This article is about intentional fasting. For involuntary fasting, seeStarvation. For fasting before a clinical or surgical intervention, seePreoperative fasting.
Intermittent fasting is any of various meal timing schedules that cycle between voluntaryfasting (or reduced calorie intake) and non-fasting over a given period.[1][2] Methods of intermittent fasting includealternate-day fasting,[3]periodic fasting, such as the5:2 diet, and dailytime-restricted eating.[1][4]
Intermittent fasting has been studied to find whether it can reduce the risk of diet-related diseases, such asmetabolic syndrome.[1][5][6][7] It may improve several health outcomes inoverweight orobese adults; its effect on loweringblood pressure appears to be less pronounced than that ofcontinuous energy restriction.[8] There is preliminary evidence that intermittent fasting is generally safe.[9]
Adverse effects of intermittent fasting have not been comprehensively studied, leading some academics to point out its risk as adietary fad.[10] The United StatesNational Institute on Aging states that current research is limited and inconclusive; they do not recommend intermittent fasting to the general public.[11] TheNew Zealand Ministry of Health states that intermittent fasting may aid weight loss but is not recommended for people withinsulin-dependent diabetes and can cause low energy and hunger.[12]
Fasting is an ancient tradition, having been practiced by many cultures and religions overcenturies.[10][14][15]
Therapeutic intermittent fasts for the treatment of obesity have been investigated since at least 1915, with a renewed interest in the medical community in the 1960s after Bloom and his colleagues published an "enthusiastic report".[16] Intermittent fasts, or "short-term starvation periods", ranged from 1 to 14 days in these early studies.[17] This enthusiasm penetrated lay magazines, which prompted researchers and clinicians to caution about the use of intermittent fasts without medical monitoring.[18]
Intermittent fasting calendar for 1 week, alternating feasting days (green), in which the person eats a normal diet, with fasting days (red), in which the person performs intermittent fasting
There are multiple methods of intermittent fasting.[1][2][4][19][20]
Time-restricted eating involves eating only during a certain number of hours each day, often establishing a consistent daily pattern of caloric intake within an 8–12-hour time window. This schedule may align food intake withcircadian rhythms (establishing eating windows that begin after sunrise and end around sunset).[21][22]
One-meal-a-day fasting is having just one meal a day, and not having anything for the rest of the day.
Alternate-day fasting involves 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.[4][9][20] There are two subtypes:[10][23]
Complete alternate-day fasting is total intermittent energy restriction (IER vs. CER = continuous energy restriction), where no calories are consumed on fasting days.
Modified alternate-day fasting (mADF) involves partial intermittent energy restriction which allows the consumption of up to 25% of daily calorie needs on fasting days instead of complete fasting. This is akin to alternating days with normal eating and days with avery-low-calorie diet.[24]
5:2 diet is a type of periodic fasting (that does not follow a particular food pattern) which focuses entirely on calorie content.[1] In other words, two days of the week are devoted to consumption of approximately 500 to 600 calories, or about 25% of regular daily caloric intake, with normal calorie intake during the other five days of the week. It was first documented in a 2011 article co-authored byMichelle Harvie,Mark Mattson, and 14 additional scientists.[25][26] It was later published in the UK andAustralia byMichael Mosley through the 2012BBC documentaryEat, Fast and Live Longer (where he learned about the 5:2 diet fromMark Mattson).[26][27] It also became common in Australia.[28][29][30]
Periodic fasting or whole-day fasting involves intermittent periods of water fasting longer than 24 hours.[1][23][31]
The science concerning intermittent fasting is preliminary and uncertain due to an absence of studies on its long-term effects.[1][11][32][33] Preliminary evidence indicates that intermittent fasting may be effective for weight loss, may decreaseinsulin resistance and fasting insulin, and may improvecardiovascular andmetabolic health, although the long term sustainability of these effects has not been studied.[1]
