I have noticed that a day after exercising on a fast (usually involuntary I may add! :) ), my ketostix really do go dark, and I put this down to greater demands made by the body. So actually, this may impede healing perhaps? Because the energy has to supply the muscles instead of healing?
I think the best thing to do is ignore what goes on under the bonnet to a certain extent, and just fast and rest as much as possible.