Last update: 7 February 2021

Then Jesus was led up by the Spirit into the wilderness to be tempted by the devil. And he fasted forty days and forty nights, and afterward he was hungry. And the tempter came and said to him, “If you are the Son of God, command these stones to become loaves of bread.” But he answered, “It is written,

‘Man shall not live by bread alone, but by every word that proceeds from the mouth of God.’”
Matthew 4:1-4, RSVCE Bible

Just as it is necessary to eat, it is necessary to sometimes not eat. This page is about fasting, both short-term and long-term.

Short-term (intermittent) fasting

This is what the Church traditionally promotes2 – with one full meal, and (optionally) two small ones besides. Presently, the only two days where this is required are Ash Wednesday and Good Friday, but don’t let that stop you from voluntarily fasting anyway. Especially if your lifestyle means that you’re basically eating one 16-hour meal every day3.

Intermittent fasting is roughly comparable to caloric restriction4 in terms of health benefits associated with weight loss; it’s better than doing nothing8, and if you value your health9, it’s something to try just in case you’re an abstainer than a moderator type of personality.

Long-term (extended, prolonged) fasting

As the quote from Scripture at the top of the page would imply, by extrapolation from following Christ’s example, some people do attempt to do very prolonged fasts for religious reasons. There is at least one such a case in the medical literature5, ending the fasting prematurely due to hypotension and weakness after 36 days, probably due to inadequate sodium intake. This, however, isn’t the longest fast ever recorded. That’s Angus Barbieri’s fast6 of 382 days under medical supervision, and more recently there’s also a case study of a 50-day fast10. Fasting used to be one of the methods for treating obesity7 – but don’t expect not to regain everything, if you go back to your old habits afterwards11.

While the physiology of short-term fasting is largely uninteresting, in prolonged starvation, the metabolism shifts towards a fat-burning, protein-sparing mode12. This is partly similar to a ketogenic diet – gluconeogenesis and ketogenesis are increased, and glycemia lowers. Most tissues, excepting some that cannot, begin to use ketone bodies in place of glucose. Crucially, insulinemia normalizes, which is likely the crucial part in how various diseases are reversed.

Fasting is remarkably safe to try13 if you want to lose weight really fast, so long as you don’t actually want to kill yourself.

Modified fasting

In modified fasting, you are consuming some calories, but much less than even in low calorie diets. It is similarly safe as water fasting and effective for a wide variety of illnesses15, consistent with the idea that the main problem of contemporary populations is energy toxicity. In particular, small amounts of carbohydrate are beneficial to guard against protein loss, given the requirement for gluconeogenesis doesn’t go away in fasting16, and sodium supplementation will help avoid hyponatremia20.

Dry fasting

Dry fasting is the most extreme form of fasting, where you abstain from drinking as well as eating. There have been few published studies on the topic, but it does not seem harmful for up to five days17,18. Weight loss is greater, and animal experiments suggest that the rate of fat catabolism is much increased in such a state19.


