This is just out of curiosity, I’m not really interested in really doing this, fiber is healthy blah blah blah.
Ok so what kind of diet a human being could eat and stay relatively healthy and alive would produce the least amount of fecal matter? Research reveals that at least 1/3 of feces is dead bacteria, so obviously even with a perfectly formulated liquid diet you’ll still be producing some brown stuff.
But what about a no plant, high protein diet?
Good luck!
The kind of diet you’re looking for, is called a low residue diet. People with gastro intestinal issues are often put on a low residue diet, and the early astronauts were on a low residue diet while in space for obvious reasons. As I recall, the morning of the lunch they typically had steak and eggs which are both low residue. Here is a list of foods which are low residue:
Please excuse the typo - I am using Siri. Morning of the launch not morning of the lunch.
The general term for this is “low-residue diet”.
NASA has done research into this for obvious reasons and has some example meals.
Lunch 11:30 a.m. - Meat loaf on toast? WTF?
For the 1960s, that wasn’t really an unusual meal. You can still get a meatloaf sandwich at many diners today.
There is also a variety of liquid diets that can serve the same purpose. The ones mentioned in that link are normally short-term, but a few people must be on them continuously.
Liquid protein diets for weight loss, lasting months, were once a preferred method but fell out of favor. Today you’ll see them used more often as meal replacements.
Yeah, I’m not entirely sure what’s odd about that. I mean, what else do you do with leftover meatloaf but eat it for lunch as a sandwich the next day?
what is this “leftover meatloaf” that you are talking about?
OK, if there’s no leftover meatloaf, then you’ve gotta at least make a second meatloaf for the sandwiches. 
So, at the risk of sounding too gross, would you get super-constipated on this sort of diet? And when you finally do go, would it be rock-hard nuggets or something?
I think Dr. Oz did a show on that, with guest star Cameron Diaz. No joke.
That I understand.
The intravenous kind.