What makes you fast in the first place? If it’s autophagy, then it depends on what you choose to believe about how autophagy works since that’s not definitively known. Some believe any number of calories would halt autophagy. Others believe it’s much more grey than that. It’s up to you to choose what to believe in and decide accordingly.
If you’re fasting for weight loss, I’d say go nuts with the very-low-calorie stuff. Even if your daily caloric intake is 100 calories, you’re so drastically below the lowest recommended daily caloric intake that it would make a diminshingly small difference.
Fasting has a pretty simple procedure to follow for situations like this:
I’m sure you do miss eating. I’m sure you are craving something to taste. That is the price you have to pay to do this thing we do.