And there’s more than anecdotal experience like Corbin’s that fasting can help people lose weight. In a one-year study, 100 obese adults ages 18 to 64 were assigned to three groups. One group practiced alternate-day fasting, eating 75 percent fewer calories every other day; another group followed CR, with a 25 percent calorie restriction every day; the rest were in a control group. Compared with the control group, the fasters averaged 6 percent weight loss, and those assigned to CR averaged about 5 percent, according to the 2017 paper in JAMA Internal Medicine.

Among the Western religions, only Zoroastrianism prohibits fasting, because of its belief that such a form of asceticism will not aid in strengthening the faithful in their struggle against evil. The other Western religions—Judaism, Christianity, and Islam—emphasize fasting during certain periods. Judaism, which developed many dietary laws and customs, observes several annual fast days, primarily on days of penitence (such as Yom Kippur, the Day of Atonement) or mourning. Christianity, especially Roman Catholicism and Eastern Orthodoxy, has observed a 40-day fast period during Lent, a spring period of penitence before Easter, and during Advent, a penitential period before Christmas. Among Roman Catholics the observance has been modified since the Second Vatican Council (1962–65) to allow greater individual choice, with mandatory fasting only on Ash Wednesday and Good Friday during Lent. Protestant churches generally leave the decision to fast to individual church members. The month of Ramadan in Islam is a period of penitence and total fasting from dawn to dusk.
In addition to these, there is the fast of repentance which a person keeps after committing sin, it being imposed as a penance by the priest for seven days, forty days or one year. There is also a fast which a bishop keeps at the time he is consecrated. Also there are fasts that are widely observed but which have not been included in the canon of the church and which are therefore considered strictly optional such as the "Tsige Tsom" or Spring Fast, also known as "Kweskwam Tsom" which marks the exile of the Holy Family in Egypt.
In a physiological context, fasting may refer to the metabolic status of a person who has not eaten overnight, or to the metabolic state achieved after complete digestion and absorption of a meal. Several metabolic adjustments occur during fasting. Some diagnostic tests are used to determine a fasting state. For example, a person is assumed to be fasting once 8–12 hours have elapsed since the last meal. Metabolic changes of the fasting state begin after absorption of a meal (typically 3–5 hours after eating).
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