Just one more point: the colon depends on solid material in order to push things through (it's how it works). Get rid of solid food in there and its muscles won't create any real transit.
If we only get the nutrients without the crap to poop out, much of our digestive tract has little to do. And physiology tends to revolve around "use it or lose it". Astronauts living weightless for a significant time lose muscle and bone mass because they don't need it. If you eat nothing but Soylent for a year, will you still be able to handle normal food?
That's really the only significant problem I can think of with the Soylent approach to eating. And it could be a serious one, though I suspect it's easy to mitigate by mixing the occasional real meal into the mix.
He apparently defecated every 40-50 days.
(Though the stories about this are pretty recent, the event being described happened in 1965.)
http://www.abc.net.au/science/articles/2012/07/24/3549931.ht...