Fasting is primarily the act of willingly abstaining from some or all food, drink, or both, for a period of time. Concerning that from which one fasts, and the period of fasting, a fast may be total or partial. It may be observed unbroken for many uninterrupted days, or be observed only for certain periods during the day, as is the Muslim practice during the holy month of Ramadan. Depending on the tradition, 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. Medical fasting can be a way to promote detoxification.
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 and New Testament, the Qur'an, the Mahabharata, and the Upanishads. Fasting is also practiced in many other religious traditions and spiritual practices.
Fasting is also used in a medical context to refer to the state achieved after digestion of a meal. A number of metabolic adjustments occur during fasting and many medical diagnostic tests are standardized for fasting conditions. For most medical purposes a person is assumed to be fasting after 8-12 hours. 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. Fasting has occasionally been recommended as a therapeutic intervention by physicians of many cultures, though it is uncommonly resorted to for this purpose by modern doctors.