a rough translation from
Spektrum.de:
The chemistry teacher had warned us, "Never drink distilled water!" A miserable death with stomach bleeding and bursting cells would be in store for us if we had the idea of taking a sip from the spray bottle.
Our health-conscious neighbor, on the other hand, must have missed chemistry class. He recently ordered a distiller and now swears by the detoxifying effect of salt-free water. So what then?
It is true that cells can burst through what is known as osmosis, because their cell membranes are semi-permeable. This means that only water molecules can pass through; salts or sugar molecules are too large for passive transport through the membrane pores. Now, however, water flows into the cells in an effort to balance the higher concentration of salts and sugars inside with the concentration outside the cell. If the concentration gradient is large, sometimes so much water accumulates in the cells that the cell boundary eventually gives way: The cell bursts.
Our blood cells would not fare any differently, but we would have to be hooked up to a drip of distilled water.
Drinking it, however, has far fewer consequences. This is because chemically pure water is mixed with minerals again by the stomach acid and the ingested food, so that our cells do not even come into contact with it. Thus, even tea or coffee brewed with distilled water, which some people swear by for its fine aroma, poses no danger.
Some people even see "aqua distillata" as a health supplement, since it contains no harmful substances such as lead, nitrates and pesticides and is said to purify the body. The German Society for Nutrition - and with it most scientists - warn against the exclusive use of distilled water. This is because, particularly in the case of an unbalanced diet, the distillate removes potassium and sodium ions from the cells in the long term, thus upsetting the body's electrolyte balance.