5 Stages of Intermittent Fasting Easily Explained

stages of intermittent fasting

There is a large inter-individual variability in how people respond to intermittent fasting (IF). Genetics, lifestyle choices, and state of health… all play a role in helping you achieve your goals.

But for the most part, we can only estimate what will happen when a person hops on an intermittent fasting plan using the 16/8 plan. The body has a set of predictable metabolic events to intermittent fasting that profoundly impact one’s health and weight. Intermittent fasting can significantly improve mental health, life expectancy, cognition, hypertrophy, and so on.

We will break down these responses into five stages of intermittent fasting here.

The Five Stages of Intermittent Fasting (using the 16/8 Plan)


According to Harvard professor Dr. George Cahill, all stages of intermittent fasting are defined by a timeline. It is important to discuss the body’s principal sources of energy:

  • Glucose:-The body’s main source of energy
  • Triglycerides:-The body converts any calories it doesn’t need into triglycerides, a type of fat

The body stores carbohydrates in the form of glycogen. It is a molecule that contains glucose chains and water. Glycogen is broken up into glucose molecules that are sent into the bloodstream.

The body breaks down triglycerides from fat cells once glycogen is low enough. This produces fatty acid chains and glycerol. These are then used by the body to convert into glucose for the brain.

With that out of the way, let’s discuss the five major stages involved in 16-hour intermittent fasting.

Stage 1: 0-4 Hours

Also known as the fed state, this stage of intermittent fasting starts within the first four hours of eating. The body has access to a lot of glucose. Of course, assuming that the last meal consisted of ample carbohydrates and proteins.

Insulin is released when blood sugar levels rise. Insulin blocks the enzyme responsible for breaking down stored fat.

Eating more carbohydrates and proteins triggers the nutrient hormone. The body starts storing some of the supplied energy for later.

For obvious reasons, the body needs energy even when you are not eating. This is why the body stores excess glucose as triglycerides in fat cells or as glycogen in the liver.

Note:- You’ll often hear experts refer to this stage as an anabolic stage, or growth period. It’s when the body has access to excess nutrients that it can use for energy or building muscle.

Stage 2: 4-16 Hours

Fresh ingredients

The second stage of fasting is known as the post-absorptive phase.

The body switches to a catabolic state and digests stored nutrients for energy. The body’s insulin levels are lowered. This causes the body to convert stored glycogen into glucose to be used as energy.

At the end of this phase, the body will gradually run out of glycogen in the liver. This will prompt the search for an alternative source of energy.

Most of the energy will now come from stored fat. Note:- If you eat a low-carbohydrate meal, the glycogen stores would deplete faster. As a result, your body will turn to stored fat earlier.

The second stage of fasting is useful for preventing chronic diseases. High insulin levels cause the body to retain salt (which stores more water weight).

Low insulin levels can prevent diseases like diabetes, cardiovascular disease, and cancer.

For this reason, intermittent fasting works best if the fasting window is maintained for at least 16 hours.

With insulin down, the body releases hormones like cortisol, adrenalin, glucagon, and HGH. These hormones tell the body to derive more energy from fat stores.

Important Terms

  • HGH (or human growth hormone) plays a role in building and repairing
  • Cortisol is nature’s alarm system.
  • Glucagon controls glucose levels in the blood
  • Adrenaline prepares the nervous system to fight or flee.

Stage 3: 16-30 Hours

The third stage of intermittent fasting is known as gluconeogenesis. It’s a process that allows the liver to synthesize new glucose from protein.

It’s worth mentioning that the body is not in full ketosis during this time and glucose is needed. With glycogen levels depleted, the liver uses protein to make more glucose.

Don’t worry, though. This protein does not come from lean tissue because of the activation of autophagy.

Autophagy is when the body recycles defective proteins and organelles. Health benefits include the removal of pathogens and prevention of atypical protein accumulation. The body rebuilds cell parts from amino acids obtained by breaking defective proteins.

It’s important to discuss an important detail here.

Many obese individuals carry an excess amount of protein. It’s bad because protein accumulation may cause cancer and polycystic ovary syndrome.

