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Home arrow Nutrition arrow Intermittent Fasting arrow Stages of Fasting Explained and the Health Benefits They Bring

Stages of Fasting Explained and the Health Benefits They Bring

Written by Edibel Quintero, RD
Fact checked by Rosmy Barrios, MD
Last update: October 21, 2023
6 min read 1483 Views 0 Comments
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Ever wondered what goes on in your body while fasting intermittently? In this article, you can learn all you need to know about the 5 stages of fasting and how your body adapts to these metabolic changes to bring weight loss results and other health benefits.

Stages of fasting

Weight loss, cell renewal, and disease prevention are among the purported benefits of intermittent fasting. People try these unique eating patterns for various health reasons. There are several variations, but each method cycles between periods of eating and food abstinence.

You might be fully in tune with your intermittent fasting regimen, but do you know what’s happening inside your body during the fasting period?

Intermittent fasting triggers multiple changes that prepare your metabolism for entering the fat-burning mode. A lot is going on that many of us don’t even realize. So, it’s interesting to learn about the science that delivers the many health benefits associated with the eating style.

Keep reading to discover the 5 primary stages of intermittent fasting.

5 Stages of Fasting and Their Benefits

During prolonged periods without food, different processes occur in the human body. It is these changes that trigger fat burning so you can start losing weight effectively. The benefits throughout the stages of intermittent fasting vary depending on the length of the fasting window.

Below, we look at the 5 stages of intermittent fasting in detail.

#1 Ketosis

Ketosis is a metabolic state that is characterized by higher levels of ketone bodies in the bloodstream. It occurs when glycogen stores are depleted, which pushes the body to burn fat for fuel instead of glucose. As the body breaks down fat, the liver converts fatty acids into ketones in a process called ketogenesis.

When glucose levels diminish, most organs can use ketone bodies as an alternative energy source. The brain utilizes ketone bodies for energy as ordinarily, the brain cells rely on blood glucose to function. Improved cognitive function is one of many intermittent fasting benefits.

For most people, the process occurs after fasting for around 24 hours. Although, some people can enter ketosis in as little as 12 hours. Early signs of ketosis include bad breath, known as keto breath, reduced hunger, greater mental clarity, fatigue, muscle cramps, and increased thirst.

When ketosis occurs, the fat-burning stage of fasting begins. Many people follow a ketogenic diet to get into ketosis because the very low-carb diet forces the body to use up stored glucose. Practicing keto and intermittent fasting simultaneously can accelerate ketosis and weight loss.

As well as helping you burn through stubborn fat stores, ketosis can suppress your appetite and boost your energy levels. This stage is essential for those who want to lose weight by fasting.

#2 Autophagy

After fasting for around 24–48 hours, the body begins a process called autophagy. Autophagy is a natural clean-up operation where your body recycles old, damaged cells and turns them into new, usable cells. It is a critical recovery process that triggers immune system regeneration and enables your cells to survive and thrive.

IF induces autophagy by triggering a stress response through food deprivation. It deprives the body of important nutrients and forces the cells into survival mode. The term literally translates to “self-eating.”

Autophagy declutters cell parts that are taking up space and slowing cellular performance. It can reduce inflammation, improve brain health, and kill cancer cells. Autophagy declines naturally as we age, and inducing it through fasting may slow aging and improve longevity.

#3 Growth hormone

The human growth hormone (HGH) is a natural hormone responsible for growth in children and adolescents. In adulthood, it has several roles, including maintaining the tissues and organs, and regulating body composition and fat distribution for a healthy body structure.

After around 72 hours, growth hormone levels begin to change. Intermittent fasting increases growth hormone secretion, which can help preserve lean muscle mass and speed up muscle repair. Meanwhile, fasting decreases levels of IGF-1, or insulin-like growth factor – another hormone involved in growth.

#4 Insulin reduction

Insulin is a peptide hormone produced in the pancreas. It allows glucose from carbohydrates to enter cells and provide energy, and is responsible for managing and lowering blood sugar levels. Regularly high insulin levels can cause insulin resistance and lead you to gain weight.

Fasting gives the body time to decrease insulin levels, and after around 16 hours of fasting, your insulin levels drop significantly. Lower insulin levels can improve diabetes and other factors linked to metabolic syndrome, such as high cholesterol and high blood pressure.

