Fasting is a willing abstinence or reduction from some or all food, drink, or both, for a period of time. An absolute fast (dry fasting) is normally defined as abstinence from all food and liquid for a defined period, usually a period of 24 hours, or a number of days. Water fasting allows drinking water, but nothing else.
Other fasts may be partially restrictive, limiting only particular foods or substances. A fast may also be intermittent in nature. Fasting practices may preclude intercourse and other activities as well as food.
In a physiological context, fasting may refer to the metabolic status of a person who has not eaten overnight, or in the metabolic state achieved after complete digestion and absorption of a meal. Several metabolic adjustments occur during fasting, and some diagnostic tests are used to determine a fasting state. For example, a person is assumed to be fasting after 8–12 hours from their last meal. Metabolic changes to the fasting state begin after absorption of a meal (typically 3–5 hours after a meal); “post-absorptive state” is synonymous with this usage, in contrast to the postprandial state of ongoing digestion.
A diagnostic fast refers to prolonged fasting (from 8–72 hours depending on age) conducted under observation for investigation of a problem, usually hypoglycemia. Also, extended fasting has been recommended as therapy for various conditions by health professionals of many cultures, throughout history, from ancient to modern.
Fasting is also a part of many religious observances.