Intermittent fasting, also known as intermittent energy restriction, is an umbrella term for various meal timing schedules that cycle between voluntary fasting (or reduced calories intake) and non-fasting over a given period. Non-caloric, and sometimes low-caloric, drinks can be used during intermittent fasting, contrary to strict fasting which disallows fluid intake in some religious practices.
Three methods of intermittent fasting are alternate-day fasting, periodic fasting, and time-restricted feeding. Intermittent fasting may be similar to a calorie restriction diet. Although being studied in the 21st century as a practice to possibly reduce the risk of diet-related diseases, intermittent fasting is also regarded as a fad.
The science concerning intermittent fasting is preliminary and inconclusive. 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 (NIA) recommends against intermittent fasting because of uncertainties about its effectiveness and safety, particularly for the elderly.
Diverse forms of intermittent fasting exist in various religious practices, including Vrata in Hinduism, Ramadan fasting (Islam), Yom Kippur fasting and other Jewish fasts (Judaism), Orthodox Christian fasting, Fast Sunday (The Church of Jesus Christ of Latter-day Saints), and Buddhist fasting.
- Alternate-day fasting involves alternating between a 24-hours "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 24h, such as the 5:2 diet where there are 1 or 2 fast days per week, to the more extreme version with several days or weeks of fasting. During the fasting days, it may be allowed approximately 500 to 600 calories or about 25% of regular daily caloric intake 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. There is preliminary evidence 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.
Although intermittent fasting showed weight loss success in several studies on obese or overweight individuals, the NIA does not recommend intermittent fasting for non-overweight individuals because of uncertainties about its effectiveness and safety, especially for older adults.
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 among Silicon Valley companies. It was the most popular diet in 2018 according to a survey by the International Food Information Council. 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". However, 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 re-habilitation with unlimited access to food, and found they tended to indulge in binge eating.
As of 2019, interest in intermittent fasting led some companies to commercialize diet coaching, dietary supplements, and full meal packages. These companies have been criticized for offering products or services that are expensive and not backed by science.
There is some limited evidence that intermittent fasting produces weight loss comparable to a calorie restricted diet. Most studies on intermittent fasting in humans observed weight losses, 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 for people who are overweight, obese or have a metabolic syndrome.
Intermittent fasting has not yet been studied in children, the elderly, or underweight people, and could 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.
Night-time eating has been linked to impaired sleep quality. There is no evidence that fasting has any beneficial effect (prevention, treatment, drugs interaction) for cancer and is thus not recommended in France, the United Kingdom, or the United States, although a few small-scale clinical studies suggest that intermittent fasting may reduce chemotherapy side effects. Periodic fasting may have minor effect on chronic pain and mood disorders. 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 feelings of weakness and hunger, headaches, fainting, or dehydration. However, the data remain sparse, as most studies did not analyze adverse effects specifically. A 2018 systematic review found no major adverse effect. Periodic fasting prolonged for several days or weeks may cause eating disorders, malnutrition, increased susceptibility to infectious diseases, moderate damage to organs or sudden death by cardiac failure, either during fasting or upon refeeding. Deaths also happened following therapeutic prolonged periodic fasting. These effects happened after several weeks of fasting, intermittent fasting should not produce these adverse effects, and has not been reported to do so in clinical trials as of 2018[update], but it may still cause harm when practiced too frequently or for too many consecutive days. Intermittent fasting is not recommended for pregnant or breastfeeding women, or children and adolescents during maturation, or individuals vulnerable to eating disorders.
Tolerance of a diet is a determinant of its 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 a calorie restriction diet.
