ELI5: Autophagy is when cells kill and recycle some or all of themselves. If your cells were a car, it would be similar to replacing worn parts, or even scrapping the whole car and getting a new one if the damage is too severe.
There is a low level of this happening all the time.
Fasting causes cells to increase Autophagy in animal models. Evidence shows it starts within 24 hours.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3106288/
We think this is good because it results in more functional, healthier cells.
There are some great videos on YouTube on autophagy. It’s about 16/18 hours with nothing but water/coffee/tea…. Zero food. The time it starts can vary in people. And it can get stronger the longer you fast.
Really interesting chemistry in your body.
And I’ll be the first to apologize on behalf of all research on autophagy. You’ll come across “it begins in 16-18 hours” as much as you will “between 24-48.” It’s annoying, and I still don’t know what the definitive answer is.
This article, despite the silly sounding domain name, seems well written, although it does go a bit into the weeds in some parts:
https://www.charmingzebra.com/autophagy-fasting-timeline/
Here’s an informative video about autophagy from someone who actually knows what they are talking about.
Short version; there is no evidence that fasting stimulates more autophagy than equated caloric restriction using a regular eating pattern. Not to say it doesn’t, but we don’t know. Also short periods of exercise increases autophagy way more than fasting.
So autophagy is essentially your body taking the damaged larger organelles and proteins in your cells and using them as fuel. It’s like a recycling program in your body, As far as how long do you need to fast for it to kick in can vary. Since everyone’s body is different some people can feel the effects/ benefits in a smaller fasting window like 16 hours but for many they tend to see peak levels between 24-48 hours.