Adrenaline (epinephrine) increases during fasting. Resting energy expenditure rises rather than falls in short-term fasting — about 14% over three days in one small study, though the larger study puts it at 6% by 36 hours, gone by 72, and absent in men. The tempting explanation, that catecholamines drive it, is one that study’s own authors decline: metabolic rate peaks when noradrenaline has not yet moved. This is the body mobilizing energy stores, signaling fat cells to release fatty acids. What none of it establishes is how fasting feels — a review argues that alertness while foraging would have been adaptive, but that is its framing rather than a result, and the studies here measured metabolic rate and catecholamines, not alertness or energy.
This companion covers the counter-intuitive reality of fasting energy, what happens hormonally during fasting, the evolutionary logic, the experience many report, and the metabolic effect on lipolysis and thermogenesis. (5 min read)
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