Thanks for the interest in this! I am on a fairly strict keto diet, usually hitting 85-90% fat @ around 3000 kcals per day. This has not really changed after fasting. I do not try to make up for anything, meaning that I will not be eating 6000 kcals even though I now fasted for 42 hours.
There is a positive and a negative side to my current status. On the plus side, I am lean and insulin sensitive, which means I cannot really screw up by breaking fast, health-wise. On the down side, my relatively lean status paired with my insulin sensivity seems to make fasting really hard for me. Moments where the hunger really subsides are seldom - my body constantly pushes me to eat. I definitely have to train my fasting muscles. One exception was the meal I has around a week ago, where I ate around 6500+ kcals and could fast without the hunger.
The second issue is that I have to use an app and count calories, because I really do not have any satiety signals. I can literally eat 500g of butter or drink a liter of olive oil without even noticing. Also, my relationship with food is messed up due to my former eating disorder (anorexia nervosa). Ketogenic eating has allowed me to overcome a good deal of it, since I no longer “think” in calories but can listen to my body a little bit better. Very slowly, some satiety signals are coming back, but I still have a long way to go.
That said, fasting is not a means for me to lose weight (obviously), but a therapeutic means to change my relationship with food. Food has always been my key reward, ultimately culminating in obsession over it. I want to find pleasure in other things and fasting helps my do that to some extent, by forcing me to break the cycle of daily food reward. And keto on its own is not powerful enough for that to happen, because I love the ketogenic food.