Intermittent fasting (intermittent energy restriction or intermittent calorie restriction) is an umbrella term for various eating diet plans that cycle between a period of fasting and non-fasting over a defined period. Intermittent fasting is under preliminary research to assess if it can produce weight loss comparable to long-term calorie restriction.
Types and variants
Three methods of intermittent fasting are alternate-day fasting, whole-day fasting, and time-restricted feeding.
- Alternate-day fasting (ADF) is the strictest form of IF besides religious fasting. This involves 24-hours complete fasting followed by a 24-hour non-fasting period.
- An adjusted form of ADF is alternate-day modified fasting (ADMF) which allows the consumption of approximately 25% of daily calorie needs on fasting days instead of full fasting.
- Whole-day fasting involves regular one or two fasting days per week. A 5:2 diet requires five non-fasting days and 2 fasting days in a week. During the fasting days, it allows approximately 500 to 600 calories or about 25% of regular daily caloric intake.
- Time-restricted feeding involves eating only during a certain number of hours each day. An example can be 16:8 diets which advocate 16 fasting hours cycled by 8 non-fasting hours.
As of 2019, there is only limited evidence of long-term effectiveness of these fasting methods, preventing conclusions about their relative efficacy for obese people or normal-weight people trying to lose some weight.
Forms of intermittent fasting exist in religious practices in various groups across the world. Religious fasting regimens include, but are not limited to, Vrata in Hinduism, Ramadan fasting (Islam), Yom Kippur fasting (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 the body.
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 fasting is also undertaken by laypeople who undertake the eight precepts, optional rules laypeople can take to get an impression of what it is like to live as a monastic. Taiwanese physician Ming-Jun Hung and his co-authors have analyzed early and medieval Chinese Buddhist Texts and argue that the main purposes of the half-day fast is to lessen desire, improve fitness and strength, and decrease sleepiness.
Islam engages in a fasting practice reflective of intermittent fasting in terms of both food consumption and diet consistency. The duration of the Ramadan fast is between 29 and 30 days, depending on the year, and consists of not eating or drinking from sunrise until sunset. During the holiday, Muslims eat twice per day: once in the morning before dawn and once 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." Negative effects of Ramadan fasting include increased risk of hypoglycemia in diabetics as well as inadequate levels of certain nutrients.
Intermittent fasting (specifically the 5:2 diet) 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 has become a trend among Silicon Valley companies. According to NHS Choices as of 2012, 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".
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.
A 2018 review of intermittent fasting in obese people showed that reducing calorie intake one to six days per week over at least 12 weeks was effective for reducing body weight on an average of 7 kilograms (15 lb); the results were not different from a simple calorie restricted diet, and the clinical trials reviewed were run mostly on middle-aged women from the US and the UK, limiting interpretation of the results. Intermittent fasting has not been studied in children, the elderly, or underweight people, and could be harmful in these populations.
Intermittent fasting is under preliminary research for how it may affect cardiovascular and metabolic biomarkers, such as body fat, LDL cholesterol, triglycerides, insulin sensitivity, and blood pressure. Alternate day fasting does not appear to affect lean body mass.
Understanding the potential adverse effects of intermittent fasting is limited by an inadequate number of rigorous clinical trials. One 2015 review 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. Long-term, periodic fasting may cause eating disorders or malnutrition, with increased susceptibility to infectious diseases.
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