If I fasted for 90 hours beforehand (instead of eating regularly every day), would it affect my 23 and me DNA spit test kit results?
It will not affect your results.
As long as you are hydrated enough to produce enough spit, you're good.
From numerous conversations on various genetic genealogy groups, where employees of different testing companies have weighed in, the only situations to avoid are:
Eating too soon before filling the tube. Because of contamination, not because it would give you the DNA results of a carrot. You may drink water of course and also people having trouble producing saliva can have a spoonful of sugar to help out.
Chemotherapy. Mostly the companies recommend waiting until after your treatments.
Ancestry goes back and forth about chemo. FTDNA has said to wait (per Facebook conversations). ISOGG says "Cancer treatments do not seem to affect DNA results. However, it's recommended that you wait a few months after treatment before collecting a sample for testing."
23andme says "Do not eat, drink, smoke, chew gum, brush your teeth, or use mouthwash for at least 30 minutes prior to providing your sample."
While I'm not finding anything specific about fasting, I am certain it is not an issue, provided you can produce the necessary saliva. You might be concerned about autophagy, but this will only purge cells, not change the DNA in existing ones.
Remember, when people talk about "changing your DNA" they mean one of two things. Either they mean overcoming the constraints imposed by your DNA or they mean affecting DNA methylation.
The DNA tests we take for genetic genealogy or nearly all the tests we take for medical reasons, only test the underlying DNA. You can not change this! You need a specialized test to look for methylation and none of the regular tests do that.