This article is intended solely for informational purposes. It is not meant to be taken as, and should not be construed, as medical advice. Any changes to your lifestyle or diet should be done in consultation with your doctor or health care professional.

In PART 1 of this article, I described what happens when we fast. In this part, we’ll go deeper into what autophagy is, and how long it takes to kick in – and what happens after the fast is over.

After your fast, when you start eating again, your body grows new stem cells to replace the cells that were used as fuel. Think of this as planting a new, young, healthy tree in the forest to replace the old dying tree you burned.

How long it takes to fully activate autophagy appears related to our glycogen stores.  As we finish burning through stored glycogen, autophagy can start taking place. If we typically eat a ketogenic diet (low in carbohydrates, high in fat, moderate in protein), burning ketones rather than glucose as our main source of fuel, autophagy can start very quickly, because there’s not much glycogen in our bodies to burn. If we typically eat a high-carb diet, with lots of fruit, grains, and other starches and sugars, it can take 24 hours or more before autophagy begins. 

(As a side note, both ketogenic diets and fasting are beneficial to both the body and brain, both of which love running on ketones. The brain actually prefers ketones to glucose as a source of fuel, and there’s some evidence that conditions like Alzheimer’s may be slowed down or prevented by switching to a ketogenic diet.) (During apoptosis, mis-folded proteins get cleared out of our brain, and neurons that are broken and senescent can get cleared away too.)

Autophagy is a kind of cellular deep cleaning that kicks in toward the beginning of an extended fast. Mild autophagy can kick in after just twelve hours. (Some autophagy is always going on in the body; but fasting shifts it into high gear.) The longer our fast, the deeper it goes.

If you fast for too long, of course, you’ll eventually metabolize your own body. But according to Dr. Jason Fung, fasts of several days should be safe for everyone except those with health problems that counter-indicate fasting.