through further changes.

Most people assume that this is when your body starts to use protein for energy by breaking down muscle tissue. However, this is definitely not the case. Research shows that your body‘s protein breakdown rates will drop significantly within just the first day of fasting (1) and drops even further the longer you fast.

One starvation study showed that during the first 3 days of fasting, protein breakdown rates dropped to only 75 grams per day for an adult man weighing about 150lbs. This is less than 1/2 a gram of protein per pound of body weight.

What’s even more surprising is that after 3 to 4 days of not eating, protein breakdown drops to a tiny 20 grams per day (10).

Effects on Growth Hormone Levels

The body‘s ability to conserve muscle while fasting for longer lengths can be explained by another major change that will happen to your hormones while fasting, which is that you’ll have a huge spike in your human growth hormone levels.

The change in human growth hormone won’t really help you build more muscle since you’ll still have a negative protein turnover rate. However, it will help prevent muscle loss while fasting. In one prolonged fasting study, growth hormone levels increased by over 1200 percent at its peak (11)

Just keep in mind that this only applies to prolonged fasting. Most of you will only be fasting for up to 16 hours a day, so your body won’t really hit that point where it’s super conservative with protein; nor will you have that significant of an effect on your growth hormone levels.

Effects on Metabolism 

Another thing that won’t happen when you fast is your metabolism won’t shut down or become permanently damaged. This is a very common assumption that people have about fasting, especially if you’re going without food for longer than 24 hours. However, this is a total myth.

First of all, if your metabolism does slow down from consuming fewer calories while fasting, it won’t slow anymore than it would if you cut away the same amount of calories with a traditional calorie-restricted diet. Second, there are a few studies out there that show that fasting can actually help improve your metabolism by increasing levels of a neurotransmitter known as norepinephrine (12).

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However, like I said, most research shows that fasting and regular calorie restriction both have a very similar effect on metabolic slowdown when calorie deficits are equally matched.

Effects on your Digestive System

Now, even though we’ve established that your metabolism won’t slow down any more than normal while fasting, your digestive system definitely will.

This is because you won’t be eating any food, so your stomach will finally get a break. Most people don’t realize just how much energy is required to simply digest the food you eat throughout the day. This is why you feel tired after eating a lot of food in one sitting. It is also why some people feel like they have more energy when they work out in a fasted state instead of working out right after eating a big meal.

Other than saving some energy on digestion, shorter fasts that are 16 hours in length won’t require you to take a certain series of steps just so you can break your fast in a way that you can avoid an upset stomachWhile many people obsess about how to break their fast, it’s really unnecessary since most people are only fasting for 16hour lengths.

It’s only with longer, prolonged fasts, usually lasting 3 days or more, that you may want to break your fast a certain way to help your digestive system ease back into breaking down food.

If you are doing a prolonged fast, starting by reintroducing easy to digests liquids like bone broth and vegetable juice can help your stomach not become upset. After that, you can start reintroducing vegetables, fruit, and lean sources of protein.

At that point as long as your stomach is fine you can start eating normally, there’s no need to make this process crazy complicated.

Effects on Cancer Cells

One last, potentially miraculous, effect that fasting may have on your body is the possible prevention of the formation and spreading of cancer. However, this does require much further research.

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 There have been a couple interesting initial studies.

One of them was a test-tube study which showed that exposing cancer cells to several cycles of fasting delayed the formation of tumors in the same way chemotherapy did. Fasting also increased the effectiveness of chemotherapy drugs on the development of cancer.

Another study on rats following an alternate day fast, showed that the fasting helped block tumor formation. Unfortunately, human beings are not rats and they’re obviously not test tubes, so it’ll probably be a long time before we have solid randomized control trials on the positive effects that fasting may have on cancer formation in human beings.

Concluding Notes

That about wraps it up guys!

I hope you enjoyed this article and hopefully you understand a bit more about what happens inside the body while you fast.

If you’re looking for a done-for-you fasting plan, that on average is helping my clients lose 20 pounds or 5 percent of their body fat while replacing it with lean muscle in just 6 weeks, try my challenge. You’ll select from a number of diet approaches including intermittent fasting, one meal a day, keto, vegetarian, vegan, and much more. All of them can be customized based on your body.

To find out more you can click the link below:

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You’ll also get a 42 day workout plan, a recipe book that goes hand in hand with your diet plan, and a private accountability coach to motivate and push you through the whole process.

My passion for fitness began when I was 14 years old. I naturally fell in love with training and haven’t stopped since. At 18 years I acquired my first personal training certification from ACE after which I opened my first of 3 transformation studios in 2011. I love to share my knowledge through personal training, my online courses, and youtube channel now with over 3,000,000 subscribers! I can happily say that we've helped over 15,000 people get in great shape over the years. I'm always here for my customers so if you need help don't hesitate to send your questions to support@gravitychallenges.com

Founder // Gravity Transformation, Max Posternak


  1. Comparative Physiology of Fasting, Starvation, and Food Limitation (Graph)


  2. Hunter-gatherer anthropoids, including those living today, often eat intermittently depending upon food availability


  3. Elizabeth Marshall Thomas, The Old Way, ( New York, Picador, 2007)

  4. Ghrelin (Hunger Hormone) Released During Normal Feeding Times (ie. breakfast, lunch, dinner)


  5. RIF attenuates inflammatory status of the body by suppressing proinflammatory cytokine expression


  6. EOD group represented an 83% increase over that of the AL group


  7. An important characteristic of IF is that it can increase lifespan even when there is little or no overall decrease in calorie intake.


  8. Rates of adult neurogenesis in the intact brain increase after energetic challenges, such as exercise and intermittent fasting & Intermittent fasting (IF) …. protects against pathological increases with stroke.


  9. IF can boost BDNF by 50 – 400%:

    https://www.ncbi.nlm.nih.gov/pubmed/12558961 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2622429/

  10. Protein breakdown drops to 70g/24h when fasting and after fasting for 3-4 days drops further to only about 20g/24h:


  11. Growth Hormone peaks at a 1250% increase with prolonged fasting


  12. Resting energy expenditure increases in early starvation, accompanied by an increase in plasma norepinephrine.


  13. Multiple cycles of fasting promote differential stress sensitization in a wide range of tumors and could potentially replace or augment the efficacy of certain chemotherapy drugs in the treatment of various cancers


  14. Long-term IF regimen exerts an anti-promoting effect on rat hepatocarcinogenetic induced by DEN


  15. Bodybuilders had a 14% higher metabolism than a similar control group (see chart):