Here is how I do my extended fasts. YMMV
I start with doing strict keto, then carnivore. I wait to begin my fast when I know I am fat adapted. It keeps my hunger down and helps the transition because I am already in strong ketosis.
I then look at my reason for fasting. Am I wanting to lose some weight, wanting to lose some inflammation, or am I wanting to heal? If it is weight loss or inflammation I will allow myself to have coffee and green tea during the fast. If it is for healing I ONLY allow myself water with electrolytes.
Your body will cleanse itself out pretty well by about day 3-5. I like to do long fasts. 2 weeks feels really good for health reasons. Just remember that electrolytes are required…do not skip this. And it’s easier than you think it is.
Many feel any calories of any sort impede the increase in autophagy that fasting brings on. The “purist” model that is supposed to be the most healing only includes non caloric electrolytes, and even some think some electrolytes should be avoided to heal or cleanse bone and cartilage.
But it’s all still evolving science with many opinions.