Willing abstinence from, or reduced consumption of, food and/or drink
A glass of water on an empty plate
Fasting is the act of refraining fromeating, and sometimesdrinking. However, from a purelyphysiological context, "fasting" may refer to themetabolic status of a person who has not eaten overnight (before "breakfast"), or to the metabolic state achieved after completedigestion andabsorption of a meal.[1] Metabolic changes in the fasting state begin after absorption of a meal (typically 3–5 hours after eating).
Alternate-day fasting (alternating between a 24-hour "fast day" when the person eats less than 25% of usual energy needs, followed by a 24-hour non-fasting "feast day" period) has been shown to improvecardiovascular and metabolicbiomarkers similarly to acalorie restriction diet in people who areoverweight, obese or havemetabolic syndrome.[2][3][4][5]
A 2021 review found that moderate alternate-day fasting for two to six months was associated with reductions of body weight,body mass index, and cardiometabolicrisk factors in overweight or obese adults.[6]
Fasting is almost always practiced prior to surgery or other procedures that requiregeneral anesthesia because of the risk ofpulmonary aspiration of gastric contents after induction of anesthesia (i.e., vomiting and inhaling the vomit, causing life-threateningaspiration pneumonia).[7][8][9] Additionally, certain medical tests, such ascholesterol testing (lipid panel) or certainblood glucose measurements require fasting for several hours so that abaseline can be established.
In one review, fasting improvedalertness,mood, and subjective feelings of well-being, possibly improving overall symptoms ofdepression, and boosting cognitive performance.[10]
There is little evidence to suggest that intermittent fasting for periods shorter than 24 hours is effective for sustainedweight loss in obese adults.[11][12]
Prolonged fasting (also called extended fasting or water fasting) involves periods of fasting above 24 hours, typically in the range of 5–20 days.[13] In early fasting, the body operates under a relatively high level ofgluconeogenesis, though this eventually decreases as the body's metabolism switches intoketosis, causingketones to replaceglucose as the primary energy source in thecentral nervous system.[14] As prolonged fasting continues, drastic decreases insodium,potassium,magnesium, and otherminerals are noted, which can lead tohyponatremia.[14] In some diet systems, these losses are offset with non-caloricelectrolyte supplements, such as electrolyte beverages.[15] Typical observed weight loss under prolonged fasting averages at 0.9 kg per day during the first week and 0.3 kg per day by the third week.[14] In early fasting, during periods of high gluconeogenesis, roughly two-thirds of weight lost is lean muscle mass as opposed to fat.[13][16] After the gluconeogenic phase, however, the ratio of body fat lost to lean tissue lost becomes roughly 7:6.[16]
Refeeding syndrome is ametabolic disturbance which occurs as a result of reinstitution of nutrition in people and animals who arestarved, severelymalnourished, or metabolically stressed because of severe illness. When too much food or liquid nutrition supplement is eaten during the initial four to seven days following amalnutrition event, the production ofglycogen,fat andprotein in cells may cause lowserum concentrations ofpotassium,magnesium andphosphate.[18][19] Theelectrolyte imbalance may cause neurologic, pulmonary, cardiac, neuromuscular, and hematologic symptoms—many of which, if severe enough, may result in death.
Refeeding syndrome can occur when someone does not eat for several days at a time usually beginning after 4–5 days with no food.[20]
Fasting can increase the risk of developing gallstones for some people. This is thought to occur due to decreased gallbladder movement with no food to be digested, which can cause the bile to become over-concentrated with cholesterol, combined with the liver secreting extra cholesterol into bile as the body metabolizes fat during rapid weight loss, further exacerbating the situation.[21]
Fasting is often used to make a political statement, toprotest, or to bring awareness to a cause. Ahunger strike is a method ofnon-violent resistance in which participants fast as an act of political protest, or to provoke feelings of guilt, or to achieve a goal such as a policy change. Aspiritual fast incorporates personal spiritual beliefs with the desire to express personal principles, sometimes in the context of social injustice.[22]
The political leaderGandhi undertook several long fasts as political and social protests. Gandhi's fasts had a significant impact on theBritish Raj and theIndian population generally.[23]
In Northern Ireland in 1981, a prisoner,Bobby Sands, was part of the1981 Irish hunger strike, protesting for better rights in prison.[24] Sands had just been elected to the British Parliament and died after 66 days of not eating. 100,000 people attended his funeral, and the strike ended only after nine other men died. In all, ten men survived without food for 46 to 73 days.