There is limited evidence that intermittent fasting producesweight loss comparable to acalorie-restricted diet.[5][6][34][35] Most studies on intermittent fasting in humans have observed weight loss, ranging from 2.5% to 9.9%.[36][37]
The reductions in body weight can be attributed to the loss of fat mass and some lean mass.[38][39] For time restricted eating the ratio of weight loss is 4:1 for fat mass to lean mass, respectively.[9][20] Alternate-day fasting does not affectlean body mass,[4][40] although one review found a small decrease.[41]
Intermittent fasting has not been studied in children, elderly, or underweight people, and may be harmful in these populations.[5][7][11][44] Intermittent fasting is not recommended for people who are not overweight, and the long-term sustainability of intermittent fasting is unknown as of 2018[update].[11][45]
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.[46]
A 2025 review found no consistent effect of short-term fasting (12 hours) on cognitive performance across various tasks, includingattention,working memory,inhibitory control,cognitive flexibility, and long-term memory.[47] The review concluded that cognitive function remains stable during short-term fasting, indicating that skipping breakfast or time-restricted eating do not impair thinking skills or concentration in adults.[47]
Athletic performance does not benefit from intermittent fasting.[57] Overnight fasting before exercise increaseslipolysis, but reduces performance in prolonged exercise (more than 60 min).[58][59]
There is preliminary evidence that intermittent fasting appears safe for people withoutdiabetes oreating disorders.[9]
Reviews of preliminary clinical studies found that short-term intermittent fasting may produce minor side effects, such as continuous feelings of hunger, irritability, dizziness, nausea, headaches, and impaired thinking, although these effects disappear within a month from beginning the fasting practice.[1][60] A 2018systematic review found no major adverse effects.[5] Intermittent fasting is not recommended forpregnant orbreastfeeding women, growing children and adolescents, the elderly, or individuals with or vulnerable toeating disorders.[19]
Tolerance of a diet is a determinant of the potential effectiveness and maintenance of benefits obtained, such asweight loss or biomarker improvement.[35] A 2019 review found that drop-out rates varied widely from 2% to 38% for intermittent fasting, and from 0% to 50% forcalorie restriction diet.[5]
Preliminary research indicates that fasting may induce a transition through four states:
The fed state orabsorptive state duringsatiety, when the primary fuel source isglucose and body fat storage is active, lasting for about 4 hours;[61][62]
A 2019 review of weight-change interventions, including alternate day fasting, time-restricted feeding, exercise and overeating, found thatbody weight homeostasis could not precisely correct "energetic errors" – the loss or gain of calories – in the short-term.[69]
Another pathway for effects of meal timing on metabolism lies in the influence of thecircadian rhythm over theendocrine system, especially onglucose metabolism andleptin.[70] Preliminary studies found that eating whenmelatonin is secreted – during darkness and commonly when sleeping at night – is associated with increased glucose levels in young healthy adults, and obesity and cardiovascular disorders in less healthy individuals.[71] Reviews on obesity prevention concluded that "meal timing appears as a new potential target in weight control strategies" and suggest that "timing and content of food intake, physical activity, and sleep may be modulated to counteract" circadian and metabolic genetic predispositions to obesity.[72][73]
Other feeding schemes, such ashypocaloric feeding[74] and intermittent feeding, also called bolus feeding were under study.[75] A 2019 meta-analysis found that intermittent feeding may be more beneficial for premature infants, although better designed studies are required to devise clinical practices.[76] In adults, reviews have not found intermittent feeding to increase glucose variability or gastrointestinal intolerance.[77][78] A meta-analysis found intermittent feeding had no influence ongastric residual volumes and aspiration, pneumonia, mortality nor morbidity in people with a trauma, but increased the risk of diarrhea.[75][78]
Intermittent fasting, or "skip-a-day" feeding, is supposedly the most common feeding strategy forpoultry inbroiler breeder farms worldwide, as an alternative to addingbulky fibers to the diet to reduce growth. It is perceived as welfare-reducing and thus illegal in several European countries including Sweden. Intermittent fasting in poultry appears to increase food consumption but reduce appetitive behaviors such asforaging.[79]
