Intermittent fasting (IF) is a pattern of eating that alternates between periods of fasting (usually meaning consumption of water and sometimes low-calorie drinks such as black coffee) and non-fasting.
There is evidence suggesting that intermittent fasting may have beneficial effects on the health and longevity of animals—including humans—that are similar to the effects of caloric restriction (CR). There is currently no consensus as to the degree to which this is simply due to fasting or due to an (often) concomitant overall decrease in calories, but recent studies have shown support for the former. Alternate-day calorie restriction may prolong life span. Intermittent fasting and caloric restriction are forms of dietary restriction (DR), which is sometimes referred to as dietary energy restriction (DER).
Scientific study of intermittent fasting in rats (and anecdotally in humans) has been conducted at least as far back as 1943.
A specific form of intermittent fasting is alternate day fasting (ADF), also referred to as every other day fasting (EOD), or every other day feeding (EODF), a 48-hour routine typically composed of a 24-hour fast followed by a 24-hour non-fasting period.
In common usage, intermittent fasting describes any diet that includes a period of fasting and a period of non-fasting, even if the diet involves consuming a limited amount of calorie-containing beverages such as coffee or tea during the fasting period.
Another variation on intermittent fasting is to consume limited calories (e.g., 20% of normal) rather than none at all on fasting days – so-called 'modified fasting'. This regimen may provide many of the benefits of intermittent fasting while being much more acceptable and likely to be adhered to.
Another possibility is eating only one meal per day without caloric restriction. When overall calorie intake is not reduced, this diet worsens some cardiovascular disease risk factors.
The BBC2 Horizon documentary Eat, Fast and Live Longer showed another plan: during days of fasting, people eat 400–500 calories (women) or 500–600 calories (men), and during feed days, the diet was unrestricted. This was done either alternately (one day fasting, one day feeding) or by fasting two days per week: the 5:2 diet.
A 2007 review of alternate day fasting in said "the findings in animals suggest that ADF may effectively modulate several risk factors, thereby preventing chronic disease, and that ADF may modulate disease risk to an extent similar to that of CR. More research is required to establish definitively the consequences of ADF."
Studies on humans suggest possible benefits:
Intermittent fasting may function as a form of nutritional hormesis.
Alternate-day fasting may encourage fat oxidation.
Alternate-day fasting may reduce body weight, LDL, and triglyceride levels to the same degree regardless of maintenance of low fat or high fat diet on the feeding day.
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