Fasting seems like the opposite of stress- your digestive tract isn't actually doing anything
Fasting invokes all your counter-regulatory hormones: cortisol, adrenaline, others but they are the big players - because not having easy access to sugar leads to a stress response.
So it isn't gut rest, it's physiological phase change that is causing the stress response
Thyroid hormone is also going to be implicated somewhere along with a host of other hormones so I have simplified but suffice to say it's shades of grey on the way in and out
The digestive tract may not do anything, but the body gets into "we're fighting possible starvation" mode.