Water typically generated for use in a laboratory setting is deionized by resins which pull out ions and produce "pure water". This water, sometimes called 18 mega-ohm, for its resistivity, can be considered corrosive, as it has virtually no dissolved ion content and thus will strip it out of certain materials, thus soft iron and copper pipes are not recommended for use in transporting such water. Using such materials will "recontaminate" your water supply. Treated stainless steel or certain plastics (PVDF) are commonly used.
A similar process would likely occur if a person drinks DI water. While it is not acutely toxic, i.e. one glass won't hurt you, if used as the only water source, over time it may cause damage from electrolyte imbalance, etc. I saw it described as the "slow death" by some guys on an engineering forums page . Perhaps a little dramatic, but it could reasonably happen.
I also found a paper looking at treatment of zebra mussels with DI water as a means of abatement, it seems to be a relatively good killing agent, especially in combination with chemical pesticides.
We work with 18 meg water in our lab and use a 2-stage Millipore Deionizer.
While it's true that DI water will leach salts out of your body, this will not kill you over a few sips, as the Doc suggests. I have drunk off our millipore several times (totalling about 2-3 glasses over 4 years) and have even offered a sip of water from the first stage (which is significantly lower than 18 meg) to someone who I might be explaining the system to. The taste difference is easy to notice. On the other hand, continuous consumption is not likely to be a good idea at all.
On a side note, since we work a lot with DI (especially boiling DI) we are familiar with the corrosiveness of this beast, and it's no joke what it can do. We've spent thousands of bucks on stainless components (PPE or PVDF won't handle the temperatures), for our DI needs (we consume, at peak, about 5 gallons an hour).
I meant that many metals (and some other materials) rust/corrode/degrade several times faster on exposure to DI water than to regular water. Even some of our stainless fittings have to get replaced periodically because of rusting. And once, we had a large DI spill on the floor, and over the weekend following the spill, the floor tiles had been so badly destroyed that they had to be replaced. This never happened with regular water, which gets spilled quite often.
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