Fasting-Caloric Restriction: Fasting is a willing abstinence or reduction from some or all food, drink, or both, for a period of time. An absolute fast or 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. Many people may also fast as part of a medical procedure or check-up such as a colonoscopy.

Fasting is also a part of many religious rituals.

Calorie restriction, or caloric restriction, or energy restriction, is a dietary regimen that reduces calorie intake without incurring malnutrition or a reduction in essential nutrients. "Reduce" can be defined relative to the subject's previous intake before intentionally restricting calories, or relative to an average person of similar body type.

In a number of species calorie restriction without malnutrition may slow the biological aging process, resulting in longer maintenance of youthful health and an increase in both median and maximum lifespan. However, the life-extending effect of calorie restriction is not shown to be universal. In humans, the long-term health effects of moderate caloric restriction with sufficient nutrients are unknown.

Using rhesus monkeys, a collaboration of the United States National Institute on Aging and the University of Wisconsin found that caloric restriction without malnutrition extended lifespan and delayed the onset of age-related disorders; older age, higher diet quality, and female sex were positive factors affecting the benefits realized from lower caloric intake.