Stages of Intermittent Fasting

Stages of Intermittent Fasting

You’ve probably heard of intermittent fasting, but do you know what the stages of intermittent fasting are?

While it may appear overly-simplistic at first glance, fasting causes significant physiological changes within the body, each with its unique positives and negatives.

Today, we’ll go over the four stages of fasting, what characterizes each, and how they impact us.

Let’s dive in.

The Four Stages of Intermittent Fasting (and Prolonged)

Fed State (Postprandial State)

The fed state comes after we finish a meal and can last for up to several hours. The duration of the postprandial state depends on the size and composition of the meal. For example, protein and fats tend to digest more slowly, so having more of them will prolong the fed state.

During this period, your blood sugar levels are higher, and your body releases insulin to shuttle the glucose to your body’s various tissues. How much insulin you release depends on your sensitivity to the hormone, how big the meal was, and its composition.

Excess glucose gets stored in the muscles (as glycogen). The remaining serves to maintain normal blood sugar levels and provide fuel for the brain.

Meanwhile, protein and fats get broken down into amino acids, peptides, and fatty acids. These are then sent to various parts of the body for energy, muscle repair, growth, and more.

Initiation of the Fasted State

Depending on your meal’s size and composition, this phase will typically begin to set in within three to five hours of eating. This stage lasts for up to 18 hours after your last meal and is characterized by gradually declining insulin and blood sugar levels.

Near the end of this stage, your body will run out of liver glycogen and will start looking for another significant source of energy.

At that time, lipolysis (the breakdown of fat) begins to set in, and fatty acids become a major energy source for the body. At the same time, the body will start to use amino acids for energy.

What Are Fasting Stages

Fasted State

The fasted state typically lasts from 18 to 48 hours after having your last meal. At that point, insulin and blood sugar levels are at baseline, and your body is rapidly exhausting any remaining glycogen you have.

At that point, your body is also breaking fat and lean tissue to get amino acids and other nutrients to get enough energy for the many processes it has to carry out.

Interestingly, at this point, your body also begins to convert fatty acids into ketone bodies in the liver. This is when you transition into a state of ketosis, where fats and ketones become the primary fuel source for your body. (1)

Through a process called gluconeogenesis, your body also converts some proteins and fats into glucose, mostly to fuel the brain.

Starvation (Longer Term Fasting)

After about two days of not eating anything, your body enters what is known as starvation (or long-term fast). On a deep physiological level, little separates a fasted state from starvation. In fact, one could argue that the two conditions are practically identical.

Levels of ketone bodies keep rising, while insulin and blood sugar levels remain at baseline. Thanks to gluconeogenesis, the body can get some glucose from fats and proteins to sustain the brain. (2)

Depending on the person, two to three days of not eating anything should be enough to enter a ketosis state. Initially, folks tend to feel tired, irritable, and unable to focus, but the effects tend to pass after a while. This state is also known as keto flu.

At this point, the body becomes primed for burning fat and lean tissue as it needs to get the energy it needs to stay alive.

Summary of Stages of Intermittent Fasting

Intermittent fasting can be beneficial. It helps us control our food intake, makes our days easier, and research finds that it can help us live longer.

But, it’s essential to keep things moderate. While fasting can be useful for us, not eating for days is not exactly healthy. It can work at times if you are trying to drop significant weight for a big event, like a wedding or reunion, but it’s not a long-term strategy.

In any case, always check with your physician before starting a new diet. Explain your goals, what you hope to get out of a regimen, and heed their advice.

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David Williams

A diet and fitness enthusiast, David is an ex-Army Airborne Ranger and Infantry soldier with decades of fitness and wellness experience. A West Point graduate with a degree in engineering, he focuses on technical research related to fitness, nutrition, and wellness. He loves the beach and working out, and spending time with his wife and daughters.

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  1. MediLexicon International. (n.d.). Ketosis: Symptoms, diet, and more. Medical News Today.
  2. Gluconeogenesis. Gluconeogenesis – an overview | ScienceDirect Topics. (n.d.).

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