You should read Dr. Fung’s book “The Complete Guide to Fasting.” It discusses alternate day & extended fasting & how to break a fast & all that stuff. You might find a good used copy on AbeBooks. The simple answer is, go easy when breaking a fast.
Depends on the length of fast. The longer you go, the more sensitive your body will be at the time you break the fast. For longer fasts (>72 hours), best to be gentle with your body - I do bone broth, pea protein shake, then wait about 30 mins before having the actual meal.