The American civil rights activistCésar Chávez undertook several spiritual fasts, including a 25-day fast in 1968 promoting the principle of nonviolence and a fast of 'thanksgiving and hope' to prepare for pre-arranged civil disobedience by farm workers.[22][25] Chávez regarded a spiritual fast as "a personal spiritual transformation".[26] Other progressive campaigns have adopted the tactic.[27]
Fasting is practiced in various religions, and details of fasting practices differ.
Yom Kippur,Tisha B'av,Fast of Esther,Tzom Gedalia, theSeventeenth of Tamuz, theTenth of Tevet, andFast of the Firstborn are examples of fasting inJudaism.[28] Yom Kippur and Tisha B'Av are 25-hour fasts in which observers abstain from consuming any food or liquid from sunset until nightfall the next day and include other restrictions. The fasts of Esther, Gedalia, Tamuz, and Tevet all last from dawn until nightfall and therefore length varies depending on the time of the year. The Fast of the Firstborn is not biblically mandated and can therefore be ended early in the case of aseudat mitzvah.
Lent is a common period of fasting inChristianity. In the Catholic Church, the current practice of fast and abstinence is regulated by Canons 1250–1253 of the 1983 code.[29] They specify that all Fridays throughout the year, and the time of Lent are penitential times throughout the entire Church. All persons who have completed their fourteenth year are bound by the law of abstinence on all Fridays unless they are solemnities, and again on Ash Wednesday and Good Friday. Fasting must be observed by those who have attained their majority, until the beginning of their sixtieth year. The precept to both fast and abstinence must be observed onAsh Wednesday andGood Friday. Certain communities ofLutheran Churches advocate fasting during designated times such as Lent,[30][31] especially on Ash Wednesday and Good Friday.[30][32][33] Therogation days are traditional days of fasting, as well as abstinence of meat, in Lutheranism.[34]A Handbook for the Discipline of Lent delineates the following Lutheran fasting guidelines:[35]
Fast on Ash Wednesday and Good Friday with only one simple meal during the day, usually without meat.
Refrain from eating meat (bloody foods) on all Fridays in Lent, substituting fish for example.
Eliminate a food or food group for the entire season. Especially consider saving rich and fatty foods for Easter.
Consider not eating before receiving Communion in Lent.
Abstain from or limit a favorite activity (television, movies, etc.) for the entire season, and spend more time in prayer, Bible study, and reading devotional material.[35]
In addition to the fasts mentioned above, Catholics must also observe the Eucharistic Fast, which in the Latin Church involves taking nothing but water or medicine into the body for one hour before receiving theEucharist[36]. In Lutheranism, theSmall Catechism, with regard to the Eucharistic Fast, states: "Fasting and bodily preparation are indeed a fine outward training".[37][38] Though voluntary in Lutheranism, the Eucharistic Fast is kept from midnight until the reception of the Eucharist. Theologically, the "Eucharistic Fast is always in anticipation for the Eucharistic Feast, a reminder of the Last Day when all poverty is abolished, the necessity of fasting has ceased and every prayer answered as all of God's people celebrate the marriage supper of the Lamb."[39]
Eastern Orthodox Christians fast during specified fasting seasons of the year, which include not only the better-knownGreat Lent, but also fasts on every Wednesday and Friday (except on special holidays), together with extended fasting periods before Christmas (theNativity Fast), after Easter (theApostles' Fast) and in early August (theDormition Fast).
Members ofthe Church of Jesus Christ of Latter-day Saints generally abstain from food and drink for two consecutive meals in a 24-hour period, on the first Sunday of each month, and members are invited to donate the money they would have used for those meals to assist others in need (called afast offering).[40]
Muslims fast during the month ofRamadan each year. The fast includes refraining from consuming any food or liquid from dawn until sunset. It is a religious obligation for all Muslims unless they are children or are physically unable to fast.
