Fasting

Fasting is the voluntary abstinence from some or all food, or from food and beverages, for a defined period. The term covers a range of practices that differ in duration and in whether water or limited energy intake is permitted. Physiologically, fasting creates a period in which absorbed nutrients from a recent meal are no longer the body's main source of metabolic fuel .¹

As fasting progresses, insulin concentrations generally fall and the relative influence of glucagon increases. The liver initially helps maintain blood glucose by breaking down glycogen through glycogenolysis. At the same time, gluconeogenesis produces glucose from precursors such as lactate, glycerol, and amino acids. Adipose-tissue lipolysis also increases, releasing fatty acids that can be oxidized by many tissues for energy .²

With longer fasting, liver glycogen becomes increasingly depleted and reliance on fat-derived fuels rises. The liver converts some fatty-acid-derived acetyl-CoA into the ketone bodies acetoacetate and beta-hydroxybutyrate. As their concentrations increase, ketone bodies become an increasingly important fuel, including for the brain, helping reduce its dependence on glucose during prolonged fasting .²

Fasting is not synonymous with calorie restriction. Fasting is defined primarily by periods of abstinence from food or energy intake, whereas calorie restriction refers to reducing overall energy intake and does not necessarily require prolonged periods without food. Terms such as intermittent fasting, short-term fasting, and prolonged fasting describe particular patterns or durations within the broader concept .¹