Health Stream Literature Summary - Issue 58 - June 2010

Relationship between tap water hardness, magnesium, and calcium concentration and mortality due to ischemic heart disease or stroke in the Netherlands.
Leurs LJ, Schouten LJ, Mons MN, Goldbohm RA and Van Den Brandt PA. (2010) Environmental Health Perspectives, 118(3); 414-420.

An evaluation published in 2007 of the epidemiological evidence on the effects of calcium and magnesium in drinking water on cardiovascular disease rates, concluded that the available evidence was not sufficiently strong to infer that low levels of calcium or magnesium in drinking water are significant factors in the causation of cardiovascular disease. However a meta-analysis of case-control studies published in 2008 found evidence of a significant inverse relationship between magnesium levels in drinking water and cardiovascular mortality. Due to the inconclusive results of previous studies, this study was conducted to investigate the association between water hardness and ischaemic heart disease (IHD) or stoke mortality in the ongoing Netherlands Cohort Study (NLCS). This is a large prospective cohort study on diet and cancer that began in 1986 with a total of 58,279 men and 62,573 women between 55 and 69 years.

Emerging cases were collected from the whole cohort whereas accumulated person-years in the cohort were estimated from a subcohort. A subcohort of 5,000 subjects from the NLCS was randomly sampled immediately after baseline. At baseline, all cohort members completed a self-administered questionnaire on dietary habits, beverage consumption, anthropometry (weight and height) and other risk factors related to cancer and the presence of cardiovascular disease. There was a 150-item semi-quantitative food-frequency section of the questionnaire which concentrated on habitual consumption of food and beverages during the year preceding the start of the study. Information on intake of minerals was collected from this food-frequency section of the questionnaire. Data on calcium and magnesium concentrations in tap water was obtained in 1986 from all of the 364 pumping stations in the Netherlands. The calcium and magnesium concentrations of tap water were estimated for each home address by postal code. Tap water hardness was categorised as soft (less than 1.5 mmol/L calcium carbonate), medium hard (1.6-2.0 mmol/L calcium carbonate) and hard (greater than 2.0 mmol/L calcium carbonate). The 1986 information linking postal code and pumping station was verified for 1996 (the end of the investigation period) by the water companies as several pumping stations had introduced pellet softening in this period. Mortality data were obtained between January 1978 and December 1996 by linking the NLCS database to the Central Bureau of Genealogy. There were 18,091 deaths identified in this period. Of all deaths, 6,735 were primarily related to cardiovascular disease. A multivariate case-cohort analysis was conducted based on 1,944 IHD mortality cases and 779 stroke mortality cases and 4,114 subcohort members.

In 1986 in the Netherlands, tap water calcium and magnesium concentrations ranged from 15 to 157 mg/L and from 1.7 to 26.2 mg/L, respectively. Among the subcohort members, 36% lived in an area with soft water, 26% received medium hard water and 38% received hard tap water. In both the age-adjusted and multivariate analysis, no association was found between the concentration of calcium and magnesium in tap water or the total hardness of the water and IHD mortality or stroke mortality among men or women. When the analysis was restricted to those with the 20% lowest dietary magnesium intake (upper limit of the lowest quintile = 285 mg/day for men and 255 mg/day for women), no statistically significant relationship between increasing intake of magnesium from tap water and IHD mortality among men or women was found. For stroke mortality, a statistically significant inverse association was found among men in the fourth quintile versus the first quintile (beneficial association with higher levels of magnesium in water) whereas for women the opposite was found (adverse association with higher magnesium levels in water). When the analysis was restricted to subjects with the 20% lowest dietary calcium intake, no associated was found between the calcium concentration in tap water and IHD mortality or stroke mortality in either men or women.

The results from this study obtained in subjects with low dietary magnesium intake need to be interpreted with caution as results were opposite for men and women and this has not been previously reported, the number of cases and person-years on which this sub-analysis was based was limited and no other studies have analysed the relationship between tap water magnesium concentration and cardiovascular mortality in a subpopulation with low dietary intake. In the general population, food consumption contributes significantly more than tap water to the total intake of calcium and magnesium. It has been suggested however that waterborne magnesium is more easily absorbed by the body than dietary magnesium. Further research is required to examine the effect of tap water magnesium on IHD mortality or stroke mortality among men and women with low dietary magnesium intake.

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