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) was carried out at least as early 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.