It is like asking what if I take poison and then try the antidote for it. Sorry being harsh. Why would you eat fast food even though you were not going to fast.
If you have good nutrients in your body it is easier for your body to clean the junk that is sitting there for years. If you start with junk and add more junk, your symptoms will be harder to deal with, especially you want to go for a long fast.
IMO, eat raw plant based diet and be ready girl the fast.