Hard drinking water does not protect against cardiovascular disease: new evidence from the British Regional Heart Study.

BACKGROUND: It has been previously suggested that hard drinking water in general, and in particular high calcium and magnesium intake from drinking water, protect against cardiovascular disease.
DESIGN: Prospective study of men from 24 British towns, with widely differing levels of hardness in drinking water.
METHODS: A total of 7,735 men aged 40-59 years were recruited during 1978-1980. Estimates of town-level water hardness were available and tap water samples, taken from 947 participants who also answered a questionnaire about water consumption, were used to calculate individual calcium and magnesium intakes. Men were followed for incident of major coronary heart disease (CHD) and stroke, and CHD mortality for 25 years.
RESULTS: Water hardness varied from 0.27 to 5.28 mmol/l in the 24 towns. A weak inverse association was found between water hardness and incidence of cardiovascular disease (CVD) [hazard ratio (HR), 0.96 per two-fold increase, 95% confidence interval (CI), 0.91-1.01, P=0.08 after adjustment for age and seven established coronary risk factors]. No association was observed with CHD incidence (adjusted HR, 0.99, 95% CI, 0.94-1.04, P=0.62) or mortality (adjusted HR, 0.96, 95% CI, 0.90-1.02, P=0.18). Individual magnesium intake showed a positive, rather than an inverse, association with CHD incidence (adjusted HR, 1.10 per two-fold increase, 95% CI, 1.01-1.20, P=0.045); individual calcium intake was unrelated to CHD or CVD end points.
CONCLUSIONS: This study suggests that neither high water hardness, nor high calcium or magnesium intake appreciably protect against CHD or CVD. Initiatives to add calcium and magnesium to desalinated water cannot be justified by these findings.