If you buy a bottle of DI water to put in your battery/steam iron etc, it says on it "Do not drink"
The water we produce is the same stuff. Incidentally, tropical fish afficionados buy RO water from pet shops at vast expense - possible earner?
The reason you shouldn't drink it is that pure water has an aggressive need to revert to its natural impure state - as soon as it comes into contact with any substance it starts dissolving material, such is the nature of naturally occurring water.
If you drink it, what's the first soluble substance it comes into contact with? That's right - YOU - or rather your mouth/throat/gullet/stomach.
Hence Sparklebright's guts ache

Drink enough and it can seriously damage your stomach lining