How fuel sourcing shifts through a fast
As a fast lengthens the body moves through overlapping fuel phases. In roughly the first four hours after eating it is in the fed, digestive state with insulin elevated. Over the next several hours it draws on stored liver glycogen to keep blood glucose steady. As glycogen depletes, gluconeogenesis (making glucose from amino acids and glycerol) takes over, and with continued fasting the liver ramps up ketone production so the brain and tissues can use fat-derived fuel.
Commonly cited markers place glycogen use through about 4 to 12 hours, gluconeogenesis around 12 to 18, and rising ketosis past roughly 24 hours, with deeper autophagy associated with longer fasts.
Why the hour markers are approximate
These transitions are gradual and overlapping, not sharp switches at exact clock times. The timing depends heavily on the last meal's size and composition, activity level, muscle glycogen, and individual metabolism, so two people fasting the same number of hours can be in different states.
This is general educational information about metabolic physiology, not a fasting prescription. Extended fasting is not appropriate for everyone and should involve a qualified professional.