Intermittent Fasting Phases You Must Know

Intermittent Fasting Phases

What are the phases of intermittent fasting? With so much information floating around the internet, intermittent fasting can seem a bit more complex than it is.

For this reason, I want to explain how your body reacts at each stage of intermittent fasting.

The “fasting” state sends different signals to the body. In each of the phases of intermittent fasting, the signals are different.

Knowing this, you will be able to take full advantage of the benefits of fasting to improve your health and achieve your goals and in this post I explain everything to you.

The purpose of this post is to give you a simplified guide to the phases of intermittent fasting for hours so that you better understand how it works. Do not give you medical or professional advice.


When you abstain from eating by fasting for 12 , 14, 16 , or even 24 hours, your body goes through different phases that send out different signals and impact your metabolism and hormone levels.

You may already know that the female body is a bit more complex, usually due to the hormonal cycles that we naturally experience each month. For this reason, understanding the phases of intermittent fasting will help you achieve your health goals.


In the first phase of intermittent fasting, your body focuses on digesting the last meal you ate and absorbing all the nutrients from the food.

At this point, your body “doesn’t know” about intermittent fasting. You are doing what you usually do after eating.

During this first phase, this happens in your body:

  • You soak up the nutrients from your last meal, so during intermittent fasting it’s important to eat nutritious and healthy foods.
  • It increases blood sugar levels and releases insulin(depending on the type of food you ate).
  • Extra glucose(sugar) is stored in the liver and muscles as glycogen (stored carbohydrates).
  • Your body stores this glycogen as a future source of energy.
  • Hormones change, including ghrelin, the appetite-stimulating hormone, which increases after eating; and leptin, the hormone that reduces appetite (these two hormones work together).


Once the body finishes digesting, about 3-4 hours after eating, it goes into an early fasting phase, which lasts until 16-18 hours after the last meal.

During this second phase, this happens in your body:

  • Blood sugar and insulin levels begin to drop.
  • Begins to convert stored glycogeninto glucose (sugar) to use as an energy source .
  • Once it’s used up glycogen stores, it looks for alternative sources of “fuel fuel”to function, such as stored fat cells.
  • Your body also turns amino acids, the building blocks of protein, into energy.

During this phase, your body converts the glycogen, amino acids, and fatty acids it receives from the food you eat into energy to provide the “fuel” it needs to function.

Of course, the better the food you give your body for fuel, the more efficient this process will be .

This is one of the most important phases for those who want to experience the benefits of intermittent fasting without making major lifestyle changes.

This phase is when benefits such as cell repair, increased basal metabolic state, hormonal regulation, and much more are obtained.

If you are doing intermittent fasting and at times you feel a lack of energy, in this post I explain what you can eat and drink during intermittent fasting .

Sounds complicated to you? Don’t worry, getting started fasting can be easy with this guide.


Glycogenesis? Complicated word but easy to explain: In this phase, your body switches from using sugar for energy to using fat for fuel.

Once the body realizes that “no more food is on the way,” it fully enters a fasting state that can last anywhere from 18 hours to 2 days.

During this third phase, this happens in your body:

  • It begins to produce ketone bodies, a chemical the body produces when it converts fat into energy.
  • Your body begins to enter ketosis (ketosis), a state in which it uses fat for fuel.

The latter is what many people who follow a ketogenic diet (keto diet) are looking for: that their body enters a state of “ketosis” and transforms fat into fuel.


Approximately 48 hours into the fast, your body begins to go into a state of prolonged fasting.

During this phase, insulin levels drop, ketone levels rise, and protein breakdown is reduced, to preserve muscle mass.

Prolonged fasting without medical supervision is not recommended.

Especially if you are a woman, prolonged fasting can disrupt natural hormonal processes and metabolic states, causing slow metabolism, altered thyroid function, mood swings and sleep cycles, etc.

Abbas Jahangir

I am a researcher and writer with a background in food and nutritional science. I am the founder of, our reputable online platform offering scientifically-backed articles on health, food, nutrition, kitchen tips, recipes, diet, and fitness. With a commitment to providing accurate and reliable information, we strive to empower our readers to make informed decisions about their health and lifestyle choices. Join us on's journey toward a healthier and happier lifestyle.

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