The second phase kicks off after four hours, and lasts up until about 16 hours from your last meal. This is the ‘catabolic’, or breakdown phase, when all those extra nutrients start being released from storage to be used for energy.
Once the energy stored in your cells runs out, your body starts to rely instead on stored fat. The process of releasing fat and burning it up for energy releases chemicals known as ketone bodies for energy, which usually happens around the 16-hour mark.
The rate at which you reach this stage really depends on what you ate for the last couple of meals before your fast. If you ate a lot of carbs and starch, it will take a bit longer than if you ate mostly fats and protein.
One of the most powerful features of fasting, called autophagy, also kicks off during this phase.
Autophagy is triggered by a reduction in a growth regulator called MTOR, and this process is basically a spring clean for your cells. It gets rid of any dead or damaged cellular material, which can otherwise contribute to aging, cancer, and chronic disease.