I've never heard of that 18 hour rule. It's funny because I've read a lot about fasting, but this is news to me, and it seems like this is reported from multiple sources. But I can't seem to find any authoritative references.

The Japanese researcher that got a Nobel Prize for discovering the autophagy mechanisms has a course on edx. Doesn't get much more authoritative than that.

can you please provide the link to the course?

Google seems to suggest this is it:

https://www.edx.org/course/autophagy-research-behind-the-201...

Autophagy is always active, the real question is how that activity changes during fasting. The number of 18 hours is fairly arbitrary, as even after a few hours of fasting autophagy increases.

Depends on what you're eating... if you're on a keto (fast mimicking) macro level intake, you'll reach autophagy a bit faster. Heavy exercise/resistance/lifting can also help speed this up, which can also lend to timing of exercise and food intake.

I’ve been looking for a good source but haven’t found one. Has it not been studied in humans yet?

I think I heard it on the FoundMyFitness podcast which has excellent discussions about fasting science. I think it may have been the last interview with Valter Longo. I’ll see if I can dig it up, but their website would be a good starting point.

"Someone said so in an interview" isn't really compelling evidence. Do you know of actual evidence of this? Studies?

No idea why you're being downvoted. Even when videos/podcasts cite studies, it's difficult to verify that the study actually supports their claim since it's often difficult to find the study they mention.

The 18 hour claim may be supported by evidence, but there isn't any in this subthread so far.

Feel free to pore over thousands of scientific papers about the benefits of fasting on NCBI for us and let us know. Until then, I'll take NutritionFacts.org videos' word for it: https://www.youtube.com/channel/UCddn8dUxYdgJz3Qr5mjADtA which cites scientific papers as it makes claims.

Autophagy is not a binary thing. Every body does autophagy all the time, it's the intensity/volume that increases in longer fasts.