Fasting

Fasting is a willing abstinence or reduction from some or all food, drink, or both, for a period of time. An absolute fast (dry fasting) is normally defined as abstinence from all food and liquid for a defined period, usually a period of 24 hours, or a number of days. Water fasting allows drinking water but nothing else. Other fasts may be partially restrictive, limiting only particular foods or substances. A fast may also be intermittent in nature. Fasting practices may preclude intercourse and other activities as well as food.

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, and some diagnostic tests are used to determine a fasting state. For example, a person is assumed to be fasting after 8–12 hours from their last meal. Metabolic changes toward the fasting state begin after absorption of a meal (typically 3–5 hours after a meal); "post-absorptive state" is synonymous with this usage, in contrast to the postprandial state of ongoing digestion.

A diagnostic fast refers to prolonged fasting (from 8–72 hours depending on age) conducted under observation for investigation of a problem, usually hypoglycemia.

Fasting is also a part of many religious observances.

Fasting is often practiced prior to surgery or other procedures that require general anesthetics because of the risk of pulmonary aspiration of gastric contents after induction of anesthesia (i.e., vomiting and inhaling the vomit, causing life-threatening aspiration pneumonia). Additionally, certain medical tests, such as cholesterol testing (lipid panel) or certain blood glucose measurements require fasting for several hours so that a baseline can be established. In the case of a lipid panel, failure to fast for a full 12 hours (including vitamins) will guarantee an elevated triglyceride measurement.

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