Have you read anything on the gut biome? Then there are the nerve plexus and the vagus nerve.

If one consumes lots of sugars and carbs fasting is going to result in a much higher degree of cravings, largely due to the resulting gut biome. If one is at the opposite end, a keto nutrition plan, not only will the cravings be much, much less but it's a jump-start on fasting-induced ketosis.

I don't do keto, but I don't do sugar or refined carbs. I'm south of 20% and north of 10% carb intake. Fasting is pretty much a breeze, but I've not gone beyond 36 hours and don't intend to do so.