During an extended fast, the body progresses through five metabolic stages: fed state (0-4 hours), early fasting (4-16 hours, liver glycogen), fasting state (16-24 hours, fat burning increasing), ketosis (24-72 hours, ketones as primary fuel), and deep ketosis (72+ hours, maximal fat oxidation). Research by Cahill and Longo documented these transitions. Understanding the stages removes mystery from fasting—your body has evolved mechanisms for each phase.
This companion covers each stage in detail, what’s happening hormonally, how it feels, why this matters, and practical implications for different fasting lengths. (5 min read)