Mahayana traditions that follow theBrahma's Net Sutra may recommend that the laity fast "during the six days of fasting each month and the three months of fasting each year".[41]
Although practitioners ofalternative medicine promote "cleansing the body" through fasting,[42] (as though it were a diagnostic fast) the concept of "detoxification" is a marketing myth with little scientific basis for its rationale or efficacy.[43][44]
Linda Hazzard, put her patients on such strict fasts that some of them died ofstarvation. She was responsible for the death of more than 40 patients under her care.[47][48]
In 1911,Upton Sinclair authoredThe Fasting Cure, which made sensational claims of fasting curing practically all diseases, includingcancer andsyphilis.[49] Sinclair states he recommended fasting for all diseases excepttuberculosis.[50] Sinclair has been described as "the most credulous of faddists. In 1932, physicianMorris Fishbein listed fasting as afad diet and commented that "prolonged fasting is never necessary and invariably does harm".[51]
^Papamichou D, Panagiotakos DB, Itsiopoulos C (June 2019). "Dietary patterns and management of type 2 diabetes: A systematic review of randomised clinical trials".Nutrition, Metabolism, and Cardiovascular Diseases (Systematic review).29 (6):531–543.doi:10.1016/j.numecd.2019.02.004.PMID30952576.S2CID86497236.
^Doig, GS; Simpson, F; Heighes; Bellomo, R; Chesher, D; Caterson, ID; Reade, MC; Harrigan, PWJ (1 December 2015). "Restricted versus continued standard caloric intake during the management of refeeding syndrome in critically ill adults: a randomised, parallel-group, multicentre, single-blind controlled trial".The Lancet Respiratory Medicine.3 (12):943–952.doi:10.1016/S2213-2600(15)00418-X.ISSN2213-2619.PMID26597128.
^abGassmann, Günther; Oldenburg, Mark W. (10 October 2011).Historical Dictionary of Lutheranism. Scarecrow Press. p. 229.ISBN978-0-8108-7482-4.In many Lutheran churches, the Sundays during the Lenten season are called by the first word of their respective Latin Introitus (with the exception of Palm/Passion Sunday): Invocavit, Reminiscere, Oculi, Laetare, and Judica. Many Lutheran church orders of the 16th century retained the observation of the Lenten fast, and Lutherans have observed this season with a serene, earnest attitude. Special days of eucharistic communion were set aside on Maundy Thursday and Good Friday.
^Pfatteicher, Philip H. (1990).Commentary on the Lutheran Book of Worship: Lutheran Liturgy in Its Ecumenical Context. Augsburg Fortress Publishers. pp. 223–244, 260.ISBN978-0-8006-0392-2.The Good Friday fast became the principal fast in the calendar, and even after the Reformation in Germany many Lutherans who observed no other fast scrupulously kept Good Friday with strict fasting.
^Jacobs, Henry Eyster; Haas, John Augustus William (1899).The Lutheran Cyclopedia. Scribner. p. 110.Archived from the original on 9 April 2010. Retrieved14 November 2019.By many Lutherans Good Friday is observed as a strict fast. The lessons on Ash Wednesday emphasize the proper idea of the fast. The Sundays in Lent receive their names from the first words of their Introits in the Latin service, Invocavit, Reminiscere, Oculi, Lcetare, Judica.
^"Fasting Guidelines"(PDF). Grace Evangelical Lutheran Church. p. 3. Archived fromthe original(PDF) on 30 June 2020. Retrieved5 July 2025.
^Porter, Sian (May 2016)."Detox diets"(PDF). British Dietetic Association.Archived(PDF) from the original on 17 October 2016. Retrieved29 January 2019.The whole idea of detox is nonsense. The body is a well-developed system that has its own built-in mechanisms to detoxify and remove waste and toxins. Our body constantly filters out, breaks down and excretes toxins and waste products like alcohol, medications, products of digestion, dead cells, chemicals from pollution and bacteria
^David Gorski (23 May 2011)."Fashionably toxic".Science-Based Medicine.Archived from the original on 30 January 2019. Retrieved29 January 2019.
^abGriffith, R. Marie. (2000).Apostles of Abstinence: Fasting and Masculinity during the Progressive Era.American Quarterly 52 (4): 599-638.
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