The Stages of Intermittent Fasting: Fed to Autophagy
As a fast progresses your body passes through distinct stages: fed (0–4 hours), fat-burning (4–12 hours), ketosis (12–16 hours) and autophagy (16 hours and beyond). Knowing where you are helps you choose a fasting window that matches your goal.
Fed (0–4 hours)
Your body is digesting and absorbing the last meal. Insulin is elevated and you're primarily burning glucose.
Fat-burning (4–12 hours)
Glycogen stores deplete and your body shifts toward burning fat for fuel. This is where a low-carb diet gives you a head start.
Ketosis (12–16 hours)
The liver begins producing ketones in earnest. For people already keto-adapted, this stage arrives faster.
Autophagy (16+ hours)
Cellular clean-up ramps up as the body recycles damaged components. This is a key reason longer fasts are popular.
Frequently asked
Does keto speed up fasting stages?
Yes — because you start with low glycogen and are fat-adapted, you typically reach fat-burning and ketosis stages sooner than someone on a high-carb diet.