Deionized water is very pure by one measure and is not sold or handled as a drinking water product, and the reason has nothing to do with the mineral content people usually worry about. It is that ion exchange removes only things carrying an electrical charge, and a good deal of what would make water unsafe carries none.
What ion exchange leaves behind
Cation and anion resins swap dissolved ions for hydrogen and hydroxide, which combine into more water, so dissolved salts go essentially completely and conductivity falls close to zero. What passes straight through is everything uncharged: dissolved organic compounds, particulates, and crucially bacteria and viruses. So deionized water can read as extremely pure on the usual meter while being microbiologically unsuitable, and the meter gives no hint of it.
The resin bed as a habitat
This is the part that makes it more than theoretical. A resin bed is a large moist surface area with no disinfectant residual, held at room temperature, often for long periods between uses. That is close to ideal conditions for bacterial growth, and a cartridge that has stood unused can deliver water with a considerably higher bacterial count than the tap water that went into it. Industrial systems handle this with sanitization regimes that domestic setups do not have.
The mineral question, in proportion
The absence of calcium and magnesium is the objection usually raised and it is the lesser one. Drinking water contributes a small share of dietary minerals next to food, and the claim that mineral free water leaches minerals from the body is not supported. The USGS classes water as soft at 0 to 60 mg/L as calcium carbonate through to very hard above 180, so what is forgone depends on the supply it replaced. That is a nutritional footnote rather than a safety issue.
What to drink instead
If the goal is water with very little dissolved in it, distilled water is the product sold for drinking, because the phase change removes organisms and organics as well as ions. If the goal is a household drinking supply, reverse osmosis achieves a comparable result far more cheaply and with carbon stages handling organics. Either way, a remineralizing stage answers the flat taste. Our softener guide covers the mineral question at property scale.
Questions people ask about can you drink di water
Why is deionized water not sold for drinking?
Ion exchange removes only charged particles, so bacteria, viruses and uncharged organics pass through untouched.
But it reads as extremely pure, does it not?
On a conductivity meter, yes. That measure responds only to dissolved ions and says nothing about micro organisms.
Is the missing mineral content the problem?
No, that is the lesser objection. Food supplies the great majority of dietary calcium and magnesium.
What should I drink instead?
Distilled water is sold for drinking, and reverse osmosis achieves a similar result far more cheaply for a household.