I am not familiar with the process anorexics go through, but it is likely very different - thousands, possibly hundreds of thousands, do fasting of 2-7 days on a regular basis with no ill effect (on the contrary - often with good effect).

Good bacteria can likely be reinstated by the body itself (the appendix is a cache for them), from food, or if all else fails - by FMT. I’m not an expert on this, and may be wrong, but from a lot of reading - any anorexia symptoms and processes seem to be entirely absent from mindful fasting.