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.Water fasting allows drinking water but nothing else.Other fasts may be partially restrictive, limiting only particular foods or substances. Fasting practices may preclude intercourse and other activities as well as food.The American Cancer Society used to recommend that chemotherapy patients increase their intake of protein and calories; however, current research suggests "fasting for up to 5 days followed by a normal diet..the potential to be translated into effective clinical interventions for the protection of patients" undergoing cancer treatment especially compared to calorie restriction, Some scientists have indicated that a fast will cause white blood cells to break down during the fasting, resulting in new ones needing to be built when the fast is broken, resulting in the replacement of old damaged ones.
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.
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.
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).