Wondering What Actually Happens Inside Your Body When Intermittent Fasting?

Wondering What Actually Happens Inside Your Body When Intermittent Fasting? - YAstrength.com

Many new intermittent fasters, and even people who’ve been intermittent fasting for several years, aren’t too sure of what exactly happens inside their bodies as they fast.  And yes, many research studies have suggested that intermittent fasting can lower insulin levels, enhance recovery, reduce inflammation, boost metabolism, increase fat burning, help with cell regeneration, and more.  All that is great information but understanding that alone is not helpful with regard to painting the full picture for many of us.

The fact is that people fast for different lengths of time. Some people fast for just 12 hours to start out, others do it for about 16 hours, some do it for 18-20hrs, some people do alternate day fasting, etc. The various intermittent fasting styles and lengths can seem endless. But a common question remains, regardless of the type of interment fast you do – When do the different benefits occur with regard to fasting length?

Continue reading and I will tell you about the different stages of intermittent fasting and what happens inside your body as you do it, broken down by hours. Feel free watch the included YouTube video if you prefer.

What Happens Inside Your Body, as You Fast – YAstrength.com


Right after you eat your last snack or meal, your body begins the digestion process immediately afterwards and the carbohydrates you consumed are being processed and sent into your bloodstream as sugar (or in scientific words Glucose). This process increases your blood sugar levels to rise, and as a result, your body begins to produce insulin.

What’s the purpose of Insulin?

Insulin a protein pancreatic hormone secreted by the beta cells of the islets of Langerhans that is essential especially for the metabolism of carbohydrates (1). Insulin’s purpose is to stimulate the absorption of glucose (aka, blood sugar) into the tissues to provide energy – it also stores excess amounts as energy. This is because we eat more than enough carbohydrates that our bodies usually need at a given moment and our bodies simply do not need all of that energy that’s being release from those extra carbs/ sugars.

As your body produces insulin to convert the extra glucose into glycogen. Glycogen is stored in your liver and muscles and it gets used when the energy is needed again. But even your liver and muscles can reach its capacity for glycogen storage. When this happens, the excess glycogen is stored throughout the tissue in your body, in the form of fat

Wondering What Actually Happens Inside Your Body When Intermittent Fasting? - YAstrength.com


About 4 hours after your last meal or snack, the level of glucose in your blood begins to drop and your blood sugar will decrease, coming back from those previously high levels. However, your insulin hormone is still at work during this time, and as long as glucose is continually released into the bloods stream (which is occurring because Insulin is still activated), glycogen is still being stored for energy.

That said, fat is not being used as energy yet.  But, as insulin drops, your glucose will become completely used up and glycogen stores will be depleted. It is at that point that your body will finally switch its energy source from glucose, to fat. I will let you know when this change officially occurs.


After about 8 – 9 hours, your body will have fully digested all foods and stopped producing insulin. During this time, your body rests very briefly – but as your organs, muscles, and brain continually demand energy, your body reacts by releasing something called Glucagon.

  • So, What’s Glucagon?

Glucagon is a protein hormone that is produced especially by the islets of Langerhans and that promotes an increase in the sugar content of the blood by increasing the rate of glycogen breakdown in the liver (2). Glucagon releases all the glycogen that was previously stored from the excess carbs and sugars, back into the blood stream. This keeps your bloods sugar levels steady as an energy supply source is continuously provided.

As your body constantly demands for energy, this begins to stress the body (in a good way), and your body prepares to find an alternate energy source because your glycogen stores will soon become exhausted. Once that occurs, your Fat reserves are next in line.

To kick off this process, your body begins producing several fat burning hormones. These hormones include:

  • Testosterone
  • Human Growth Hormone
  • IGF-1 (which is an insulin-like growth hormone)
  • T3
  • Adrenaline
  • And More Glucagon

FAT BURNING & KETOSIS (12 – 16HOURS After Last Meal)

After about 12-16 hours, your body is going full throttle with those fat burning hormones. Those fat burning hormones eventually trigger your liver to begin producing something called ketones because of these hormonal changes.

Wondering What Actually Happens Inside Your Body When Intermittent Fasting? - YAstrength.com
  • What are Ketones?

A ketone is a class of organic compounds (such as acetone) characterized by a carbonyl group attached to two carbon atoms (3). They are fatty acid molecules that get formed after your body breaks down fat cells.

During this time, your body loves ketones because they provide energy to your vital organs, heart, and brain. You may also notice a that you feel extremely focus, zoned in and productive during this time. This is because Ketones also help active nerves cells, develops new cells from your brain’s stem cells, and strengthens your intellectual capacity. Plus, ketone production increases more and more, the longer you fast!

AUTOPHAGY (16 – 18HOURS+ After Last Meal)

After about 16-18 hours, Autophagy begins. The Greek word “Auto” means self, and “Phagy” means eat. That said, the literal meaning of autophagy is “self-eating”, which is what’s actually happening inside of your body when you reach this point. But do not worry, this process if extremely beneficial. Autophagy is the body’s way of cleaning out. It recycles damaged older cells and recycles them to generate renewed cells.  This makes your cells more efficient and longer lasting.

Some studies show that autophagy can even begin sooner, around the 14-hour mark, depending on how long you have practiced intermittent fasting overall (just beginning or for a couple of years).


So, long-story-short, there is a lot that goes on within your body when you do intermittent fasting. I encourage you to continue learning about fasting and pick up a few best practice approaches or tips along the way. The more you get to understand the intermittent fasting process, from a biological perspective, the more you will enjoy the process overall.

I hope you found this blog post informative and helpful. Be sure to like, comment, and subscribe to our email list to be keep up with future posts from YAstrength.com.


  1. https://www.merriam-webster.com/dictionary/insulin
  2. https://www.merriam-webster.com/dictionary/glucagon
  3. https://www.merriam-webster.com/dictionary/ketones#other-words
  4. Photo by Pixabay on Pexels.com
  5. Photo by Roger Brown on Pexels.com
  6. Photo by Anna Tarazevich on Pexels.com

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