Glad to hear that you can push back your blood test. You are going to be surprised and so will be your doc, your blood test results will be amazing.
I am reading this book, Brain Maker and the author, Dr. Perlmutter says that during a fast the body makes ketone molecules from fat and junk. Particularly, Beta-Hydroxybutyrate (Beta-HBA), which is a superior fuel for our brain and body.