I was just thinking this too as I was reading this. If you go further than a 3 day fast you do have to be careful when it comes to refeeding syndrome, it is no joke. I would honestly talk with your doctor and plan if you want to do more than a 3 day fast.
Only problem with that is you may have a dr that just tells you not to do it and won't really help you figure it out. You can find a dr who will help you though if yours doesn't want to.
I find that people often carry the attitude of "my Dr knows best", now this attitude is waning with the availability of infinite information, but still the attitude persists, understandably, because it is well-ingrained. My attitude is more "a dr probably knows best" -- but not necessarily my dr -- he's just one guy amongst millions of drs, -- information, education, opinions, and practical experience varies greatly among them.
My advice is to find a dr who makes sense to you and listen to her or him. The world is full of access to drs who will help you. Now there my be a price to pay if you want personal attention, but it may be worth it. You can still keep your dr too. I suppose my point is that advice from many drs is better than advice from one dr -- the thing is it is a personal decision determining who to listen to, it takes time and effort to figure it out for yourself when there're so much different guidance to choose from.