Currently, I am on Day 8 of a 12 Day fast, being interested much more in the astounding benefits of autophagy and mitophagy than losing weight (which IS very important). (I will post in a more appropriate thread my reasons for selecting this length of time.)
Regarding refeeding, Valter Longo (Professor of Biological Science and Director of the Longevity Institute at University of Southern California), who has studied fasting extensively, recommends the following transitional diet:
Transition diet For 24 hours following the end of the five-day FMD, patients should follow a diet based on complex carbohydrates (vegetables, cereals, pasta, rice, bread, fruit, etc.), and minimize the consumption of fish, meat, saturated fats, pastries, cheeses, milk, etc.
(“FMD” = “Fasting Mimicking Diet”, which he developed as a way to maximize autophagy while still having fasting be acceptable to the mainstream medical profession.)