How Long Is a Prolonged Fast? Defining Extended Fasting Windows
A prolonged fast typically means fasting beyond 24–48 hours. Here's how science defines the stages of fasting and what changes at each threshold.
The word "prolonged" means something different to someone who has never fasted before than it does to a researcher measuring respiratory quotient in a laboratory. Understanding how science defines prolonged fasting — and what actually changes in the body at each stage — helps you make sense of the options and the risks. A prolonged fast is generally defined as any fast lasting beyond 24–48 hours, though some researchers draw the threshold at 72 hours or longer. The most scientifically…
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