Wikipedia cites one 1970 study by Voors claiming that drinking hard water lessens the odds of dying from atherosclerosis. The stud itself even makes some bolder claims stratified by race, namely that lithium was protective for whites and vanadium for non-whites.

A much more recent (2008) article by Burton makes no mention of either lithium or vanadium, but says:

The idea that hard water—particularly that with higher magnesium concentrations—helps ward off cardiovascular problems has been around for 50 years. However, due to the ecologic nature of most studies, uncontrolled confounding factors, and the different variables and outcomes measured, no firm conclusions have ever been drawn. The WHO is therefore coordinating worldwide efforts to compare cardiovascular morbidity before and after changes in the calcium/magnesium content of water supplies.

Approximately 10 years later, has anything conclusive come out this? Is hard water in general protective of cardiovascular problems? Is any mineral in particular important?

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    This is interesting because magnesium is a critically important electrolyte in cardiology and it's generally estimated that 60% of the western population is magnesium deficient due to diet and modern farming practices. Therefore, water containing high levels of magnesium would be beneficial to many.
    – Carey Gregory
    Dec 21 '17 at 2:10

Nothing much new.

Potential Health Impacts of Hard Water (PubMed Central, 2013)

Although, there is some evidence from epidemiological studies for a protective effect of magnesium or hardness on cardiovascular mortality, the evidence is being debated and does not prove causality. In spite of this, drinking-water may be a source of calcium and magnesium in the diet and could be important for those who are marginal for calcium and magnesium intake.

The World Health Organization (2011) mentions several studies, none of which found any causal relationship between water magnesium or calcium content or total hardness and cardiovascular health.

Cardiovascular diseases and hard drinking waters: implications from a systematic review with meta-analysis of case-control studies (PubMed, 2017)

Hard water consumption seems to be protective against CVD. However, the high heterogeneity and the existence of publication bias limits the robustness and generalizability of these findings.

Anyway, hard water may not contribute that much to your calcium and magnesium intake. 2 liters of hard water (an average daily requirement) will likely contain only about 100 mg of calcium (Recommended Daily Allowance is 1,000 mg/day) and 50 mg of magnesium (RDA = 400 mg/day).

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