I must diagree with you about it being "slightly off-topic." Fasting does help sometimes when it comes to food allergies or sensitivities. So not off topic at all.

I have severe nutritional deficiencies and sometimes I will have a minor health problem. I have had ulcers in the past from too much stress. When I don't feel well I usually don't feel much like eating. So, sometimes I know that the body may benefit from easing up on digestion by fasting. Feed a cold and starve a fever.

I find that sometimes when I fast for maybe a day or two it does seem to bring everything back to the same level like KimS indicated. In my case though I can have problems with the nutritional supplements I take. So just stopping the supplements for a day or two can basically allow me to start fresh again.

The only medical reason I have seen to do a fast for more than a couple of days is to start the rotation diet. There seems to be a real benefit in this situation where you are getting basically everything (all foods) out of your system.

I have read that if you do need to fast for more than a couple of days glutamine, an amino acid, can provide energy to your brain. This amino acid is also great mixed in water to soothe the GI tract. So glutamine might be good for people with GI problems, like leaky gut, or those who do a fast sometimes. But glutamine would prevent the problems some can feel when their brain is starved of sugar basically.