I don’t want to derail your question regarding heavy water, but the notion that ultrapure (or deionized) water is unsafe to drink is false.
Statements like this are floating all over the internet and repeated in many laboratories. I can only attribute its basis to a poorly designed thought experiment. “Valuable minerals” leech out of the body? To where?
In theory and in practice, yes, ions will diffuse from high concentrations to areas of low concentrations. So if you take pure water, devoid of all ions, and introduce it to your mouth and gut, the ionic concentration of all contacted fluids will decrease. This does not mean it is dangerous, or even problematic. The water you drink from the tap also has a much lower ionic concentration than any of your bodily fluids. Not a big deal. For the most part, any water that is introduced to your gut is absorbed into the blood stream. Any excess is expelled by your kidneys. So even if deionized water pulled, via osmotic forces and/or diffusion, ions from your body, they would just be reabsorbed. Net loss of “valuable minerals” is zero.
To make deionized water dangerous, you would have to drink massive quantities, quantities that would lead to water intoxication (which would occur at said quantities whether it was deionized or not).
I personally have drank dozens (possibly hundreds) of gallons of 18 mega-ohm, deionized water. In my old lab, the DI system was more easily accessible than the water fountains, so I’d drink a pint or two a day.
DI water does taste different, much in the way bottled water tastes different from tap water. I personally wouldn’t call it bitter, but it doesn’t taste as good as most bottled water. It was certainly better than the tap water in our building.
Dangerous? No. One could make try to make the argument that it could be bad for your teeth because it could leach calcium from them. However, I would be very surprised if it was more damaging than acidic beverages like soda or orange juice.
As for health effects of residual ion exchange resin in DI water, I’ll take my chances. The amount of resin present in DI water is trivial anyway.
I apologize if this is seen as a highjack, but seeing as your OP was based partly on incorrect information, I felt it was appropriate to address this.
On edit: I see lazybratsche addressed similar points. Also, my spelling errors.