Speaking for myself, I didn't fast. But I did eat little a few days leading up to my flushes. This is because I was drinking apple juice, so I wasn't hungry, anyway.
On the day of my potion, I was hungry enough to have a little steamed broccoli for dinner (around 6). I was wondering what this would do. But since the hunger was there, I thought I'd better eat.
The good news is, I had a great flush the next day.
Many people just fast from 2 p.m. on on the day of drinking their potion. I believe this is good if you're going to be doing the epsom salt, which might make you sick.