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 daytime period. Other fasts may be partial in scope of food and drink, such as restricting only meat. The fast may also be intermittent, spanning an irregular set of days. Fasting practices may preclude sexual and other activities as well as food.
In a medical 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 many medical diagnostic tests are standardized for fasting conditions. For most ordinary diagnostic purposes a person is assumed to be fasting after 8–12 hours. Many of the metabolic shifts of fasting begin as absorption of a meal is complete (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 medical observation for investigation of a problem, usually hypoglycemia. Finally, extended fasting has been recommended as therapy for various conditions by physicians of most cultures, throughout history, from ancient to modern.