“WHY A LONGER FAST? Three to seven days of fasting sounds extreme, but when you weigh in all the benefits above, it becomes worthwhile. We here at Ujido chose to do a longer five-day fast partly for a larger, prolonged effect on our weight and health, but also partly for the ratio of good days to bad days. The first couple days of a water fast aren’t great. You’re hungry, moody, focused on food, and spacy. Once ketosis kicks in, you get a boost of energy, your mind clears, and you can even exercise. Those fat stores are no joke. A five or seven-day fast results in more time in ketosis versus time suffering through the transition.”

https://ujido.com/blogs/matcha-insider/14-reasons-to-do-a-longer-3-to-7-day-water-fast#:~:text=WHY%20A%20LONGER,through%20the%20transition.