What you eat has a big impact on your gut flora - they subsist on the contents of your lower intestine and the balance of the population changes depending on what you eat.
I’m pretty sure that gut flora aren’t only interested in fibre - they’re opportunistic bacteria and will evolve to eat whatever is around, changing themselves and their populations to suit the available food.
Anything you eat that doesn’t get digested and absorbed further up the gut, ends up passing through the lower intestine - the colon - where most intestinal flora live. Different people and nationalities, with different diets, have gut flora that has adapted to match. Difference in diet predicts most of the difference (~60%) in gut flora between people.
@nthmost mentioned Butyric acid which is produced by intestinal flora - amongst many other things. These contribute quite substantially to caloric intake - rodents raised in a sterile environment and lacking a microbiome require ~30% more calories to maintain weight than controls.
Gut flora also mediate other metabolic effects such as the syntheses of vitamins like biotin and folate, as well as absorption of ions including magnesium, calcium and iron.
Gut flora also have a big impact on health, regulating the immune system in the gut, protecting against opportunistic infections by pathogens, preventing inflammatory bowel disease, allergies, etc.
TLDR: gut flora are a large and important part of the Human organism and they change if you change your diet. A major change of diet, like Soylent, cannot fail to have an effect on your microbiome - probably a fairly pronounced one, especially if you keep it up long term.
This isn’t necessarily a bad or good thing - it entirely depends on the outcome - but it would be one of my major concerns with long term soylent use.
I can also see that Soylent might be an ideal way to change your gut flora on purpose. I used to have Ulcerative Colitis - but I cured it by changing the balance of my gut flora through diet. Doing this while eating whole food is very hard, requires enormous strictness and takes years of work - individual bacteria don’t eat much, so you have to be very strict - and while it’s easy to affect your gut flora through diet, it takes a long time to completely change it for good.
Soylent could make that process much easier by allowing you to finely control what ends up in the lower intestine and therefore deliberately change the bacterial population much more quickly and easily.
TLDR;TLDR: @rob, I think you might have the makings of a much more accessible/workable treatment for IDB/IBS/Colitis/Crohns on your hands.
Please note, the original version of this post had lots of links to references, but although I’m apparently only allowed to have two links per post, it wouldn’t work unless I took all of them out. I can supply the rest in follow up comments if anyone is interested.