Fasting is primarily the act of willingly abstaining from some or all food, drink, or both, for a period of time. A fast may be total or partial concerning that from which one fasts, and may be prolonged or intermittent as to the period of fasting. Fasting practices may preclude sexual activity as well as food, in addition to refraining from eating certain types or groups of foods; for example, one might refrain from eating meat. A complete fast in its traditional definition is abstinence of all food and liquids except for water.

Fasting for religious and spiritual reasons has been a part of human custom since pre-history. It is mentioned in the Bible, in both the Old Testament (the Tanach) and New Testament, the Qur'an, the Mahabharata, and the Upanishads. Fasting is also practiced in many other religious traditions and spiritual practices.

In a medical context fasting may refer to (1) the metabolic status of a person who has not eaten overnight, (2) to the metabolic state achieved after complete digestion and absorption of a meal, or (3) an unusually extended period of starvation. 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.

Health effects

Glucose is the body's primary fuel source and is essential for the brain's functioning. When denied glucose for more than 4–8 hours, the body will turn to the liver for glycogen, a storage form of glucose, to be used for fuel. A process called glycogenolysis converts glycogen into a usable form of fuel. At this point, the body will also use small amounts of protein to supplement this fuel. This fuel will last for up to 12 hours before the body needs to turn to muscle stores of glycogen, lasting for a few more days. If glucose is still denied at this point, muscle wasting is prevented by temporarily switching to fat as the fuel source, meaning fat is converted into ketone through catabolism. Ketones, while not sugars, are able to be used by the brain as a fuel source as long as glucose is denied.

As a byproduct of metabolism, some toxins are captured in adipose tissue. During fat catabolism these toxins are released into the blood stream This increases the likelihood of acetaminophen poisoning for example, possibly because of depletion of hepatic glutathione reserves.Whitcomb DC, Block GD. Association of acetaminophen hepatotoxicity with fasting and ethanol use. JAMA 1994;272(23):1845-50 and present the body with a second challenge for their elimination. The body will continue to use fat for as long as there is fat to consume. The body will generally indicate to the faster when fat levels are running extremely low (less than 7% of bodyweight for males 10% for females) with an increased urge for food. Fasts are usually broken long before this point. If the fast is not broken, starvation will begin to occur, as the body begins to use protein for fuel. Death can occur if fasting is pursued to the point of complete starvation.