One form of intermittent fasting, alternate day fasting (ADF), involves a 24-hour fast followed by a 24-hour non-fasting period. Alternate-day calorie restriction is under basic research for whether it can prolong life. One can choose fasting 23 hours with one meal per day.
In some contexts, fasting allows the consumption of a limited amount of low-calorie beverages such as coffee or tea. Modified fasting involves limiting caloric intake (e.g., 20% of normal) on fasting days rather than none at all, possibly retaining most of the benefits of intermittent fasting.
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 regular.