Check out August Dunn. He has a Youtube channel and wrote a book on how to safely dry fast called “The Phoenix Protocol” which is what I followed on a 7-day dry fast last month (my first). He studied with Russian doctors and astronauts who use dryfasting extensively for radiation exposure.
He spells out what to before and after the fast and who should not dry fast. I do not recall him saying being slender or having low-body fat should be a disqualifier but there were other conditions.
The book + videos were super helpful as I was going through the fast. It was fascinating to learn what my body was doing on each day of the fast. I’m definitely going to do it again next year.