Fasting is primarily the act of willingly abstaining from some or all food, drink, or both, for a period of time. An absolute fast is normally defined as abstinence from all food and liquid for a defined period, usually a single day (24 hours), or several days. Other fasts may be only partially restrictive, limiting particular foods or substance. The fast may also be intermittent in nature. Fasting practices may preclude sexual intercourse and other activities as well as food.
In a physiological context, fasting may refer to (1) the metabolic status of a person who has not eaten overnight, and (2) to 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. Metabolic changes toward 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 "post-prandial" 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. Finally, extended fasting has been recommended as therapy for various conditions by health professionals of most cultures, throughout history, from ancient to modern.