I’ve done 2 water fasts (first time was for 15 days and the second time was for 12 days). I broke each of my fasts with a slice of watermelon and that made me feel tired. It took me 4 days to get back to eating cooked food and each meal consumption was exhausting. Anyone who water fasts realises pretty quickly that there is a lot of truth to how much energy is spent on digestion which is also one of the reasons why we are told not to exercise after a meal. Read dr Fuhrman’s book on fasting. It clears up a lot of misconceptions