Intermittent fasting (IF) describes diets that cycle between a period of fasting and non-fasting. In some contexts, fasting allows the consumption of a limited amount of low-calorie beverages such as coffee or tea.
Intermittent fasting and caloric restriction are forms of dietary restriction (DR), which is sometimes referred to as dietary energy restriction (DER). 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). Some studies suggest that benefits could be the result of an overall reduction in caloric intake.
Scientific study of intermittent fasting in rats (and anecdotally in humans) has been conducted at least as far back as 1943.
One form of intermittent fasting, alternate day fasting (ADF), involves a 24-hour fast followed by a 24-hour non-fasting period. This is sometimes referred to as every other day fasting or every other day feeding. Alternate-day calorie restriction may prolong life span.[unreliable source?]
Modified fasting involves limiting caloric intake(e.g., 20% of normal) on fasting days rather than none at all. A study suggests that this regimen may retain most of the benefits of intermittent fasting.[unreliable source?]
Another form involves eating only one meal per day. In cases that do not restrain consumption, overall calorie intake may increase which worsens some cardiovascular disease risk factors.
More generally, forms may choose to specify various ratios of fasting to non-fasting periods. The BBC2 Horizon documentary Eat, Fast and Live Longer  covered people who committed to fasting two non-consecutive days per week. Known as the 5:2 diet, people consumed 400–500 calories (women) or 500–600 calories (men) during the days of fasting. During feed days, the diet was unrestricted.
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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