From one of the first articles to come up:
"Altogether, these results indicate that coffee triggers 2 phenomena that are also induced by nutrient depletion, namely a reduction of protein acetylation coupled to an increase in autophagy. We speculate that polyphenols contained in coffee promote health by stimulating autophagy."https://www.tandfonline.com/doi/abs/10.4161/cc.28929
I don't think that intermittent fasting results in much if any autophagy since the time frame is so short. What it does do brilliantly is help to maintain insulin sensitivity. So in the case of coffee if you are drinking it without
cream and sugar/artificial sweetener then there shouldn't be much if any insulin response. In that case you are maintaining the intermittent fasting state with no significant insulin spikes. Drinking plain, black coffee should have little if any negative impact on your fast with regard to insulin sensitivity. At least that's my takeaway from what I've seen from Rhonda Patrick and Peter Attia.
If you want autophagy you will likely need to fast longer, although from what I have gathered we can't draw too many conclusions for humans at this point as the studies with solid data are on other species like mice. I do five day fasts primarily because I don't want to do daily intermittent fasts and because Valter Longo has done a lot of research on fasting and promotes a five day fast every 6 months for those who are reasonably healthy and in good shape. I don't know that I'm getting any real uptick in autophagy, but I can see from my ketone meter and from my blood glucose readings that I am in ketosis and that my blood sugar level has dropped off significantly.
After my last 5 day fast I hit 3.2 mmol/dL for ketones and 72 blood glucose. I got my blood glucose measured at a recent doctor's appointment and it was at 88. It's been at that level for the past year whenever I am not doing a five day fast. So the tradeoff I make is one five day fast every six months and then eat fairly regularly every day between the fasts. I just don't like intermittent fasting so I go a different route.
Intermittent fasting is clearly
working for you so keep up the great work! Regarding autophagy, I think we're all extrapolating from animal studies and trying to figure out what would be the equivalent amount of time for humans. Another thing to keep in mind is the mice in these autophagy studies lose something like 20-25% of their bodyweight during these fasts. That is difficult if not outright dangerous for many humans to attempt.