ORIGINAL ARTICLES
Scand J Work Environ Health 1991;17:91-4
Magnesium and calcium in drinking water and cardiovascular
mortality
Ragnar Rylander, MD, Håkan Bonevik, MD, Eva Rubenowitz,
MD
Department of Environmental Hygiene, University of
Göteborg, Göteborg, Sweden.
Data on the hardness of drinking water were collected from 27
municipalities in Sweden where the drinking water quality had
remained unchanged for more than 20 years. Analyses were made of
the levels of lead, cadmium, calcium, and magnesium. These
water-quality data were compared with the age-adjusted mortality
rate from ischemic heart and cerebrovascular disease for the
period 1969-1978. Lead and cadmium were not present in detectable
amounts except in one water sample. A statistically significant
inverse relationship was present between hardness and mortality
from cardiovascular disease for both sexes. Mortality caused by
ischemic heart disease was inversely related to the magnesium
content, particularly for the men (P<0.01). The rather small
set of data supports results from previous studies suggesting
that a high magnesium level in drinking water reduces the risk
for death from ischemic heart disease, especially among men,
although the possible importance of confounding factors needs
further evaluation.
Key terms: cerebrovascular disease,
ischemic heart disease, magnesium, water hardness.
Several epidemiologic investigations performed during recent
decades have demonstrated an inverse relationship between water
hardness and death from cardiovascular disease. The first
observation was made in 1957 (1) and was subsequently elaborated
upon in investigations in many other countries (2-4). A
particularly relevant study was reported by Crawford et al (5),
who followed the mortality rate in 11 English cities where the
water hardness had changed between 1950 and 1960. Hardness had
increased in five cities and decreased in six. Mortality from
cardiovascular disease increased about 10% in the general
population during the period of study. In the cities where
hardness had decreased, mortality had increased by 20%. In the
cities where hardness had increased mortality had increased by
8.5 %.
Various suggestions leave been offered concerning, the
causative agent for the relationship between death from
cardiovascular disease and water hardness. Two principles have
been discussed, namely, a toxic effect induced by the
contamination of lead or cadmium or a protective effect from the
water content of calcium or magnesium (2).
The importance of magnesium has been investigated in some
previous field studies. Schroeder (6) found a negative
relationship between mortality from heart disease among 45- to
64-year-old men and the amount of magnesium in drinking water (r=
-0.30). In a Canadian study, Anderson (7) found a similar
association, as did Leary et al (8) in South Africa (r = -0.68).
Punsar & Karvonen (9) studied mortality from cardiovascular
disease and water quality in two areas of Finland. They suggested
that mortality from ischemic heart disease could be related to
magnesium and chromium, but no relationship was found for sudden
death. In a case-referent study of 30- to 64-year-old men with
their first acute myocardial infarction, the myocardial disease
rates were inversely related to the level or magnesium and
fluoride in the drinking water (10). The risk ratio for low
amounts of magnesium in drinking water was 2.0 for cases versus
hospital referents and 4.7 for cases versus general population
referents.
The present study was designed to evaluate the possible roles
of lead, cadmium, magnesium, and calcium in cardiovascular
mortality. Data on water quality were obtained from 27 Swedish
municipalities, and the water was analyzed for its content of
these elements. The data on water quality were compared with the
mortality rates for ischemic heart disease and cerebrovascular
disease among the male and female residents of the
municipalities.
Material and methods
For the study, 44 Swedish municipalities were selected which
had a range of water hardness from 0.8 to 20.7 dH (German
hardness degrees).
The waterworks in each municipality received a questionnaire
requesting information on water hardness during the period
1955-1985 and on whether any changes had occurred in the water
supply or treatment of the water during that period. Certain
municipalities were excluded as changes in water quality (eg,
change from well water to surface water) had been made during
that period. The final material comprised hardness data from 32
municipalities.
Water samples were requested from these areas for the
determination of the levels of calcium, magnesium, lead, and
cadmium. The analyses were made at Chalmer's Institute of
Technology with atomic absorption spectrophotometry using the
flame technique (magnesium and calcium) and oven technique (lead
and cadmium). The detection threshold was 1 µg/l for lead
and 0.5 µg/l for cadmium. Complete analysis data were
obtained from 27 municipalities.
Data were obtained on mortality from ischemic heart disease
and cerebrovascular disease (International Classification of
Diseases, codes 410-414 and 430-438) in these 27 municipalities
for the period 1970-1980 from the Swedish Planning and
Rationalization Institute for Health and Social Services. The
data for each municipality were standardized for age groups in
10-year intervals and related to mortality data for the whole
country during the period 1969-1978. The total population of the
municipalities ranged between 7794 and 100,875 inhabitants (mean
values 1969 to 1978). The total number of inhabitants in the
municipalities was 809,655.
The mortality index for each municipality was compared with
the water hardness data and the results of the analysis regarding
calcium, magnesium, lead, and cadmium.
The relationship between the water-quality variables and
mortality was tested with a special test designed for problems in
which environmental factors and Poisson distributed variables are
studied (11). The statistical model used has the realistic
property that the expected numbers of deaths are allowed to vary
between municipalities because of reasons other than water
quality, age, or sex distribution. The size of the municipalities
was also taken into account. Two-sided tests were used.
