Intermittent fasting, also known as intermittent energy restriction, is an umbrella term for various meal timing schedules that cycle between voluntary fasting (or reduced calorie intake) and non-fasting over a given period.
Three methods of intermittent fasting are alternate-day fasting, periodic fasting, and daily time-restricted feeding. Intermittent fasting may be similar to a calorie-restriction diet, and has been studied in the 21st century as a practice to possibly reduce the risk of diet-related diseases, such as metabolic syndrome. However, intermittent fasting has been criticised as a fad.
The science concerning intermittent fasting is contested. The American Heart Association (AHA) states that intermittent fasting may produce weight loss, reduce insulin resistance, and lower the risk of cardiometabolic diseases, although its long-term sustainability is unknown. The US National Institute on Aging states that there is insufficient evidence to recommend intermittent fasting and encourages speaking to one's healthcare provider about the benefits and risks before making any significant changes to one's eating pattern. A 2019 review concluded that intermittent fasting may help with obesity, insulin resistance, dyslipidemia, hypertension, and inflammation.
- 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. It is the strictest form of intermittent fasting because there are more days of fasting per week. There are two subtypes:
- Complete alternate-day fasting (or total intermittent energy restriction), where no calories are consumed on fast days.
- Modified alternate-day fasting (or 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 a very-low-calorie diet.
- Periodic fasting or whole-day fasting involves any period of consecutive fasting of more than 24 hours, such as the 5:2 diet where there are one or two fast days per week, to the more extreme version with several days or weeks of fasting. During the fasting days, consumption of approximately 500 to 700 calories, or about 25% of regular daily caloric intake, may be allowed instead of complete fasting.
- Time-restricted feeding involves eating only during a certain number of hours each day. Skipping a meal and the 16:8 diet (16 fasting hours cycled by 8 non-fasting hours) are examples. This schedule is thought to leverage the circadian rhythm.
The science concerning intermittent fasting is preliminary and uncertain due to an absence of studies on its long term effects. Preliminary evidence indicates that intermittent fasting may be effective for weight loss, may decrease insulin resistance and fasting insulin, and may improve cardiovascular and metabolic health, although the long term sustainability of these effects has not been studied.
The AHA recommends intermittent fasting as an option for weight loss and calorie control as part of an "intentional approach to eating that focuses on the timing and frequency of meals and snacks as the basis of a healthier lifestyle and improved risk factor management". For overweight people, fasting may be integrated into a wider dietary change, such as "placing snacks strategically before meals that might be associated with overeating", planning meals and snacks throughout the day to help manage hunger and control meal portions, and "promote consistent overnight fast periods". The AHA noted that eating some food on a fast day (instead of a complete fast) produced the greatest weight loss and decreases in insulin resistance, when at least 4% weight loss was achieved by obese individuals.
The American Diabetes Association "found limited evidence about the safety and/or effects of intermittent fasting on type 1 diabetes" and preliminary results of weight loss for type 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".
New 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 that healthy food must be chosen on non-fast days.
The NIA stated that although intermittent fasting showed weight loss success in several studies on obese or overweight individuals, it does not recommend intermittent fasting for non-overweight individuals because of uncertainties about its effectiveness and safety, especially for older adults.
According to NHS Choices, people considering the 5:2 diet should first consult a physician, as fasting can sometimes be unsafe. A news item in the Canadian Medical Association Journal expressed concern that promotional material for the diet showed people eating high-calorie food, such as hamburgers and chips, and that this could encourage binge eating since the implication was that "if you fast two days a week, you can devour as much junk as your gullet can swallow during the remaining five days".
Intermittent fasting (specifically the 5:2 diet of Michelle Harvie and Mark Mattson and popularized by Michael Mosley) became popular in the UK in 2012 after the BBC2 television Horizon documentary Eat, Fast and Live Longer. Via sales of best-selling books, it became widely practiced.
In the United States, intermittent fasting became a fad in Silicon Valley. It was the most popular diet in 2018 according to a survey by the International Food Information Council. The idea of binge eating following dietary restrictions stems from an extrapolation of a post-war famine study, where nineteen prisoners of war were observed during their rehabilitation with unlimited access to food, and found they tended to indulge in binge eating.
As of 2019[update], interest in intermittent fasting led some companies to commercialize diet coaching, dietary supplements and full meal packages. These companies were criticized for offering expensive products or services that were not backed by science.
There is limited evidence that intermittent fasting produces weight loss comparable to a calorie restricted diet. Most studies on intermittent fasting in humans have observed weight loss, ranging from 2.5% to 9.9%. Alternate day fasting does not affect lean body mass, although one review found a small decrease. Alternate day fasting improves cardiovascular and metabolic biomarkers similarly to a calorie restriction diet in people who are overweight, obese or have a metabolic syndrome.
