Water entering your body with a trace mineral deficit will dilute the mineral content of your body, and you'll excrete these minerals every day in your urine.
One day or one week doing this is no big deal, but if it's your main water source and you do it all the time, you will start demineralizing your body and you'll be short of electrolytes which is known as hyponatremia.
Our house came with a ridiculous reverse osmosis filter on the kitchen sink, and the included TDS meter showed < 5 ppm. It seemed great at first, but we grew concerned about the potential for mineral deficit for our kids, so I disconnected it. But, we could find no definitive source of info on this topic.
It's hard to take the water fanatics seriously: the manual talks about how the filter "reprograms" the water so that "the water is imprinted with the far-infrared frequencies." And: "the water molecules are shaken by the special field in a way that, for a short time, the clusters and the hydrogen bridges are broken up", and "the water is carried forward in an s-pattern movement, identical to that of water in nature".
Then again, I'm the guy who disconnected it out of possibly baseless fear of demineralization, so I'm not in a position to throw stones.
People tend to get a lot of sodium from their diets making the sodium content in their water a non issue. The same is generally true of most other minerals, unless your diet is horrid the water you drink is not going to make a meaningful difference.
you'll be short of electrolytes which is known as hyponatremia
The endurance scene is full of people banging on about hyponatremia but I'll wager 1000 people have gotten dehydrated trying to avoid it for every 1 that was affected.
I've been drinking non-remineralized RO water almost exclusively since literally conception and I'm just fine. My daughter has done the same and she is fine.
Water is potentially a source of minerals but a varied diet works just as well. The situation may be different for people in developing countries with very limited diets but in rich nations with access to year-round fruits, etc it's not a problem.