Results
The levels of lead and cadmium were below the detection
threshold in all the water samples analyzed except one, in which
1.05 µg of cadmium per liter of water was determined.
The hardness of the different water samples ranged between
0.80 and 20.7 dH. The amount of calcium ranged from 3.4 to 131
mg/l and that of magnesium from 0. 57 to 15.0 mg/l. There was a
high correlation between calcium and hardness (0.93). Magnesium,
however, was not as highly correlated with either hardness (0.71)
or calcium (0.55).
Table 1 shows the correlation coefficients and statistical
significance for the relation between water hardness and the
amount of calcium and magnesium with mortality from ischemic
heart disease and cerebrovascular disease. An inverse
relationship was observed for the relation between hardness and
mortality from ischemic heart disease among the men. The same
relationship, although less pronounced, was found for the women.
Mortality from cerebrovascular disease was also inversely related
to water hardness, although the relationships were less
pronounced. Magnesium was inversely correlated with ischemic
heart disease mortality among the men and the women. For
cerebrovascular disease, calcium showed the strongest inverse
relation for the men, but the relation for the women was not
statistically significant.
The relative risk for mortality from ischemic heart disease
among the men in the different municipalities and the magnesium
content in their drinking water is illustrated in figure 1.
Discussion
The analyses of the water samples were made at a time (1988)
later than the period during which the hardness data were
collected (1970-1990). In the design of the study, however, only
municipalities in which the water quality or source had not
varied during the last two decades were included. It is thus
likely that the 1988 analysis also reflected the magnesium and
calcium content over the time from which the mortality data were
obtained.
The results from the study reconfirm the previously observed
inverse relationships between mortality from cardiovascular
disease and water hardness. The relationship was stronger for the
men than the women, as has also been found in previous studies
(2).
The hypothesis that toxic materials in drinking water can be
related to cardiovascular mortality could not be tested in this
study, as significant amounts of lead or cadmium were not found
in the water of the municipalities examined.
The results demonstrated a statistically significant
relationship between the amount of magnesium and ischemic heart
disease but not cerebrovascular disease. The possible importance
of confounding factors must be considered. Cardiovascular disease
is more common in built-up areas. However the populations of the
municipalities ranged in size between about 8000 and 100000, and
there was no relationship between population size and mortality
from ischemic heart disease or cerebrovascular disease.
Magnesium in the drinking water showed a tendency towards
lower levels in municipalities in the north of Sweden, but this
geographic distribution was not uniform. Smoking habits represent
an important possible confounder, but a covariation between the
extent of smoking in different municipalities and the magnesium
content of the water is not likely. It is also unlikely that
there should be a direct relation between other traditional risk
factors such as consumption of fat and the level of magnesium in
drinking water; however the problem or possible confounding
factors should be further evaluated.
The relation between ischemic heart disease in men and water
magnesium levels illustrated in figure 1 is probably complex. For
the municipalities with low levels of magnesium in the water,
both high and low mortality could be found. This variation could
be dependent upon random variation or a variation in traditional
risk factors such as diet and smoking habits. The low mortality
in all the municipalities with high levels of magnesium (>8
mg/1) suggests that magnesium could have a protective effect
stronger than the negative influence of the traditional risk
factors.
The significant association between mortality from ischemic
heart disease among the men and the amount of magnesium in the
water is supported by knowledge of the functions of magnesium. It
is an activator for several enzyme systems and is essential for
cell membrane permeability and neuromuscular excitability. A lack
of magnesium leads to a decrease in the concentration of
intracellular potassium and an increase in calcium levels (12).
Magnesium deficiency may increase the risk for cardiac arrhythmia
(13) and the contractility of blood vessels, as shown in animal
experiments (14, 15).
The question has been raised of how the relatively small
intake of magnesium via water, about 10% of the total intake, can
have a critical significance for the amount of magnesium in the
body and heart muscle. However, several.studies support this
hypothesis. Anderson et al (16) investigated 54 cases of
accidental death in cities with soft water and 29 cases in cities
with hard water. A significantly lower amount of heart muscle
magnesium was found in the cases in cities with soft water.
Crawford & Crawford (17) showed that the amount of magnesium
in coronary arteries was lower among persons who lived in areas
with low magnesium levels.
Only limited information is available regarding the uptake of
magnesium in food from the intestines. It may be that magnesium
in a dissociated form in water has a higher uptake rate than
magnesium in food. This hypothesis is supported by the results of
two experimental pilot studies (18, 19), but more extensive
investigations are required before final conclusions can be
drawn.
In order to evaluate further the question of the proportional
uptake of magnesium from water and food, studies need to be made
in which the body balance of magnesium is prospectively measured
in individuals with controlled intake of magnesium in water and
food. Such studies are in progress.
In summary, the present study suggests that mortality from
ischemic heart disease, particularly among men, can be related to
the amount of magnesium in drinking water. Further studies on the
relative role of different sources of magnesium in food and
water, as well as expermental studies, are needed before these
conclusions can be transformed into public health practice.
Acknowledgments
This study was supported by funds from Wilhelm and Martina
Lundgren's Research Fund and the Medical Society of
Göteborg, Sweden. The magnesium analyses were performed by
Ms E Axen, Chalmer's Institute of Technology, Göteborg,
Sweden, and the statistical analysis was carried out by A Oden,
PhD.
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