Excellent literature review of the effects of intermittent and periodic fasting.

a number of studies indicating that frequent fasting cycles may … increase side effects and even mortality … daily fasting/TRF periods of approximately 12 hours appear to be associated with benefits without known negative effects

It looks like all of these are strategies (including the usual 14 or 16 hour daily fasts) best used regularly, but not ongoing, and the re-feeding period might be just as important as the fast. From the abstract:

[intermittent fasting] lasting from 12 to 48 hours and repeated every 1 to 7 days and [periodic fasting] lasting 2 to 7 days and repeated once per month

And from the conclusion:

the refeeding period that has more recently emerged as an equally important process involved in the regeneration, and possibly rejuvenation, of systems, including organs, cells and organelles.