Fasting

Fasting is the act of refraining from eating and sometimes drinking. From a physiological perspective, it refers to a specific metabolic state. This state occurs after the body has completely digested and absorbed a meal. Metabolic changes typically begin about three to five hours after eating. Fasting can serve many purposes, ranging from medical necessity to religious devotion. It is also used as a tool for political protest or personal health management.

When a person fasts, the body undergoes complex internal shifts. In the early stages, the body relies on a process called gluconeogenesis. This process creates glucose to provide necessary energy. As fasting continues, the metabolism eventually switches into a state known as ketosis. In ketosis, the body uses ketones as the primary energy source for the central nervous system instead of glucose. During prolonged fasting, which can last between five and twenty days, the body may experience drastic decreases in minerals. These include sodium, potassium, and magnesium, which can lead to a condition called hyponatremia. To prevent this, some people use non-caloric electrolyte supplements.

There are several distinct ways that people practice fasting. Intermittent fasting involves incorporating regular fasting periods into a dietary schedule, often for weight loss. Alternate-day fasting is a specific type where a person alternates between a 24-hour fast day and a 24-hour feast day. On a fast day, the person consumes less than 25% of their usual energy needs. Another type is the diagnostic fast, which is a prolonged period of fasting conducted under medical observation. These are often used to investigate health complications like hypoglycemia. Finally, there is the hunger strike, which is a form of non-violent resistance used to achieve political goals.

Historically, fasting has been used by many influential leaders to drive social change. The political leader Gandhi used long fasts as protests in India. In 1968, the American civil rights activist César Chávez undertook a 25-day spiritual fast. He used this period to promote the principle of nonviolence and to prepare for civil disobedience. In 1981, Bobby Sands participated in an Irish hunger strike to protest for prisoner rights. Sands died after 66 days without food, and the strike eventually involved ten men who died. These events show how fasting can be a powerful tool for public demonstration.

Medical professionals use fasting for safety and accuracy. Patients almost always fast before surgery involving general anesthesia. This prevents pulmonary aspiration, which is when a person inhales vomit into their lungs. Fasting is also required for certain diagnostic tests, such as lipid panels to check cholesterol. These tests require a baseline that is not affected by recent food intake. However, doctors must be careful of refeeding syndrome. This is a dangerous metabolic disturbance that occurs when someone eats too much too quickly after being starved or malnourished. It can cause imbalances in potassium, magnesium, and phosphate, which may lead to death.

Many world religions include fasting as a core practice. In Islam, Muslims fast during the month of Ramadan from dawn until sunset. Judaism observes several fasts, such as Yom Kippur and Tisha B'Av, which can last 25 hours. In Christianity, many observe fasting during Lent. The Catholic Church has specific rules regarding abstinence on Fridays and during penitential times. Eastern Orthodox Christians fast during various seasons, including the Nativity Fast and the Apostles' Fast. Members of the Church of Jesus Christ of Latter-day Saints participate in a monthly fast where they donate the money saved from meals to help others.

Fasting also relates to broader discussions about health and weight. Research on alternate-day fasting shows it can improve cardiovascular biomarkers in overweight adults. A 2021 review found that moderate alternate-day fasting for two to six months helped reduce body mass index. Regarding weight loss during prolonged fasting, the body loses about 0.9 kg per day during the first week. By the third week, this average drops to 0.3 kg per day. In the early gluconeogenic phase, about two-thirds of weight lost is lean muscle mass. After this phase, the ratio of body fat lost to lean tissue lost becomes roughly 7:6. While some use it for weight management, there is no sound clinical evidence that fasting promotes longevity in humans.