Kevin, Donald, thanks for your replies.
Intuitively, the idea of eating calorie-rich foods (whatever the macronutrient breakdown) during fasting seems like cheating to me, and your feedback supports these suspicions.
If a lot of fat is consumed during "fasting", the result might be closer to a low-carb, ketogenic diet, which could still offer some health benefits for specific issues. But to me it sounds improbable that the full range of effects associated with IF still applies.
My eating window usually is somewhere between five and seven hours long, and this works pretty well. I don't find myself especially hungry at the end of the fast (around noon). By that time, I have already surpassed my low point (mid-morning).
With the warrior diet and alternate day fasting, I have read about people consuming low-calorie foods during the undereating phase. Do any of you practice this, or is the best way really to stick to zero calories?