I’ve been on my homebrewed version of soylent for about a week and a half now. I’m no longer consuming it exclusively, but I was for the first couple days. During this period, I was chewing a lot of gum to at least partially satisfy the urge to chew something, and as has been my habit for years, I was swallowing it without thinking.
This is headed exactly where you think it is.
Basically, after all the solid food from before I started the diet left my system, there was presumably only one thing left: a big hunk of polyisobutylene. Passing this was, erm, an ordeal. Ordinarily, it isn’t noticeable, but ordinarily the gum is <1% of the solid matter in my stomach at any given time. Without (much) normal poop as a vector, the polymer (which isn’t digested) is just as gummy coming out as it is going in. It felt at times like I was blowing a bubble.
Moral: if you’re on a liquid diet, don’t swallow your gum.