Fasted

Fasted is an adjective describing the physiological state that results from a period without caloric intake. In everyday and clinical contexts, a person is considered fasted after several hours without eating, often overnight. Depending on the protocol, non-caloric fluids such as water may be allowed, but caloric intake is generally avoided during a fast.

Biochemically, fasting lowers circulating insulin and shifts metabolism away from immediate glucose use toward stored energy. The liver uses glycogen to maintain blood glucose early in a fast, and as glycogen stores decline, the body increases lipolysis to release fatty acids for energy. The liver also produces ketone bodies from fatty acids, especially during longer fasts. Hormonal changes, including higher catecholamines and growth hormone, help preserve glucose for critical tissues while promoting fat oxidation.

Fasting is used in medical, nutritional, and athletic contexts. It commonly accompanies laboratory tests that require an overnight fast, and it is a component of various dietary patterns, such as intermittent fasting. Some athletes train in a fasted state to enhance fat oxidation, though effects on performance vary. In practice, maintaining hydration and electrolytes is important, and individuals may experience dizziness, headaches, or fatigue during fasts.

Safety and suitability depend on individual health and goals. People with diabetes, pregnant or lactating individuals, those with a history of eating disorders, or those taking certain medications should consult a healthcare professional before undertaking prolonged fasting. While longer fasts can induce deeper metabolic changes, evidence for benefits such as autophagy in humans remains inconclusive and context-dependent.