  1. Matthew 4, RSVCE Bible.
  2. Catholic Encyclopedia: Fast.
  3. Gill S, Panda S. A Smartphone App Reveals Erratic Diurnal Eating Patterns in Humans that Can Be Modulated for Health Benefits. Cell Metab. 2015;22(5):789–798. doi:10.1016/j.cmet.2015.09.005
  4. Ruth Schübel, Johanna Nattenmüller, Disorn Sookthai, Tobias Nonnenmacher, Mirja E Graf, Lena Riedl, Christopher L Schlett, Oyunbileg von Stackelberg, Theron Johnson, Diana Nabers, Romy Kirsten, Mario Kratz, Hans-Ulrich Kauczor, Cornelia M Ulrich, Rudolf Kaaks, Tilman Kühn, Effects of intermittent and continuous calorie restriction on body weight and metabolism over 50 wk: a randomized controlled trial, The American Journal of Clinical Nutrition, Volume 108, Issue 5, November 2018, Pages 933–945,
  5. Kerndt PR, Naughton JL, Driscoll CE, Loxterkamp DA. Fasting: the history, pathophysiology and complications. West J Med. 1982;137(5):379–399.
  6. Stewart WK, Fleming LW. Features of a successful therapeutic fast of 382 days’ duration. Postgrad Med J. 1973;49(569):203–209. doi:10.1136/pgmj.49.569.203
  7. Drenick EJ, Swendseid ME, Blahd WH, Tuttle SG. Prolonged Starvation as Treatment for Severe Obesity. JAMA. 1964;187(2):100–105. doi:10.1001/jama.1964.03060150024006
  8. JBI Database System Rev Implement Rep. 2018 Feb;16(2):507-547. doi: 10.11124/JBISRIR-2016-003248. Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis. Harris L1, Hamilton S2,3, Azevedo LB2,3, Olajide J2,3, De Brún C2,3, Waller G2,3, Whittaker V2,3, Sharp T4, Lean M1, Hankey C1, Ells L1,3.
  9. N Engl J Med. 2019 Dec 26;381(26):2541-2551. doi: 10.1056/NEJMra1905136. Effects of Intermittent Fasting on Health, Aging, and Disease. de Cabo R1, Mattson MP1.
  10. Elliott B, Mina M, Ferrier C. Complete and Voluntary Starvation of 50 days. Clin Med Insights Case Rep. 2016;9:67–70. Published 2016 Aug 11. doi:10.4137/CCRep.S39776
  11. Arch Intern Med. 1977 Oct;137(10):1381-2. Therapeutic fasting in morbid obesity. Johnson D, Drenick EJ.
  12. Annu Rev Nutr. 2006;26:1-22. Fuel metabolism in starvation. Cahill GF Jr1.
  13. Finnell JS, Saul BC, Goldhamer AC, Myers TR. Is fasting safe? A chart review of adverse events during medically supervised, water-only fasting. BMC Complement Altern Med. 2018;18(1):67. Published 2018 Feb 20. doi:10.1186/s12906-018-2136-6
  14. (stricken)
  15. Wilhelmi de Toledo F, Grundler F, Bergouignan A, Drinda S, Michalsen A. Safety, health improvement and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects. PLoS One. 2019;14(1):e0209353. Published 2019 Jan 2. doi:10.1371/journal.pone.0209353
  16. Owen OE, Smalley KJ, D’Alessio DA, Mozzoli MA, Dawson EK. Protein, fat, and carbohydrate requirements during starvation: anaplerosis and cataplerosis. Am J Clin Nutr. 1998 Jul;68(1):12-34. doi: 10.1093/ajcn/68.1.12. PMID: 9665093.
  17. Papagiannopoulos IA, Sideris VI, Boschmann M, Koutsoni OS, Dotsika EN. Anthropometric, hemodynamic, metabolic, and renal responses during 5 days of food and water deprivation. Forsch Komplementmed. 2013;20(6):427-33. doi: 10.1159/000357718. Epub 2013 Dec 16. PMID: 24434757.
  18. Papagiannopoulos-Vatopaidinos IE, Papagiannopoulou M, Sideris V. Dry Fasting Physiology: Responses to Hypovolemia and Hypertonicity. Complement Med Res. 2020;27(4):242-251. English. doi: 10.1159/000505201. Epub 2020 Jan 20. PMID: 31958788.
  19. Increased fat catabolism sustains water balance during fasting in zebra finches. Joanna Rutkowska, Edyta T. Sadowska, Mariusz Cichoń, Ulf Bauchinger. Journal of Experimental Biology 2016 219: 2623-2628; doi: 10.1242/jeb.138966
  20. Weinsier RL. Fasting–a review with emphasis on the electrolytes. Am J Med. 1971 Feb;50(2):233-40. doi: 10.1016/0002-9343(71)90152-5. PMID: 4925695.