Don't read any further if you are at all squeamish about the functions of the body.
Sorry about the bluntness of the title. Something I was taught when very much younger was a saying along the lines of, "If you don't shit, you die". It means what it says. You have to eat to survive and there is a by-product of that eating. Maybe it has some ramifications if you are also constipated.
I would like to debunk the theory, if I may. Poo is just waste that the body can't extract any nutrients from, correct? Surely we have the technology to remove from our food anything that the body can't make use of. Then, we could survive quite happily without the need to produce poo. Don't you think? The large intestine could become irrelevant. The Greeks have found another use for it.