Prolonged and intermittent fasting are natural methods that can dramatically improve health. Although people react differently to fasting, a general sequence of stages initiates different metabolic processes.
For this reason, this article uses five stages to explain how fasting affects an average person depending on their hourly progress. This way, you can adapt your fasting plan to your individual goals.
Fasting is more than a method for losing weight. Its health benefits can balance the hormonal system, reduce inflammation and detoxify the body.
For example, fasting improves:
- Mental clarity and cognition (Witte et al. 20091)
- Life expectancy (Catterson et al. 20182)
- Immune function (Cheng et al. 20143)
- Muscle growth (Ho et al. 19884)
- Mood (Solianik et al. 20165)
However, not all methods offer the same benefits. Depending on the duration of the fasting period, various processes are triggered in the body, which can produce different effects.
Therefore, Dr. George Cahill divided fasting into five general stages.
He was a professor of medicine at Harvard Medical School, where he conducted essential studies. He published his collected findings in the article “Fuel Metabolism in Starvation.”
What Are the 5 Stages of Fasting?
The five stages of fasting are feeding, postabsorptive, gluconeogenesis, ketosis, and protein conservation stage.
Dr. George Cahill conceptualized his fasting stages according to which energy source the body predominantly uses at any given time (Cahill 20066).
He distinguishes between two primary fuel sources:
- Glucose: a carbohydrate and sugar
- Triglycerides: stored body fat
While most cells can use both energy sources, the brain is an exception because it cannot use triglycerides.
Other cells, such as those in the liver, kidneys, muscles, or heart, can use both energy sources.
While the body stores fat as triglycerides, carbohydrates are stored as glycogen.
This molecule consists of glucose chains and water. Glycogen is stored primarily in the liver and partially in skeletal muscle.
When the body needs blood sugar, it splits the molecule into smaller glucose molecules and relieves them into the bloodstream.
The body must draw on fat reserves when glycogen is no longer available. For this purpose, triglycerides from the fat cells enter the liver, where they are broken down.
In addition to fatty acid chains, glycerol is produced, which your body can convert into glucose for the brain.
1. Feeding (0-4 Hours)
The early fasting stage begins immediately after a meal and lasts until about 4 hours afterward. For this reason, Dr. Cahill has referred to the phase as feeding (Cahill 20067).
Much glucose is available after an average meal of carbohydrates, proteins, and fats.
When blood sugar levels are high, all cells use glucose as their primary energy source.
Insulin is one of the essential nutrient sensors in the body and our primary storage hormone.
Consequently, it prevents fat breakdown by enzymes (lipolysis) and initiates the storage mode in the body (Meijssen et al. 20018).
When you eat carbohydrates or proteins, you trigger the nutrient sensor, and insulin rises. This way, insulin signals the body to store some of the supplied energy for later.
After all, it needs energy even when you are not eating. For this reason, the body stores excess glucose as glycogen in the liver or as triglycerides in fat cells.
2. Postabsorptive Phase (4-16 Hours)
According to Cahill, the second fasting stage is the post-absorptive phase.
The body digests the ingested food energy during these four to 16 hours after eating. However, during this process, insulin levels begin to fall again.
Although most cells still use glucose for fuel, it now comes from glycogen stores rather than directly from food (Cahill 20069).
Because the liver breaks glycogen down into glucose, it can be transported throughout the body via the bloodstream.
The capacity of these carbohydrate stores depends on stature and body size. Therefore, they cover almost precisely the individual daily requirement of each person.
The glycogen stores will not be full if you eat a low-carbohydrate diet. Consequently, the body can fall back on stored fat even earlier.
Like fasting, therefore, ketogenic diets aim to lower insulin levels.
The post-absorptive fasting stage is essential since high insulin levels are a significant risk factor for chronic diseases.
- Metabolic syndrome
- Type 2 diabetes
- Cardiovascular disease
- Alzheimer’s disease
- Parkinson’s disease
Moreover, high insulin levels cause the body to retain salt and water (Quiñones-Galvan et al. 199714).
Therefore, fasting for at least 16 hours also makes sense during intermittent fasting.
While insulin decreases, so-called counter-regulatory hormones rise, which counteract the storage hormone.
These include in particular:
- Human growth hormone (HGH)
Thus, these hormones signal the body to mobilize energy from glycogen and fat stores.
While high growth hormone levels ensure the body can build and repair cells, the other hormones ensure an increased basal metabolic rate (Zauner et al. 200015).
3. Gluconeogenesis (16-30 Hours)
After we are in the third stage of fasting between sugar and fat metabolism, Cahill called it gluconeogenesis (Cahill 200616).
Gluconeogenesis is a process by which the liver can make new glucose from protein.
Since the body is not yet in full ketosis 16 to 30 hours after eating, glucose is still needed. After glycogen is nearly depleted, the liver needs to make more glucose.
However, the depleted protein does not come from muscle mass because autophagy kicks in during this intermittent fasting stage.
When food is absent, the body activates this intracellular recycling system, which has been awarded the Nobel Prize in Medicine (Levine et al. 201717).
Autophagy is the maintenance service of our cells induced by fasting.
