According to Dr. Satchin Panda consumption of anything other than water, including coffee (no specification of caffeinated or not) will have an impact which you could interpret as breaking a fast. Link and key quote follow.
“When we drink coffee, is it going to trigger metabolism or certain things in our gut so that the gut will think, "Well, now I have to start working, the rest is over”? And we think that’s where the metabolism or the function of the gut to absorb, or digest this coffee, send that caffeine to liver, and then to brain does kick start right after we drink coffee. Because that’s how we are feeling the effect of coffee in the rest our body, because the stomach started working, it absorbed coffee, it sent it to liver, liver might have metabolized it slightly and started to send it to the rest of the brain and body. And then it gets back to kidney, it gets metabolized and excreted. So then the question is, forget about circadian clock, now if we think about just metabolism and, say, mitochondria function, or even, say, go back to autophagy, and then ask, “Is caffeine breaking the fasting so that it stops autophagy, or it stops something else? Or is there a crosstalk between, say, caffeine receptor and glucagon receptor so that it does?” No, fasting is kind of slightly over. You may not be in 100% fast, but in 40% or 50% fast. So that’s where things become murky, so that’s why we say, “Well, if you can, drink your coffee within this 8-hour, 10-hour, it’s better.” But at the same time we know, going back to the study that we discussed, Ruth Patterson study, they did not consider coffee as food. So when they considered 13 hours overnight fasting, that 13 hours actually included coffee and tea. So in that we know for cancer, reducing breast cancer risk, this 13 hours of fasting can include coffee, black coffee, and tea. So this is where things are really murky. And we tend to error on the safe side, so we tell, well, if you can have that coffee within your eating window, that’s much better. If you can’t, then just have black coffee. At least that will not trigger your insulin response or glucose response. So that’s what we do, we recommend.
Rhonda: Just sort of as a side note because you mentioned it, I recently spoke with Dr. Guido Kroemer, who is an expert on autophagy, and he was telling me about a study he had published a few years ago where the specific polyphenols in coffee, decaf or caffeinated… So irrespective of caffeine, it’s just it’s the polyphenols.
Rhonda: They triggered protein de-acidulation, which is one of the triggers for autophagy. So it actually increased autophagy.
Satchin: Increased autophagy, yeah. So that’s why we never know, because coffee, or any natural compound, has so many different ingredients that we don’t know the activity."
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