What Are The Different Stages Of Fasting?


Intermittent fasting is a very old concept that has recently grown in popularity. Breakfast comes from the term “break fast,” which is what occurs in our sleep. While we sleep, our body does not consume food and gets a much-needed break from digestion. This time is intended to focus on cellular repair, rest, and autophagy. Autophagy is the body’s ability to recycle old cells and make newer, improved ones. This process is activated when we fast. Autophagy is beneficial to the body because it helps the body’s cells perform a natural cleaning process. 

A Guide To Intermittent Fasting

Intermittent fasting occurs when we refrain from consuming food for a period of time. We all do some form of intermittent fasting cycles while we sleep. Some choose to increase this time by eating within certain time windows. For example, a person may choose to start their eating at noon and stop eating at eight pm. This would leave them with 16 hours of no eating and 8 hours of eating. People may choose to customize this plan with whatever works best for them. 

There are many different ways to intermittently fast, and you’ll have to choose which method works best for your body and lifestyle. Some people find that abstaining from food for a whole day or two helps them feel great, while others prefer intermittent fasting daily where they consume all of their food during an eight hour period. If you want to achieve real benefits from restricting your food intake, there are certain rules you have to abide by. General Intermittent Fasting Stage Rules:

  1. No Eating During The Fasting Timeline.
  2. You Can Drink Water, Black Coffee, and Tea.
  3. Taking Supplements Is Allowed
  4. Fast for at least 12 hours.
  5. 16/8 is ideal (16 hour fasting period, 8 hour eating period)
  6. Fast for at least 2 days a week
  7. Eat a healthy, nutrient-dense diet. 

What Are The Stages Your Body Goes Through When Fasting?

There are four fasting stages that the body encounters. Anabolic, Catabolic fasting zone, Partial Ketosis, and Active Ketosis. When we think of fasting, we often think of starvation or deprivation. There is a great deal of research around using this process as a tool for cell renewal, weight loss, and immune support. Through these stages of restrictive eating, the body has the ability to heal itself. Stages of  a fast by Day: 

  • Anabolic Stage - The Anabolic stage of fasting is the first four hours after eating. During this digestive state, your body uses all the carbohydrates, sugars, proteins, and fats from your last meal. Once these resources are distributed to fuel your body, the remaining nutrients are stored as glycogen or fat. 
  • Catabolic Stage - The Catabolic phase of fasting occurs four to twelve hours after eating. The process of breakdown diminishes insulin and glucose levels. Glycogen fuels the body with energy. It helps regulate blood sugar in the body. 
  • Partial Ketosis - Partial ketosis takes place twelve to fourteen hours after ingesting food. Glycogen is depleted from the body. The body must utilize another form of fuel. This process of gluconeogenesis creates other avenues to attain glucose. Certain body parts need glucose to function, like the liver and kidneys. During this metabolic process, the body creates glucose from non-carbohydrate sources. It uses what is already in the body. 
  • Active Metabolic State - Gluconeogenesis differs from ketosis which is also happening at the time. In ketosis, the body burns stored fat and converts it to energy. Active ketosis occurs 18-36 hours after beginning a fast. During this time frame, the body detoxifies and uses autophagy to recycle damaged or old cells. Your body is not busy digesting food, so it can focus on other needed operations in the body. Many studies conducted on fasting show multiple health benefits. Evidence shows that fasting can help with age-related issues, skin health, cancers, dementia and Alzheimer’s, memory, and much more.
  • Replenishing Stage - The final stage of this process and debated as one of the most important phases. When reintroducing food after a prolonged fast, it’s important to choose easily digestible food. Your body has not had food for many hours, so your stomach acid and digestive enzymes are low. Introducing a high carbohydrate or sugary food will surely spike your insulin. Your body creates a balance of electrolytes in your system. By introducing carbohydrates too early after a prolonged fast one may cause cell damage. Depending on what you eat, you may experience stomach upset, cramping, or diarrhea. Being mindful of this, you may want to start with a nourishing soup or bone broth. This will help ease your stomach back into digestive mode. Stay away from sodium as well. If your stomach takes to the broth ok you may want to introduce some lean protein like chicken or fish. From there, you could try roasted or steamed non-starchy vegetables. 

At What Fasting Stage Does Your Body Begin To Burn Fat

Your body begins to burn fat during the Partial Ketosis stage, so let’s break down the stages of fasting by hour. At about 12 hours of fasting, your liver begins to break down fat and produce ketones. Fat burning increases from sixteen to 24 hours. Many people who fast regularly report seeing the best results after a sixteen-hour fast. 16 hour fast benefits include improvements in weight loss results, inflammation, and an improvement in overall health. Many people also experience increased mental clarity and focus. 

What Are Some Popular Ways To Intermittently Fast?

There are several popular windows or time frames to eat and fast. One can experiment with different schedules and find what works best for their needs. It’s always important to talk with your doctor before starting an intermittent fasting regimen. There are individuals who should not fast, especially for long periods of time. There are several different ways to set up your fasting phases.

