Author World Health Organization
The World Health Organization details the benefits for drinking water with magnesium concentrations. There is also evidence of benefit for drinking water with calcium. Studies have shown magnesium’s protective action on the cardiac system. Hard water, defined as water that contains mineral salts (as calcium and magnesium ions) that limit the formation of lather with soap, can be a source of needed calcium and magnesium. The reintroduction of calcium and magnesium into demineralized water may provide health benefits to consumers. There are no known harmful human health effects associated with the addition of calcium and magnesium and the nutritional benefits are well known. In addition, some evidence exists for benefits associated with other diseases (stroke, renal stone formation, cognitive impairment in elderly, very low birth weight, bone fractures among children, pregnancy complications, hypertension, and possibly some cancers). Adding calcium and magnesium to the demineralized water would be a relatively inexpensive preventive intervention that does not require individual behavioral change. The intervention could not only provide health benefits but also help reduce medical care costs. It has been suggested that reduced cardiovascular mortality and other health benefits would be associated with minimum levels of approximately 20 to 30 mg/l calcium and 10 mg/l magnesium in drinking water.
http://www.who.int/water_sanitation_health/dwq/nutrconsensusrep.pdf