it takes 72 hours for the gut to fully void from top to bottom, that's why it's the magic time people keep talking about

in about 48 hours you are starting to absorb dramatically less from your gut and that's when you start to feel it, and at 72 hours you start to go into ketosis, and autophagy starts, which is where cells start being digested. this is why it heals stuff, usually it's stuff that has got too ragged and needs to be purged and regrown.

also, the amount of things in our guts these days that are irritating... it was not as bad as this in the old days, it's not for nothing, if nothing else fasting gives you a break from that irritation, which also helps improve digestion afterwards.

i'm of the opinion that it should be done at least twice a year, maybe every season, i'm going to try 3 days every month. i have other issues that benefit from autophagy and ketosis (starting to get tubby and i'm pretty sure my liver is very fatty)