Fasting is primarily the act of willingly abstaining from some or all food, drink, or both, for a period of time. Fasting practices may also preclude 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.
For most ordinary diagnostic 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. Finally, extended fasting has been recommended as therapy for various conditions by physicians of most cultures, throughout history, from ancient to modern.
Research suggests there are major health benefits to caloric restriction. Benefits include reduced risks 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. Long-term studies in humans have not been conducted. However, short-term human trials showed benefits in weight loss. The side effect was that the participants felt cranky during the trial.
Excessive fasting for calorie restrictive purposes, however, may have dangerous effects and can lead to severe disorders and malfunctions. It is strongly advised therefore, that those thinking of fasting as a "diet" consult a physician or dietitian before they start due to potential health risks (see also Diet).