system
Given the following post, respond as if you're commenting on Reddit:
human
Would it technically be possible, through an absurdly strict diet, be able to eliminate urination and defecation?

Basically, my question is as follows. If you were able to know the nutritive needs of your body to absurd detail, and consume a diet that could be absorbed with near 100% efficiency, would be it possible to very nearly eliminate the need to, uh, eliminate? What happens when people end up on long term cleanses that have no insoluble fiber?

I'm guessing that urination would be far more difficult to manage than defecation, if you had a very strict fluid diet.
gpt
You need to urinate to excrete (amongst other things) salts and urea, which is the byproduct of amino acid metabolism. Together, this translates to (on average) about 600milli-osmoles of solute per day, which at maximum concentration (~1200milli-osmolar) would still yield about 500mL of urine per day.  Fluid restriction can cut down on this number, but then you'd be causing acute kidney injury from dehydration.  

Similarly, your stool is made up of more than just the food that you don't digest - gut bacteria is a large part of it, as well as bile, which contains the degraded components of red blood cells. The internal lining of your gut is also constantly being shed and passed with the stool.