In this process, defective proteins in the cells are degraded and recycled. Thus, fasting brings the following health benefits through autophagy:
- Recycles defective proteins and organelles
- Prevents atypical protein accumulation
- Removes pathogens
The body can build necessary cell parts from the amino acids obtained, resulting in a rejuvenating effect.
Because autophagy breaks down cell parts that are no longer necessary, weight loss through fasting leaves no troublesome excess skin.
In addition to fat, overweight people carry a lot of excess protein.
Therefore, it’s no surprise that protein accumulations characterize modern diseases, such as atherosclerosis, cancer, or polycystic ovary syndrome.
Fasting, on the other hand, helps to break down excess fat and protein. While doing so, it is regulated by the three essential nutrient sensors:
- mTOR: Sensitive to sensitively to proteins
- Insulin: Sensitive to carbohydrates and proteins
- AMPK: Sensitive to energy in cells
Besides proteins and carbohydrates, even fat inhibits autophagy due to AMPK. For this reason, only water, mineral water, and natural salt are allowed during strict rejuvenation fasts.
4. Ketosis (2-7 Days)
The fourth stage of fasting, according to Dr. Cahill, is ketosis. So now we slowly leave the spheres of intermittent fasting and move on to more prolonged fasting.
Once the body is entirely in ketosis, the brain is fed predominantly by fat energy sources (Cahill 200618).
Although the brain uses glucose as its go-to energy source, there is not enough left in the body two days after fasting.
Consequently, the liver must draw on triglycerides from body fat to produce ketones. Unlike conventional fatty acids, ketone bodies can cross the blood-brain barrier (Hallböök et al. 201419).
While only ¼ of the energy for the brain now comes from gluconeogenesis, the remaining ¾ is provided by ketones.
For this reason, many people also report feeling more energetic and clear-headed in ketosis.
Moreover, some research suggests that the brain may metabolize ketones more efficiently, leading to better performance, concentration, and memory.
In addition, ketones unleash anti-inflammatory properties in the body (Youm et al. 201520).
If you follow a ketogenic diet, you’ll have a head start on intermittent fasting, be able to enter the stage of perfect ketosis earlier and reap its benefits, as we’ll see shortly.
In addition, according to studies, starting from three-day fasting, the body undergoes an almost complete immune system rejuvenation. Furthermore, since fasting stimulates stem cell production, it can lead to new immune cell formation (Cheng et al. 201421).
Moreover, just two days of fasting can quintuple growth hormone release, which leads us to the next and final stage (Hartman et al. 199222).
5. Protein Conservation (>7 Days)
The fifth stage of fasting is the protein conservation phase. After about a week of fasting, the body meets its energy needs almost exclusively from fat reserves.
While minimal amounts of protein are needed for gluconeogenesis, the brain has also switched predominantly to fat energy consumption (Cahill 200623).
In this stage of fasting, hunger has genuinely disappeared. Consequently, ghrelin, the hunger hormone, is also at a low.
For this reason, the world record for fasting is 382 days, as the body now feeds only on its body fat (Stewart et al. 197324).
Contrary to popular belief, muscles do not atrophy during long-term fasts.
In the phase of protein conservation, the release of growth hormones is already enormously high. As a result, it protects bone and lean muscle mass from degeneration during fasting (Rudman et al. 199025).
Hence, this fasting stage lives up to its name.
The human growth hormone stimulates muscle growth and repair (Tavares et al. 201326).
For this reason, intermittent fasting is no stranger to bodybuilders. Occasionally not eating long can help build muscle instead of breaking it down.
Fasting, for example, can also speed up the healing process of wounds and more severe injuries (Gilpin et al. 199427).
Since growth hormones are so effective, their use in competitions is even considered doping.
How to Accelerate Fasting Stages with Keto
Based on these five stages of fasting, you can decide how long the intermittent should last for your health goals.
However, you can combine intermittent fasting with the keto diet for faster and more efficient results.
Like fasting, the keto diet aims to deplete glycogen stores and tap into body fat for energy.
You’ll have a clear starting advantage if you’re already in ketosis at the beginning of the fast. This way, you don’t have to consume glycogen for 1-2 days before burning fat efficiently.
Furthermore, a keto diet helps suppress hunger and protects against side effects such as headaches (Gibson et al. 201528).
Finally, the body is not deprived of sugar for the first time during this process, making intermittent fasting much more convenient.
Suppose you’re fasting for an extended period. In that case, breaking the fast with a meal that returns the proper nutrients to the body while not overwhelming the gastrointestinal tract is crucial.
Frequently Asked Questions
What are the stages of fasting?
You can break down fasting into feeding, postabsorptive, gluconeogenesis, ketosis, and protein conservation stage.
What is the fat burning stage of fasting?
Improved fat burning starts at the third fasting stage, gluconeogenesis, 16 hours after eating. Fat burning will improve at the fourth stage, full ketosis.
What happens when fasting?
When fasting, the storage hormone insulin drops, and the body starts depleting carbohydrate stores and burns fat for energy afterward.
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Stephan is a writer and a true man of science, holding multiple diplomas and master's degrees in different research areas. His greatest passion is closing the gap between the conventional perception of health and the latest scientific evidence – always following the data.