I have collected quite a few papers on longer-term fasting (mostly 1-14 days). As you can see from the collection, most research on fasting has been in the 1-3 day range. The longest and largest trials come from Buchinger in Europe and TrueNorth in the US.
There’s a fasting talking head named Jason Fung, he’s a nephrologist who works with diabetics. He wrote a book called “Complete guide to fasting” which has tons of information and citations. It’s definitely worth a read as he displays studies and interpretations about fasting.
I worried about this a lot. I know the anecdotal evidence is not bad, but I prefer to follow doctors on youtube who are more conservative. There’s this doctor who recommends 72 hrs as an optimal long term fasting, after that hour the benefits continue, but tend to be lower.
It is true that we don’t have sufficient evidence for longer fasts, so it’s not that their necessarily bad.