Intermittent fasting (intermittent energy restriction or intermittent calorie restriction) is an umbrella term for various diets 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.
Practice and variants
Intermittent fasting protocols can be grouped into two categories: whole-day fasting and time-restricted feeding.
- Whole-day fasting involves regular one-day fasts. The strictest form would be alternate day fasting (ADF). This involves a 24-hour fast followed by a 24-hour non-fasting period. The alternate day modified fasting (ADMF) and 5:2 diets—the latter defined as five days per week not fasting and two days per week either total fasting or modified fasting—both allow the consumption of approximately 500–600 calories on fasting days.
- Time-restricted feeding (TRF) involves eating only during a certain number of hours each day. A common form of TRF involves fasting for 16 hours each day and only eating during the remaining 8 hours, typically on the same schedule each day. A more liberal practice would be 12 hours of fasting and a 12 hour eating window, or a stricter form would be to eat one meal per day, which would involve around 23 hours of fasting per day.[medical citation needed]
Variants include modified fasting, such as ADMF, with limited caloric intake (20% of normal) during fasting periods rather than none at all. Intermittent fasting has a different duration (up to 48 hours) than periodic fasting (more than 48 hours).
Forms of intermittent fasting exist in religious practices in various groups across the world. Religious fasting regimens include, but are not limited to, Ramadan fasting (Islam), Yom Kippur fasting (Judaism), Orthodox Christian fasting, and Buddhist fasting. Certain religious fasting practices, like Buddhist fasting, 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. Islam is the only major religion that 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 28 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 2018, 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, 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 low-quality 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 fasting may cause eating disorders or malnutrition, with increased susceptibility to infectious diseases.
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