Intermittent fasting has not yet been studied in children, elderly, or underweight people, and can be harmful in these populations. Intermittent fasting is not recommended for people who are not overweight. The long-term sustainability of intermittent fasting is unknown as of 2018[update].
Night-time eating is linked to impaired sleep quality. Intermittent fasting is not recommended to treat cancer in France, the United Kingdom, or the United States, although a few small-scale clinical studies suggest that it may reduce chemotherapy side effects. Periodic fasting may have a minor effect on chronic pain and mood disorders. In preliminary research, intermittent fasting has shown signs of reducing risk factors for certain disorders, including insulin resistance and cardiovascular disease. Intermittent fasting does not affect bone health.
Reviews of preliminary clinical studies found that short-term intermittent fasting may produce minor adverse effects, such as continuous feeling of hunger, irritability, and impaired thinking, although these effects disappear within a month from the fasting practice. However, the data remains sparse, as most of the studies did not analyze adverse effects specifically. A 2018 systematic review found no major adverse effect. Intermittent fasting is not recommended for pregnant or breastfeeding women, or growing children and adolescents, or individuals vulnerable to eating disorders.
Tolerance of a diet is a determinant of the potential effectiveness and maintenance of benefits obtained, such as weight loss or biomarker improvement. A 2019 review found that drop-out rates varied widely from 2% to 38% for intermittent fasting, and from 0% to 50% for calorie restriction diet.
Preliminary research indicates that fasting may induce a transition through four states: 1) the fed state or absorptive state during satiety, when the primary fuel source is glucose and body fat storage is active, lasting for about 4 hours; 2) the postabsorptive state, lasting for up to 18 hours, when glucagon is secreted and the body uses liver glucose reserves as a fuel source; 3) the fasted state, transitioning progressively to other reserves, such as fat, lactic acid, and alanine, as fuel sources, when the liver glucose reserves are depleted, occurring after 12 to 36 hours of continued fast; 4) the shift from preferential lipid synthesis and fat storage, to the mobilization of fat (in the form of free fatty acids), metabolized into fatty acid-derived ketones to provide energy. Some authors call this transition the "metabolic switch". A 2019 review of weight-change interventions, including alternate day fasting, time-restricted feeding, exercise and overeating, found that body weight homeostasis could not precisely correct "energetic errors" – the loss or gain of calories – in the short-term.
Other feeding schemes, such as hypocaloric feeding and intermittent feeding, also called bolus feeding were under study. 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. In adults, reviews have not found intermittent feeding to increase glucose variability or gastrointestinal intolerance. A meta-analysis found intermittent feeding had no influence on gastric residual volumes and aspiration, pneumonia, mortality nor morbidity in people with a trauma, but increased the risk of diarrhea.
Intermittent fasting, or "skip-a-day" feeding, is supposedly the most common feeding strategy for poultry in broiler breeder farms worldwide, as an alternative to adding bulky 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 as foraging.
Therapeutic intermittent fasts for the treatment of obesity are 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". Intermittent fasts, or "short-term starvation periods", were ranging from 1 to 14 days in these early studies. This enthusiasm penetrated lay magazines, which prompted researchers and clinicians to caution about the use of intermittent fasts without medical monitoring.
Intermittent fasting exists in some religious practices. These include the Black Fast of Christianity (commonly practiced during Lent), Vrata (Hinduism), Ramadan (Islam), Yom Kippur and other fasts (Judaism), Fast Sunday (The Church of Jesus Christ of Latter-day Saints), and Buddhist fasting. Religious fasting practices may only require abstinence from certain foods or, last for a short period of time and cause negligible effects.
In Christianity, many adherents of Christian denominations including Catholics, Lutherans, Methodists, Anglicans and the Orthodox, often observe the Friday Fast throughout the year, which commonly includes abstinence from meat. Throughout the liturgical season of Lent (and especially on Ash Wednesday and Good Friday) in the Christian kalendar, many Christians practice a form of intermittent fasting in which one can consume two collations and one full meal; others partake of the Black Fast, in which no food is consumed until sundown.
In Buddhism, fasting is undertaken as part of the monastic training of Theravada Buddhist monks, who fast daily from noon to sunrise of the next day. This daily fasting pattern may be undertaken by laypeople following the eight precepts.
During Ramadan, 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. A meta-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. The analysis concluded that "Ramadan provides an opportunity to lose weight, but structured and consistent lifestyle modifications are necessary to achieve lasting weight loss." One review found similarities between Ramadan and time-restricted feeding, with the main dissimilarity being the disallowance of water drinking with Islamic fasting. Negative effects of Ramadan fasting include increased risk of hypoglycemia in diabetics, as well as inadequate levels of certain nutrients. Ramadan disallows fluids during the fasting period. This is hazardous for pregnant women, as it is associated with risks of inducing labor and causing gestational diabetes, although it does not appear to affect the child's weight.
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