The 5 Stages of Fasting by Hour

Fasting | Stages | #1 | #2 | #3 | #4 | #5 | Conclusion | FAQ | Studies

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. 

Accordingly, low insulin levels prevent (Herman et al. 2017¹⁰; Orgel et al. 2014¹¹; Ferreira et al. 2018¹²; Athauda et al. 2016¹³):

• Obesity
• Metabolic syndrome
• Type 2 diabetes
• Cardiovascular disease
• Cancer
• 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: 

• Cortisol 
• Adrenalin 
• Noradrenalin 
• Glucagon 
• 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. 

Additionally, combining keto and intermittent fasting offers even more health benefits. 

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.

Studies

#1-6

¹Witte AV, Fobker M, Gellner R, Knecht S, Flöel A. Caloric restriction improves memory in elderly humans. Proc Natl Acad Sci U S A. 2009 Jan 27;106(4):1255-60. doi: 10.1073/pnas.0808587106. Epub 2009 Jan 26. PubMed PMID: 19171901; PubMed Central PMCID: PMC2633586. 

²Catterson JH, Khericha M, Dyson MC, Vincent AJ, Callard R, Haveron SM, Rajasingam A, Ahmad M, Partridge L. Short-Term, Intermittent Fasting Induces Long-Lasting Gut Health and TOR-Independent Lifespan Extension. Curr Biol. 2018 Jun 4;28(11):1714-1724.e4. doi: 10.1016/j.cub.2018.04.015. Epub 2018 May 17. PubMed PMID: 29779873; PubMed Central PMCID: PMC5988561. 

³Cheng CW, Adams GB, Perin L, Wei M, Zhou X, Lam BS, Da Sacco S, Mirisola M, Quinn DI, Dorff TB, Kopchick JJ, Longo VD. Prolonged fasting reduces IGF-1/PKA to promote hematopoietic-stem-cell-based regeneration and reverse immunosuppression. Cell Stem Cell. 2014 Jun 5;14(6):810-23. doi: 10.1016/j.stem.2014.04.014. PubMed PMID: 24905167; PubMed Central PMCID: PMC4102383. 

⁴Ho KY, Veldhuis JD, Johnson ML, Furlanetto R, Evans WS, Alberti KG, Thorner MO. Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man. J Clin Invest. 1988 Apr;81(4):968-75. doi: 10.1172/JCI113450. PubMed PMID: 3127426; PubMed Central PMCID: PMC329619. 

⁵Solianik R, Sujeta A, Terentjevienė A, Skurvydas A. Effect of 48 h Fasting on Autonomic Function, Brain Activity, Cognition, and Mood in Amateur Weight Lifters. Biomed Res Int. 2016;2016:1503956. doi: 10.1155/2016/1503956. Epub 2016 Nov 29. PubMed PMID: 28025637; PubMed Central PMCID: PMC5153500. 

⁶Cahill GF Jr. Fuel metabolism in starvation. Annu Rev Nutr. 2006;26:1-22. doi: 10.1146/annurev.nutr.26.061505.111258. Review. PubMed PMID: 16848698. 

#7-14

⁷Cahill GF Jr. Fuel metabolism in starvation. Annu Rev Nutr. 2006;26:1-22. doi: 10.1146/annurev.nutr.26.061505.111258. Review. PubMed PMID: 16848698. 

⁸Meijssen S, Cabezas MC, Ballieux CG, Derksen RJ, Bilecen S, Erkelens DW. Insulin mediated inhibition of hormone sensitive lipase activity in vivo in relation to endogenous catecholamines in healthy subjects. J Clin Endocrinol Metab. 2001 Sep;86(9):4193-7. doi: 10.1210/jcem.86.9.7794. PubMed PMID: 11549649. 

⁹Cahill GF Jr. Fuel metabolism in starvation. Annu Rev Nutr. 2006;26:1-22. doi: 10.1146/annurev.nutr.26.061505.111258. Review. PubMed PMID: 16848698. 

¹⁰Herman ME, O’Keefe JH, Bell DSH, Schwartz SS. Insulin Therapy Increases Cardiovascular Risk in Type 2 Diabetes. Prog Cardiovasc Dis. 2017 Nov – Dec;60(3):422-434. doi: 10.1016/j.pcad.2017.09.001. Epub 2017 Sep 25. Review. PubMed PMID: 28958751. 

¹¹Orgel E, Mittelman SD. The links between insulin resistance, diabetes, and cancer. Curr Diab Rep. 2013 Apr;13(2):213-22. doi: 10.1007/s11892-012-0356-6. Review. PubMed PMID: 23271574; PubMed Central PMCID: PMC3595327. 

¹²Ferreira LSS, Fernandes CS, Vieira MNN, De Felice FG. Insulin Resistance in Alzheimer’s Disease. Front Neurosci. 2018;12:830. doi: 10.3389/fnins.2018.00830. eCollection 2018. Review. PubMed PMID: 30542257; PubMed Central PMCID: PMC6277874. 

¹³Athauda D, Foltynie T. Insulin resistance and Parkinson’s disease: A new target for disease modification?. Prog Neurobiol. 2016 Oct – Nov;145-146:98-120. doi: 10.1016/j.pneurobio.2016.10.001. Epub 2016 Oct 3. Review. PubMed PMID: 27713036. 

¹⁴Quiñones-Galvan A, Ferrannini E. Renal effects of insulin in man. J Nephrol. 1997 Jul-Aug;10(4):188-91. Review. PubMed PMID: 9377725. 

