I think fasting is good to help the body repair.....but there are a few different kinds such as water fasting, juice fasting, IF fasting, 3 day fasting, etc....Juice fasting is 80% vegetables...so they are not high in sugar (minus the carrots)...otherwise you could be playing with insulin levels...although fasts are short term and you would recover fine. If I feel sick....(about 3 years ago last time I was) I do a complete water and ACV fast and sleep for a whole day....puts me on my ass but I feel 85% better the next day....my opinion, just let the body do what it needs to do. Digestion in theory is a huge drain on the energy system of the body....so if you need that energy elsewhere to fight off or repair something, then you need to minimize digestion (hence the juicing and not just eating raw fruit/veg). I've seen it work well for people with diseases such as arthritis....even just taking a 24hr fast once a week. Just be careful with diabetics.
Bragg is huge on juice fasting for health if you want to read his book.....but it will pretty much tell you the same stuff.