As near as I can tell, apple juice is not necessary for the flush.
Drinking a lot of juice can be very hard on blood Sugar and the endocrine system. If you choose to drink juice, it might be a good idea to dilute it half and half with water so it doesn't shock your system so much. I hope you are eating food, too, and that this isn't some kind of 5 day apple juice fast. JMHO.