  • 8/16 - Eating for eight hours in a day and fasting for sixteen. This window is by far the most popular. Most people who incorporate this do this daily. You can tweak this window any way you like. Some may restrict for less time and have a 14/10 window where they eat for 10 hours in a day and fast for 14. 
  • 5:2 - This diet calls for people to restrict their calorie intake for two days and eat normally the remaining five. For women, the calorie intake is 500 and for men, it is 600.
  • 24 and 2 - This eating and restricting cycle involves refraining from food for 24 hours, two days a week. Many people on the day they are supposed to begin their fast eat one meal and then fast until the same time the following day. After they have met their 24 hours, they end their fast. It does not feel as long when you are eating one meal a day in that sense. 
  • Alternating - Some people fast every other day. They either choose to restrict their calories on these days or fast completely.
  • Long-term Fasting - Experienced fasters take on the challenge of long-term fasting cycles. This may include restricted calories or going totally without food. The effects are highly debated and should only commence under a doctor’s care. 

What Kind Of Results Can Intermittent Fasting Produce?

There are many benefits to intermittent fasting. The results vary from person to person, but most people experience improved health from doing so. Some people should refrain from fasting zones. Those who are pregnant or breastfeeding, underweight, or children should not partake in this practice. Calorie restriction could become a detriment to their health and well-being. Most people who regularly fast develop a variety of positive results such as:

  1. Accelerated Weight Loss
  2. Higher Metabolism
  3. Improved Blood Sugar Levels
  4. Lower Levels of Inflammation 
  5. Less Brain Fog
  6. Lower Cholesterol
  7. Cancer Protection
  8. Combats Alzheimer's
  9. Slower Aging
  10. Better Sleep

Does Fasting Increase Your Fat Burning Rate?

When your body is fasting, it converts stored fat into energy. So you are burning more fat when you fast. People who implement restricted-calorie fasts lose weight because they are restricting calories. Their body may not be in a high ketosis state, but the calorie restriction alone will help them to lose weight. 

Each person experiences different results. If one fasts and then gorges during the eating window, they may gain weight. If a person is fasting and then breaks their fast with unhealthy food, they may not experience all the health benefits fasting offers. 

What Begins To Happen In Your Body During A 16 Hour Fast?

When you go without eating food for 16 hours, your body usually begins to convert its fat stores into energy. This releases ketones into the bloodstream, which allows your body to use them as fuel. This process is usually initiated by the liver when glycogen stores are depleted. Autophagy begins to occur throughout the body. 

This process may not feel very good at first. When beginning a fast, people often report feeling fatigued, brain fog, and a headache. 16/8 intermittent fasting results take time, and with consistency, people often have positive outcomes.  

What Is The Ideal Amount Of Time For Reaping The Benefits Of Fasting?

Usually, people go into ketosis after the first week of restricted eating. For others, it occurs after about 3-4 days. It really depends on your eating and fasting patterns prior to intentionally fasting. After 12 hours, the body begins to burn fat, and it increases from there. There are many benefits of intermittent fasting. Figuring out which windows work for you will determine the level of benefits you reap. 

What your goals are will also determine how long you should fast. If you’re wanting to reset your immune system, a three day water fast may be the most effective. If your goal is to drop some weight quickly, you may want to water fast for seven days. Adjust your fasting timeline based on your goals, and you’re more likely to meet those goals!

Immune Response To Fasting

The number one benefit of fasting is autophagy, during which the body is recycling damaged and old cells to produce new, healthier cells. Throughout this process, the immune cells that are defective get replaced and pathogens and toxins are released from cells so they can be detoxed from the body. When you fast, your body can take the energy that is usually going to digest food and divert it to cellular cleanup. In addition, when you aren’t taking in food, there are less excess calories and toxins that are being introduced into the body through the food that you eat.

Some of the benefits of fasting on the immune system include the following: 

  • A reduction in the aging of the immune system (immunosenescence)
  • Lowered numbers of white blood cells including neutrophils, lymphocytes, and natural killer cells. 
  • Reduced inflammation and lowered pro-inflammatory markers
  • Increased immune function

How long should you fast for increased immune function? One of the world’s leading authorities on fasting recommends fasting for at least 48-72 hours to reset the immune system, however, no longer than a 72-hour fast. During this time period, genes like the protein kinase A gene can be turned off. This gene is responsible for regulating fat and carbohydrate metabolism in cells, and when it’s turned off it can encourage stem cells to make new immune cells. 

Can Fasting Provide The Same Benefits As Detoxing?

Detox is something that everyone hears about, but may not be familiar with. Our bodies have built-in detoxification systems that help to keep the body healthy by removing toxins that we are exposed to on a daily basis. Our bodies come in contact with many different toxins each day, from the food we eat to the air we breathe. The body’s built-in detox systems include the digestive system that eliminates undigested food, the lungs breathe out air that contains carbon dioxide, and your skin sweats to remove toxins through sweat glands and pores. 

An intentional detox period is a time where you cut back on toxin exposure and help support your body to detox. During fasting, your body’s organs get a rest. If you do a water fast, your body doesn’t have to break down food in the digestive tract. When you fast, it’s important to keep your body properly hydrated to help your body eliminate toxins. 


  • Don Moxley - Director of Applied Science

    Don Moxley is the Director of Applied Science at Longevity Labs. Moxley draws upon his career as an athlete, a sports scientist, and an instructor to lead and educate on the science of autophagy and longevity.