Fasting makes the body transition through four states: from the fed state or absorptive state or postprandial state, when the gastrointestinal tract is full and the primary fuel source is glucose and body fat storage is activated and lasting about 4h, to the early fasted state or postabsorptive state, happening a few hours after eating and lasting up to 12-18h, when glucagon is secreted stopping fat storage and the body instead uses liver glucose reserves as a fuel source to the fasted state transitioning progressively to other reserves such as fat, lactate and alanine as a fuel source, and when the liver glucose reserves are depleted after usually 12 to 36h of continued fast, the body enters the starvation state where only the other reserves are used as fuel, which represents a shift from preferential lipid synthesis and fat storage to the mobilization of fat in the form of free fatty acids then derived into fatty acid-derived ketones to provide energy to organs. This transition to free fatty acids as the fuel source manifests as a reduction in the respiratory exchange ratio (ratio of CO2 exhaled to O2 inhaled). Sirt 1, the mammal variant from the sirtuin family of signaling proteins with a highly conserved structure throughout all kingdoms of life, is involved in the regulation of this transition and mediation of gluconeogenesis. Some authors call this transition a "metabolic switch". Whether repeatedly triggering this transition between fed, fasted and starvation states, as does intermittent fasting, may produce meaningful health benefits regardless of weight loss and caloric restriction has not yet been determined. A review of a wide variety of diets, including alternate day fasting, show they all produce similar weight loss and cardiometabolic changes, with adherence and negative energy balance being better predictors than the type of diet. IGF-1 level decreases have been observed in a systematic review during either intermittent fasting or more than 50% continuous calorie restriction. Another systematic review did not find evidence of a reduction of adaptive responses, except maybe appetite, when using intermittent fasting, although most studies were under-powered to assess this.
These mechanisms are known from studies on starvation that have been conducted for over 100 years in the 20th century, which allowed to derive a substantial amount of research examining varying durations of food deprivation on humans. However, the majority of short-term starvation studies extend through at least the fasted state, with periods of approximately 2 to 4 days of fast, which is longer than most intermittent fasting patterns.
Intermittent fasting may affect several metabolic pathways in humans, such as the cardiovascular system by effecting blood pressure; and metabolic profiles by reducing lipid, glucose, insulin levels, IGF-1 decreasing advanced glycation end-products levels, enhancing autophagy, increasing adiponectin levels, and increasing Sirt 1 levels.
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 for "energetic errors" – the loss or gain of calories – in the short-term.
As of 2019, current clinical guidelines recommend that hospitals ensure that the patients in critical health units get fed with 80–100 % of energy expenditure, the normocaloric feeding. There are investigations in other feeding schemes, such as hypocaloric feeding and intermittent feeding, also called bolus feeding. 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. Given the lack of advantage and the increased incidence of diarrhea, European guidelines do not recommend intermittent feeding.
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. Intermittant 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 propagated into lay magazines, which prompted researchers and clinicians to caution about the use of intermittent fasts without a medical monitoring.
Various forms of intermittent fasting exist in religious practices across the world. Religious fasting regimens include Vrata in Hinduism, Islamic fasting (Ramadan) (Islam), Yom Kippur fasting and other Jewish fasts (Judaism), Orthodox Christian fasting, Fast Sunday (The Church of Jesus Christ of Latter-day Saints), and Buddhist fasting. Certain religious fasting practices only require abstinence from certain foods, while others, like the Jewish fast on Yom Kippur, last for a short period of time and would cause negligible effects on body weight.
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 sunrise 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 of Ramadan ending. 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 fasting disallowing fluids during the fasting period is hazardous for pregnant women, as it is associated with risks of inducing labour and causing gestational diabetes, although it does not appear to affect the child's weight.
- 2010s in food
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Many small points were clarified regarding adipose tissue, such as the release of free glycerol with the fatty acids mobilized during fasting. [...] Therapeutic fasting of obese subjects was in vogue in the 1950s and 1960s. In an informal conversation with Rachmiel Levine, who knew of our fasting work, it became evident that the fuel substrate for brain could not continue to be glucose since gluconeogenesis from protein would consume so much muscle that longterm viability would be dramatically decreased. [...] Hepatic glycogen contribution to blood glucose is essentially zero by the second or third day of starvation. [See also Figure 1]
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Modern work stems from the enthusiastic report of Bloom on the use of starvation therapy, which stimulated many other workers to examine this type of treatment and the subject has been extensively reviewed. The idea is not new and dates back to at least 1915.:125
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