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Stages of intermittent fasting

Rachel Lett, ANutr

Head of Coaching


The quick read

  • Each stage of intermittent fasting triggers a cascade of events. These events have a unique and profound effect on health
  • 6-24 hours, insulin falls and glucagon rises. The body begins to use up stored glucose, glycogen.
  • >24 hours, the body makes new glucose from non-carbohydrate precursors. HGH rises and autophagy begins.
  • >48 hours, stored fat is metabolised for fuel. Ketosis begins
  • >72 hours, autophagy stimulates immune system rejuvenation.

What happens to your body when you begin intermittent fasting?

When we stop eating, insulin levels drop and glucagon levels rise. This hormonal shift will help restore insulin sensitivity, and thus improve PCOS, type 2 diabetes and prediabetes.

However, the story of fasting doesn't end there. A drop in insulin triggers a cascade of events that have a profound effect on health.

This article discusses the main physiological changes that underpin associated health benefits of intermittent fasting.

6-24 hours of fasting

During the post absorptive stage, food has been metabolised and glucose levels fall. As a result, insulin secretion tapers off and glucagon levels rise. Working in tandem, these hormones signal the body to metabolise energy from stored glucose (glycogen) in muscle and liver tissue. We store approximately 2,000 calories worth of glycogen, that will maintain blood glucose within a normal range for approximately 24 hours.

At the same time, low insulin and high glucagon stimulates the kidneys to excrete water and minerals (like potassium, magnesium and sodium). This process facilitates the metabolism of glycogen, as every gram of glycogen is packaged in 3 grams of water. During the post absorptive state, you will lose electrolytes, water weight and experience frequent urination. It is important to hydrate and replace electrolytes during this time — take a look at this article, where we discuss electrolytes in more detail.

After 6 hours of fasting, the digestive system begins to slow down. This 'break' allows the gut to rest and restore. Fasting favourably alters the gut microbiome population and strengthens the gut barrier lining.

>24 hours fasting

After about 24 hours of fasting, your glycogen stores will be fully drained and can no longer support a normal blood glucose level. Alternatively, the body synthesis glucose from from non-carbohydrate precursors, like amino acids and glycerol. This process is called gluconeogenesis, which literally translates as "making new glucose".

Many people worry that gluconeogenisis, and thus fasting will cause muscle loss. This is not true. It would be nonsensical for the body to use precious muscle tissue when we have an abundance of stored energy at our disposal — glycogen and fat. The body works hard to replenish energy stores in the fed state to prepare for such occasions of low food availability.

A rise in human growth hormone (HGH), preserves lean body mass. Instead, amino acids are sourced from a process called autophagy, which simultaneously begins 24 hours into fasting.

Autophagy is a natural, regeneration process for old, worn out cells. You could liken it to a self cleaning and recycling operation. The body degrades damaged, junky cells and reuses the cellular components to generate new cells or glucose — reduce, reuse and recycle at its finest!

Autophagy is the foundation of intermittent fasting and the reason it has gained a lot of attention for anti-aging, and preventing diseases like Parkinsons, Alzheimers and Cancer.

>48 Hours

Gluconeogenesis alone can’t produce enough energy to support our powerful bodies and brains; so finally, the body transitions into ketosis — fat burning mode! 🔥Ketosis is commonly referred to as 'fat adaption', whereby the body uses fat, rather than glucose, as its main energy source.

Stored fat is metabolised into glycerol and fatty acids. Glycerol is shuttled off for gluconeogenesis, which keeps our blood glucose stable. At the same time, fatty acids are used to power most tissues, except for the brain — the brain can only use glucose and ketone bodies.

The brain has a high energy demand, and glucose is in short supply, so fatty acids are further metabolised into ketone bodies.

Ketone bodies are an efficient, low inflammation fuel source that promote BDNF (brain-derived neurotrophic factor) production. BDNF helps brain cells grow and repair, and protects them against cellular stress — hello mental clarity!

‍>72 Hours

72 hours into a fast, insulin levels are at their lowest, and the body is fully fat adapted — fatty acids and ketones are the main energy source. HGH levels are high to preserve lean body mass.

By day three, autophagy is in full swing and begins to rejuvenate the immune system — old, defective immune cells are degraded, which triggers the synthesis of new immune system cells.

As you can see, fasting is a lifestyle that just keeps on giving! Understanding more about the process on a physiological level will help you overcome barriers so that you can reach your fasting goal — feeling increased thirst at the 16 hour mark, you know you're using up your glycogen stores and you'll need to replace electrolytes!

Rejuvenating the immune system sounds pretty appealing, but please don't feel disheartened if 72 hours fasting isn't manageable right now. The more you practice, the easier it will be to complete longer fasts.

Remember that with every hour of fasting, something new and positive is happening on a cellular level!

Like what you’re hearing?

Stay tuned for more guides, to help you with your health journey.

Take care, 👋

I’m sure you’re curious about the science behind all this 🔬 Here are some recent medical publications about this topic:









September 8, 2021
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