Women who have a high intake of calcium from their diet have an increased risk of death from all causes, a new Swedish study has found.
The research, published on bmj.com, also suggests that women with a low calcium intake have a heightened risk of death.
Scientists from Uppsala University in Sweden conducted the study after a number of recent trials had suggested a link between calcium supplements and a higher risk of ischemic heart disease and stroke, while another trial did not indicate there was a link and few trials have explored the association.
To put the association to the test, they studied 61,443 Swedish women, born between 1914 and 1948, over an average period of 19 years.
They used data from the Swedish Cause of Death Registry, while diet information was taken from the Swedish Mammography Cohort.
Supplemental calcium was included in the figures for calcium intake, with the mean intake in the lowest quartile found to be 572mg per day - equivalent to five slices of cheese - and the mean intake in the highest quartile was 2,137mg/day.
The researchers also obtained information from the women regarding their menopausal status, post-menopausal oestrogen therapy, parity information, weight and height, smoking habits, levels of physical activity and education.
In the 19 years of follow-up it was reported that 11,944 women (17%) died and, of those, 3,862 (32%) died from cardiovascular disease; 1,932 (16%) from heart disease, and 1,100 (8%) from stroke.
The women with a dietary calcium intake above 1,400mg/day showed the highest rates of all-cause, cardiovascular and heart disease.
And women with a sub-600mg/day intake also showed higher death rates.
Those with a dietary intake of over 1,400mg/day who also took calcium supplements had a higher death rate compared to those not taking supplements.
And those with a dietary intake of over 1,400mg/day had more than double the risk of death compared to women with an intake of between 600mg and 999mg/day.
The researchers believe that an especially high or low intake of calcium is associated with such a risk because it overrides normal homeostatic control, which in turn leads to changes in the levels of calcium in the blood.
They suggest that policies aimed at reducing the fracture risk among the elderly should focus on those who have a low intake of calcium, as opposed to a blanket approach in which those with a satisfactory intake are given supplements.
A high calcium intake is generally recommended by experts due to its key role in human physiology, with over 60% of middle-aged and older women in the USA now taking supplements.