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.
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.
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.
Such changes in blood chemistry during fasting may have dangerous effects. 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.
Research suggests there are major health benefits to caloric restriction. Benefits include a reduced risk of cancer, cardiovascular diseases, diabetes, insulin resistance, immune disorders, and more generally, the slowing of the aging process and the potential to increase maximum life span. According to Dr. Mark P. Mattson, chief of the laboratory of neurosciences at the National Institute on Aging, fasting every other day (intermittent fasting) shows as strong of beneficial effects as caloric-restriction diets in mice, though no studies have been done in humans to date. According to The National Academy of Sciences other health benefits include stress resistance, increased insulin sensitivity, reduced morbidity, and increased life span . Long term studies in humans have not been conducted. However, short term human trials showed benefits in weight loss. The side effect was the participants felt cranky during the three week trial. According to the study conducted by Dr. Eric Ravussin "Alternate-day fasting may be an alternative to prolonged diet restriction for increasing the life span".
Adherence to Greek Orthodox fasting periods contributes to an improvement in the blood lipid profile, including a decrease in total and LDL cholesterol, and a decrease in the LDL to HDL cholesterol ratio. A statistically insignificant reduction in HDL cholesterol was also observed. These results suggest a possible positive impact on the obesity levels of individuals who adhere to these fasting periods.
Prolonged fasting, generally more than 30 days, can result in serious health problems. It is highly advised that anyone thinking of fasting as a "diet" consult a physician or dietitian before they start.
Fasting leads to lowered blood sugar and blood pressure which can lead to dizziness and blackouts. People who had low blood pressure before the fast can have these problems on a one day fast. For most this becomes a problem on 5+ days water fasts. One way to avoid blackouts is to breathe deeply before standing up and to rise up slowly.