AVrata/Nombu is observed either as an independent private ritual at a date of one's choice, as part of a particular ceremony such as wedding, or as a part of a major festival such asDiwali (Lakshmi, festival of lights),Shivaratri (Shiva),Navratri (Durga or Rama), Kandasashti (Muruga),Ekadashi (Krishna, Vishnuavatars).[80][81][82]
In Christianity, many adherents ofChristian denominations including Catholics, Lutherans, Methodists, Anglicans, and the Orthodox, often observe theFriday Fast throughout the year, which commonly includes abstinence from meat.[83] Throughout the liturgical season of Lent (and especially onAsh Wednesday andGood Friday) in theChristian calendar, many Christians practice a form of intermittent fasting in which one can consume twocollations and one full meal; others practice theBlack Fast, in which no food or water is consumed until after sunset with prayer.[84][85]
InBuddhism, fasting is undertaken as part of the monastic training ofTheravada Buddhist monks, who fast daily from noon to sunrise of the next day.[86] This daily fasting pattern may be undertaken by laypeople following theeight precepts.[86][87]
DuringRamadan, Islamic practices are similar to intermittent fasting by not eating or drinking from dawn until sunset, while permitting food intake in the morning before dawn and in the evening after dusk for 30 days.[88][89] Ameta-analysis on the health of Muslims during Ramadan shows significant weight loss during the fasting period of up to 1.51 kilograms (3.3 lb), but this weight was regained within about two weeks thereafter.[90] The analysis concluded that "Ramadan provides an opportunity to lose weight, but structured and consistent lifestyle modifications are necessary to achieve lasting weight loss."[90] One review found similarities between Ramadan and time-restricted feeding, with the main dissimilarity being the disallowance of water drinking with Islamic fasting.[91] In a 2020 review, Ramadan fasting caused a significant decrease inLDL cholesterol levels, and a slight decline intotal cholesterol.[89]
Ramadan disallows fluids during the fasting period. This type of fasting would be hazardous for pregnant women, as it is associated with risks ofinducing labor and causinggestational diabetes, although it does not appear to affect the child's weight.[92][93] For these reasons, pregnant women, as well as children who have not reached puberty, the elderly, those who are physically or mentally incapable of fasting, travelers, and breast-feeding mothers are often exempt from religious fasting – Ramadan being one example.[88] Ramadan diurnal intermittent fasting is associated with healthier lifestyle behaviors and a reduction in smoking rate by more than 50% among university students.[94]
TheAmerican Heart Association (AHA) says that as with other "popular or fad diets", there is no good evidence of heart health benefits from intermittent fasting.[95]
TheAmerican Diabetes Association "found limited evidence about the safety and/or effects of intermittent fasting ontype 1 diabetes" and preliminary results of weight loss fortype 2 diabetes, and so does not recommend any specific dietary pattern for the management of diabetes until more research is done, recommending instead that "health care providers should focus on the key factors that are common among the patterns".[32]
TheNational Institute on Aging states current human research is limited, inconclusive, and not enough to recommend these diets for the general public.[11]
Given the lack of advantage and the increased incidence of diarrhea, European guidelines do not recommend intermittent feeding for people in intensive care units.[75][78]
TheNew Zealand's Ministry of Health considers that intermittent fasting can be advised by doctors to some people, except diabetics, stating that these "diets can be as effective as other energy-restricted diets, and some people may find them easier to stick to" but there are possible side effects during fasting days such as "hunger, low energy levels, light-headedness and poor mental functioning" and note thathealthy food must be chosen on non-fasting days.[33][12]
As of 2019[update], interest in intermittent fasting led some companies to commercialize diet coaching,dietary supplements, and full meal packages.[104][107] These companies were criticized for offering expensive products or services that were not backed by science.[107][108]
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