Our bodies eat the stored fat in our bodies when their is no food. A long fast will also replenish organs and skin cells. I don't think it eats bad cells, but it replaces and eliminates bad cells to give us fresh, new ones. That's why when you do a fast you should drink lots of water(it should always be consumed on a fast), or diluted juices to help the body pass the toxins out of the body through the urinary and intestinal tracts.
I am planning a fast too. I failed last week, but I will keep trying. The hardest part, I heard, is getting past the first five days. So this Friday, I will try again, wish me luck, and you the same.