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Intermittent fasting (IF), or intermittent calorie restriction, is an umbrella term for various diets that cycle between a period of fasting and non-fasting during a defined period. Intermittent fasting may produce weight loss comparable to long-term calorie restriction.

Practice and variants
Intermittent fasting can be a viable strategy to reduce caloric intake, body weight, body fat mass, and improve insulin sensitivity. 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 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.

Recommendations vary on what can be consumed during the fasting periods. Some would say only water, others would allow tea or coffee (without milk or sugar) or zero-calorie drinks with artificial sweeteners. Fasting may increase dehydration. Variants include "modified fasting", such as alternate day modified fast (ADMF), with limited caloric intake (e.g., 20% of normal) during fasting periods rather than none at all, in order to improve tolerance and mood. Intermittent fasting has a different duration (up to 48 hours) than periodic fasting (2 or more days).

Intermittent fasting is different from starvation diets, which are not recommended as a clinical treatment since they were linked to various medical complications, including sudden death syndrome during fasting or re-feeding.

Research
Intermittent fasting, as alternate day fasting, was effective for weight loss, on a scale similar to calorie restriction. Weight loss was observed in both obese and normal weight people. Improvements in several cardiovascular and metabolic biomarkers (such as body fat, LDL cholesterol, triglycerides, insulin sensitivity, blood pressure) were recorded, proportionally to the amount of weight loss. Alternate day fasting reduced little or no lean body mass. Contrary to calorie restriction, weight loss was more reliably maintained with alternate day fasting on the medium to long term. A 2014 review described that intermittent fasting has not been studied in children, the elderly, or the underweight, and could be harmful in these populations. The impact of intermittent fasting on brain functions is not yet well determined.

Intermittent fasting is associated with increased hunger and decreased feelings of fullness. For this reason, alternate day modified fasting diet (ADMF), with a reduced calorie intake during fasting days (instead of none), is investigated. This variant might be better tolerated and maintained, in particular by increasing the feelings of fullness. Although ADMF appears effective for weight loss, this scheme showed mixed effects on biomarkers.

There are several different variants of intermittent fasting diets with varying results. Several studies found that fasting may improve biomarkers and weight loss from 8-12 hours under the fast, with a peak in body fat loss and decrease in glucose loss at 18-24 hours. Clinically supervised studies detected lean body mass loss at 60 hours of fasting, but not at 20-36 hours. Resting metabolism (i.e., more energy consumed at rest) increased at 36-48 hours, but data is lacking for shorter fast duration. Time restricted diets provided lower long-term weight loss compared to other variants. Religious fasts are also being investigated.

Intermittent fasting may influence and be influenced by sleep (reduced REM sleep), circadian rythm and the gut microbiome.

Popular culture
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.

According to NHS Choices as of 2012, people considering the 5:2 diet should first consult a physician, as fasting can sometimes be unsafe. In the UK, the tabloid press reported on research claiming the 5:2 diet could reduce the risk of breast cancer, improve brain and immune functions, or extend lifespan, but there is inadequate evidence for such statements. 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".