Fasting

Fasting means not eating, and sometimes not drinking. Physiologically, it describes the metabolic state of someone who hasn’t eaten since the previous night (before “breakfast”) or the state reached after a meal is fully digested and absorbed. This metabolic shift typically begins 3–5 hours after eating. A diagnostic fast is a supervised, prolonged fast lasting 1–100 hours (depending on age) used to investigate health issues, often hypoglycemia. People also fast before medical procedures like colonoscopies or surgeries, or for tests such as cholesterol or blood glucose measurements. Intermittent fasting is a dietary pattern that includes regular fasting periods, sometimes used for weight loss or health benefits. Fasting can also be a religious practice, with specific scheduled days, or a public protest known as a hunger strike.

**Health effects** Alternate-day fasting—alternating a 24-hour “fast day” (eating less than 25% of usual energy needs) with a 24-hour “feast day”—has been shown to improve cardiovascular and metabolic markers similarly to calorie restriction in overweight, obese, or metabolically syndrome-affected individuals. A 2021 review found that moderate alternate-day fasting for two to six months reduced body weight, body mass index, and cardiometabolic risk factors in overweight or obese adults.

**Medical application** Fasting is almost always required before surgery or procedures using general anesthesia to prevent pulmonary aspiration of stomach contents. Certain medical tests, like lipid panels or blood glucose checks, also require several hours of fasting to establish a baseline.

**Mental health** One review found that fasting improved alertness, mood, and subjective well-being, possibly easing depression symptoms and boosting cognitive performance.

**Weight loss** *Intermittent fasting*: There is little evidence that intermittent fasting periods shorter than 24 hours lead to sustained weight loss in obese adults. *Prolonged fasting*: Also called extended or water fasting, this involves periods over 24 hours, typically 5–20 days. Early on, the body relies heavily on gluconeogenesis, but this decreases as metabolism shifts to ketosis, where ketones replace glucose as the central nervous system’s main energy source. As fasting continues, levels of sodium, potassium, magnesium, and other minerals drop sharply, potentially causing hyponatremia. Some diets offset these losses with non-caloric electrolyte supplements. Weight loss averages about 0.9 kg per day in the first week and 0.3 kg per day by the third week. During early high-gluconeogenesis phases, roughly two-thirds of weight lost is lean muscle mass; after that, the ratio of fat loss to lean tissue loss becomes about 7:6.

**Longevity** There is no reliable clinical evidence that fasting extends human lifespan.

**Adverse effects** *Refeeding syndrome*: This metabolic disturbance occurs when nutrition is reintroduced to someone who is starved, severely malnourished, or metabolically stressed. Eating too much food or liquid nutrition in the first four to seven days after a period of malnutrition can cause cells to produce glycogen, fat, and protein, leading to low blood levels of potassium, magnesium, and phosphate. This electrolyte imbalance can cause neurological, pulmonary, cardiac, neuromuscular, and hematologic symptoms, and in severe cases, death. Refeeding syndrome can begin after several days without food, typically starting after 4–5 days. *Gallstones*: Fasting can increase gallstone risk for some people. This is thought to happen because the gallbladder moves less without food to digest, concentrating bile with cholesterol, while the liver secretes extra cholesterol into bile as the body metabolizes fat during rapid weight loss.

**Political application** Fasting is often used to make a political statement, protest, or raise awareness. A hunger strike is a non-violent resistance method where participants fast to protest, provoke guilt, or achieve a goal like policy change. A spiritual fast combines personal beliefs with a desire to express principles, sometimes regarding social injustice. Political leader Gandhi undertook several long fasts as political and social protests, significantly impacting the British Raj and Indian population. In Northern Ireland in 1981, prisoner Bobby Sands, part of the 1981 Irish hunger strike for better prison rights, was elected to the British Parliament and died after 66 days without food. 100,000 people attended his funeral; the strike ended after nine other men died. In total, ten men survived without food for 46 to 73 days.