Intermittent fasting helps the body break down the excess but and protein. This process is regulated by the nutrient sensors such as AMPK, insulin, and mTOR.

Stage 4: 2-7 Days

After we are in the fourth stage of intermittent fasting, we are entering into a state of prolonged fasting. This stage is often referred to as ketosis.

Once ketosis is fully activated in the body, the brain is fed purely based on fat energy sources.

The brain still uses glucose for energy, which falls short after fasting for two days. The liver has to convert the body fat to produce ketones.

This is where it gets interesting. Fatty acids are unable to cross the blood-brain barrier. This process is responsible for providing at least 75 percent of the energy to the brain.

People in a state of ketosis can focus better and concentrate for longer. Moreover, ketones can have powerful anti-inflammatory properties. These make you more insulin sensitive. This is a good thing for individuals at risk of developing diabetes.

Important note:- There is a big difference between ketosis and ketoacidosis. Ketosis uses ketones for fuel. Ketoacidosis is high acidity in the blood to high ketone levels. This could be a life-threatening problem for many people. It is worth consulting your doctor to ensure you don’t hurt yourself.

Stage 5: 7 Days and Beyond

Woman at a Starbucks

The fifth stage of the intermittent fasting process involves the conservation of protein. The body will meet most of its energy from fat reserves, including the brain.

In this stage, the body’s ghrelin levels are low. Consequently, people report feeling less hungry.

Prolonged fasting can reduce IGF-1 levels in the body and PKA activity in various cells. IGF-1 is very similar to insulin and promotes growth in most cells throughout the body. IGF-1 triggers signaling pathways, including the P13K-Akt pathway that helps with cell survival and growth.

PKA has been known to activate the mTOR pathway.

Intermittent fasting reduces IGF-1 and PKA, which leads to the breakdown of older cells and proteins. Animal studies have shown that prolonged stress reduces IGF-1 and PKA. This, in turn, reduces stress and regenerates blood cell stem cells.

The Timeline of Each Stage Depends on Your Fed-Fast Cycle

We prepared this guide using the popular 16/8 diet plan. It is effective at weight loss – within the safety margins.

The five stages of intermittent fasting depend on your approach to fasting. You can experiment with meal timing and frequency to decide how long the intermittent fasting should last.

However, if your target is weight loss, you can combine intermittent fasting with the keto diet.

The goal is to deplete your glycogen stores and force the body to use stored fat for energy.

Moreover, this combination of intermittent and keto fasting has numerous health benefits.

Does intermittent fasting have to be 16/8?

No. The 16/8 plan is just one of several approaches to intermittent fasting.

Please note that if you’re just getting started with intermittent fasting, we recommend taking things slow. Instead of fasting for 2 days straight, consider fasting for 12 hours and see how it impacts your body.

You can slowly ramp things up by adding 1-hour increments every few days.  You can try the 5:2 plan with a fasting cycle of two consecutive days and eating normally for five days each week.

Pro tip: Drink lots of water and unsweetened tea or coffee to suppress your hunger.

And remember, talk to your medical provider if you have any health issues before trying intermittent fasting.

How to Feed Yourself

So there you have it – all five stages of intermittent fasting and your body’s metabolic response to them. Break your fast using a nutritious and well-balanced meal. This includes plenty of plant fibers, plant fats, vegetables, and healthy proteins. For weight loss, make sure to eat under your maintenance calories. Use this calculator to find your maintenance calories (switch to the Mifflin-St Jeor formula under advanced settings)

Feel free to reach out to us for more information here.


Author: Idrees Samih

Founder, Certified Ketogenic Diet and Ketosis Health Coach.

Access Our Full Keto Guide

Subscribe today and get full access to our exclusive Keto guide that will transform your health and weight loss dramatically for FREE.

FREE: Access Our Full Keto Guide for Beginners.

Table of Contents

Recent Posts

Leave a Comment

Your email address will not be published. Required fields are marked *

More To Explore...

Is Intermittent Fasting Good for Diabetics?

Is Intermittent Fasting Good for Diabetics?

Diabetes and Intermittent Fasting What is intermittent fasting? It is an eating plan that concentrates on reducing the eating frequency. It also involves caloric restriction