#15-21

¹⁵Zauner C, Schneeweiss B, Kranz A, Madl C, Ratheiser K, Kramer L, Roth E, Schneider B, Lenz K. Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. Am J Clin Nutr. 2000 Jun;71(6):1511-5. doi: 10.1093/ajcn/71.6.1511. PubMed PMID: 10837292. 

¹⁶Cahill GF Jr. Fuel metabolism in starvation. Annu Rev Nutr. 2006;26:1-22. doi: 10.1146/annurev.nutr.26.061505.111258. Review. PubMed PMID: 16848698. 

¹⁷Levine B, Klionsky DJ. Autophagy wins the 2016 Nobel Prize in Physiology or Medicine: Breakthroughs in baker’s yeast fuel advances in biomedical research. Proc Natl Acad Sci U S A. 2017 Jan 10;114(2):201-205. doi: 10.1073/pnas.1619876114. Epub 2016 Dec 30. PubMed PMID: 28039434; PubMed Central PMCID: PMC5240711. 

¹⁸Cahill GF Jr. Fuel metabolism in starvation. Annu Rev Nutr. 2006;26:1-22. doi: 10.1146/annurev.nutr.26.061505.111258. Review. PubMed PMID: 16848698. 

¹⁹Hallböök T, Ji S, Maudsley S, Martin B. The effects of the ketogenic diet on behavior and cognition. Epilepsy Res. 2012 Jul;100(3):304-9. doi: 10.1016/j.eplepsyres.2011.04.017. Epub 2011 Aug 27. Review. PubMed PMID: 21872440; PubMed Central PMCID: PMC4112040. 

²⁰Youm YH, Nguyen KY, Grant RW, Goldberg EL, Bodogai M, Kim D, D’Agostino D, Planavsky N, Lupfer C, Kanneganti TD, Kang S, Horvath TL, Fahmy TM, Crawford PA, Biragyn A, Alnemri E, Dixit VD. The ketone metabolite β-hydroxybutyrate blocks NLRP3 inflammasome-mediated inflammatory disease. Nat Med. 2015 Mar;21(3):263-9. doi: 10.1038/nm.3804. Epub 2015 Feb 16. PubMed PMID: 25686106; PubMed Central PMCID: PMC4352123. 

²¹Cheng CW, Adams GB, Perin L, Wei M, Zhou X, Lam BS, Da Sacco S, Mirisola M, Quinn DI, Dorff TB, Kopchick JJ, Longo VD. Prolonged fasting reduces IGF-1/PKA to promote hematopoietic-stem-cell-based regeneration and reverse immunosuppression. Cell Stem Cell. 2014 Jun 5;14(6):810-23. doi: 10.1016/j.stem.2014.04.014. PubMed PMID: 24905167; PubMed Central PMCID: PMC4102383. 

#22-28

²²Hartman ML, Veldhuis JD, Johnson ML, Lee MM, Alberti KG, Samojlik E, Thorner MO. Augmented growth hormone (GH) secretory burst frequency and amplitude mediate enhanced GH secretion during a two-day fast in normal men. J Clin Endocrinol Metab. 1992 Apr;74(4):757-65. doi: 10.1210/jcem.74.4.1548337. PubMed PMID: 1548337. 

²³Cahill GF Jr. Fuel metabolism in starvation. Annu Rev Nutr. 2006;26:1-22. doi: 10.1146/annurev.nutr.26.061505.111258. Review. PubMed PMID: 16848698. 

²⁴Stewart WK, Fleming LW. Features of a successful therapeutic fast of 382 days’ duration. Postgrad Med J. 1973 Mar;49(569):203-9. doi: 10.1136/pgmj.49.569.203. PubMed PMID: 4803438; PubMed Central PMCID: PMC2495396. 

²⁵Rudman D, Feller AG, Nagraj HS, Gergans GA, Lalitha PY, Goldberg AF, Schlenker RA, Cohn L, Rudman IW, Mattson DE. Effects of human growth hormone in men over 60 years old. N Engl J Med. 1990 Jul 5;323(1):1-6. doi: 10.1056/NEJM199007053230101. PubMed PMID: 2355952. 

²⁶Tavares AB, Micmacher E, Biesek S, Assumpção R, Redorat R, Veloso U, Vaisman M, Farinatti PT, Conceição F. Effects of Growth Hormone Administration on Muscle Strength in Men over 50 Years Old. Int J Endocrinol. 2013;2013:942030. doi: 10.1155/2013/942030. Epub 2013 Dec 8. PubMed PMID: 24382963; PubMed Central PMCID: PMC3870652. 

²⁷Gilpin DA, Barrow RE, Rutan RL, Broemeling L, Herndon DN. Recombinant human growth hormone accelerates wound healing in children with large cutaneous burns. Ann Surg. 1994 Jul;220(1):19-24. doi: 10.1097/00000658-199407000-00004. PubMed PMID: 8024354; PubMed Central PMCID: PMC1234282. 

²⁸Gibson AA, Seimon RV, Lee CM, Ayre J, Franklin J, Markovic TP, Caterson ID, Sainsbury A. Do ketogenic diets really suppress appetite? A systematic review and meta-analysis. Obes Rev. 2015 Jan;16(1):64-76. doi: 10.1111/obr.12230. Epub 2014 Nov 17. Review. PubMed PMID: 25402637.