Intermittent fasting improves insulin resistance and insulin sensitivity in people with blood sugar control issues.

#5 Immune cell rejuvenation

Prolonged fasting for up to 72 hours can trigger immune cell rejuvenation.

In this stage, the process of cell regeneration occurs, and the body begins breaking down and restoring damaged and old immune cells. This triggers the regeneration of immune cells for a healthy immune system. This stage in the fasting period also promotes stem cell production.

When Does Fasting Start?

After eating, your body remains in the fed state for a few hours with stable blood sugar. When the food you’ve eaten is fully digested, your body begins moving from the fed state to the early fasting state. Generally, this happens approximately 3–4 hours after your last meal.

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During this time, your insulin and blood glucose begin to drop. You will notice some common symptoms in this early stage, such as hunger pangs and lower energy levels. Eventually, your glycogen stores will deplete, and you will transition into a complete fasting state.

Most people will remain in the early fasted state for up to 18 hours.

When Does the Body Start Burning Fat During Fasting?

During an intermittent fast, the body starts burning stored fat once your stored glycogen is gone. It no longer has access to its usual energy source and must urgently find an alternative fuel to provide you with energy. This is when the metabolic process called lipolysis gets to work.

Lipolysis is the breakdown of triacylglycerols in fat cells to produce glycerol and fatty acids – molecules that provide an essential energy source during your fast. When you reach lipolysis, you are burning body fat. It can begin 12 hours after eating, but it may take up to 18.

The fat-burning stage usually accelerates when you’ve been fasting for 18–24 hours.

What Do the 5 Fasting Stages Do to Your Body?

The 5 stages of fasting include various internal processes that prepare your body for fat burning. The journey allows your body to digest the calories from your last meal so that your insulin and blood glucose levels drop and your body transitions into an alternative state.

Fasting also gives your body a chance to clear out old and damaged cells through autophagy and trigger immune cell rejuvenation to keep your system healthy. Without fasting long enough to reach these stages, you will not be able to acquire all of the intermittent fasting benefits.

A Word From a Nutritionist

When you abstain from eating food for prolonged periods, your body starts a series of metabolic and physiological changes. 5 significant stages happen throughout the fasting journey, which make the dietary pattern beneficial for human health.

Fasting is praised for helping people lose weight. It forces you to burn through your glycogen stores before the body burns stored fat as a substitute energy source. It can also improve insulin sensitivity, blood glucose, inflammation, weight control, and lower blood pressure.

While fasting triggers all of these advantages, you should still focus on eating a healthy diet outside of the fasting period. You can consider reducing your overall calorie intake, but you must ensure you eat enough calories to maintain good health and minimize unpleasant side effects.

A balanced diet consists of whole foods rich in essential vitamins and minerals. Aim to eat a variety of foods from different sources, including healthy fats, lean proteins, and complex carbohydrates. Fiber is another essential nutrient for losing weight and strengthening the gut.

Your stomach is more sensitive after an intermittent fast. So, you should choose sensible foods to break a fast, such as soups, broths, and green smoothies. Rushing to fill up on high-carb foods and sugary products can upset your stomach and cause sharp blood sugar spikes.


Now you’re aware of the 5 major stages of fasting, you can better understand what your body is going through and how it influences your metabolism and hormone levels. These stages also provide clarity on the anti-aging, weight loss, and heart health effects of intermittent fasting.

You can get the most significant health benefits from longer fasts, but you should not attempt extended fasting without medical supervision. Prolonged fasting can bring adverse side effects, such as dehydration and an electrolyte imbalance. You must drink water throughout your fast.

Written by Edibel Quintero, RD
Edibel Quintero is a medical doctor who graduated in 2013 from the University of Zulia and has been working in her profession since then. She specializes in obesity and nutrition, physical rehabilitation, sports massage and post-operative rehabilitation. Edibel’s goal is to help people live healthier lives by educating them about food, exercise, mental wellness and other lifestyle choices that can improve their quality of life.
The article was fact checked by Rosmy Barrios, MD
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Written by Edibel Quintero, RD
Fact checked by Rosmy Barrios, MD
Last update: October 21, 2023
6 min read 1483 Views 0 Comments

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