From Ramadan to Yom Kippur, fasting is both a ritual and a metabolic switch
Muslims fast from dawn to sunset through Ramadan, Jews go 25 hours without food or water on Yom Kippur, and Bahá'ís fast for nineteen days each March. Inside the body, fasting starts only hours after a meal, and a long fast pushes the brain to run on ketones instead of glucose.
Religious fasting takes many forms. In Judaism, Yom Kippur and Tisha B'Av run from sunset to nightfall the following day, while lesser fasts last only from dawn until dark, so their length shifts with the seasons. Catholic canon law marks every Friday and the season of Lent as penitential, with both fasting and abstinence required on Ash Wednesday and Good Friday. Eastern Orthodox Christians add fasts on most Wednesdays and Fridays plus long seasons before Christmas, after Easter and in early August. Latter-day Saints skip two meals on the first Sunday of each month and donate what they would have spent to people in need.
Physiologically, the fasting state begins once a meal has been absorbed, typically three to five hours after eating. In fasts beyond 24 hours the body at first makes glucose at a high rate, then shifts into ketosis, and ketones take over as the central nervous system's main fuel. Levels of sodium, potassium and magnesium can fall sharply over extended fasts, and restarting food too fast after several days without it can trigger refeeding syndrome, a dangerous electrolyte imbalance.
The research picture is mixed. A 2021 review linked moderate alternate-day fasting over two to six months with lower weight and cardiometabolic risk factors in overweight or obese adults. However, there is little evidence that intermittent fasts of under a day produce lasting weight loss, and no sound clinical evidence that fasting extends human life. The popular idea that fasting detoxifies the body is described as a marketing myth.
Fasting has also been a form of protest. Gandhi's long fasts had a real impact on the British Raj, and César Chávez fasted for 25 days in 1968 to promote nonviolence.
Source: Fasting
