Fasting

Fasting is the voluntary abstention from some or all foods and beverages for a defined period, practiced across cultures for religious, spiritual, health, or personal reasons. It ranges from time-restricted eating patterns, such as a daily non-fasting window of 8 hours with a 16-hour fast, to alternate-day and periodic prolonged fasts, as well as religious fasts such as Ramadan or Yom Kippur.

Physiologically, fasting shifts energy metabolism. Liver glycogen is depleted, insulin levels decline, and the body increases lipolysis and ketogenesis to supply energy. In longer fasts, cellular processes such as autophagy are proposed to occur, though evidence in humans is indirect and remains a topic of research. Fasting can affect hunger hormones, glucose regulation, and gut function, with responses that vary between individuals.

Evidence on health outcomes is mixed. Some randomized trials report modest weight loss and improvements in certain cardiometabolic markers for some people, but results are heterogeneous and depend on what is consumed during nonfasting periods. The quality and duration of evidence vary, and long-term effects are not yet well established.

Fasting is not suitable for everyone and carries risks. It should be avoided or undertaken only under medical supervision by pregnant or breastfeeding individuals, children and adolescents, people with a history of eating disorders, those with certain chronic conditions or on medications that require consistent meals, and individuals with severe underweight status or diabetes requiring medication that can cause hypoglycemia.

Practical considerations include staying hydrated, maintaining electrolyte balance, breaking fast gradually, and monitoring for adverse effects such as headaches, fatigue, or dizziness. Fasting remains a topic of ongoing research and should be personalized rather than viewed as a universal solution.