Purified water can come from any source, including spring water, well water, seawater, or municipal water. This source water is then processed by reverse osmosis or deionization to produce a water that is indistinguishable from distilled water from any other source. Purified water contains no dissolved solids. Purified water may also be unhealthy because minerals and ions are being pulled out of it due to reverse osmosis.
The human body needs ions for the brain to function properly. Many of these ions are attained through water, often under the name electrolytes. Distilled and purified water lack these ions, so prolonged ingestion of distilled or purified water may lead to brain malfunction.
The drinking of distilled water has been both advocated and discouraged for health reasons. The lack of naturally-occurring minerals in distilled water has raised some concerns. The Journal of General Internal Medicine published a study on the mineral contents of different waters available in the US. The study concluded, "drinking water sources available to North Americans may contain high levels of Calcium, Magnesium, and Sodium and may provide clinically important portions of the recommended dietary intake of these minerals," and further encouraged individuals to "check the mineral content of their drinking water, whether tap or bottled, and choose water most appropriate for their needs." Since distilled water is devoid of minerals, supplemental mineral intake through diet is needed to maintain proper health.
It is often observed that consumption of "hard" water, or water that has some minerals, is associated with beneficial cardiovascular effects. As noted in the American Journal of Epidemiology, consumption of hard drinking water is negatively correlated with atherosclerotic heart disease. Since distilled water is free of minerals, it will not have these potential benefits.
It has been suggested that -- because distilled water lacks fluoride ions that are added by many governments (e.g. municipalities in the United States) at water treatment plants using fluoridation for its supposed effect on the inhibition of cavity formation -- the drinking of distilled water may increase the risk of tooth decay due to a lack of this element.