Fasting

Fasting is the act of refraining from eating, and sometimes drinking. However, from a purely physiological context, "fasting" may refer to the metabolic status of a person who has not eaten overnight (before "breakfast"), or to the metabolic state achieved after complete digestion and absorption of a meal. Metabolic changes in the fasting state begin after absorption of a meal (typically 3–5 hours after eating).

A diagnostic fast is a prolonged fast, from 1–100 hours long (depending on age), conducted under observation, to facilitate the investigation of a health complication (usually hypoglycemia). Many people may also fast as part of a medical procedure or a check-up, such as preceding a colonoscopy or surgery, or before certain medical tests. Intermittent fasting is a technique sometimes used for weight loss or other health benefits that incorporates regular fasting into a person's dietary schedule. Fasting may also be part of a religious ritual, often associated with specific scheduled fast days, as determined by the religion, or be applied as a public demonstration for a given cause, in a practice known as a hunger strike.

Alternate-day fasting (alternating between a 24-hour "fast day" when the person eats less than 25% of usual energy needs, followed by a 24-hour non-fasting "feast day" period) has been shown to improve cardiovascular and metabolic biomarkers similarly to a calorie restriction diet in people who are overweight, obese or have metabolic syndrome.

A 2021 review found that moderate alternate-day fasting for two to six months was associated with reductions of body weight, body mass index, and cardiometabolic risk factors in overweight or obese adults.

Fasting is almost always practiced prior to surgery or other procedures that require general anesthesia 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 one review, fasting improved alertness, mood, and subjective feelings of well-being, possibly improving overall symptoms of depression, and boosting cognitive performance.

There is little evidence to suggest that intermittent fasting for periods shorter than 24 hours is effective for sustained weight loss in obese adults.

Prolonged fasting (also called extended fasting or water fasting) involves periods of fasting above 24 hours, typically in the range of 5–20 days. In early fasting, the body operates under a relatively high level of gluconeogenesis, though this eventually decreases as the body's metabolism switches into ketosis, causing ketones to replace glucose as the primary energy source in the central nervous system. As prolonged fasting continues, drastic decreases in sodium, potassium, magnesium, and other minerals are noted, which can lead to hyponatremia. In some diet systems, these losses are offset with non-caloric electrolyte supplements, such as electrolyte beverages. Typical observed weight loss under prolonged fasting averages at 0.9 kg per day during the first week and 0.3 kg per day by the third week. In early fasting, during periods of high gluconeogenesis, roughly two-thirds of weight lost is lean muscle mass as opposed to fat. After the gluconeogenic phase, however, the ratio of body fat lost to lean tissue lost becomes roughly 7:6.