Latest research examining the potential hazards of a high calcium intake finds an increased mortality risk among women with a high intake through diet and supplements.
The study found an increased risk for death from both all cause and cardiovascular disease (CVD) above a certain threshold of calcium intake, although the authors say the risk increase was only moderate for women whose intake was exclusively through diet - a more pronounced risk was seen among women with a high dietary intake who also used calcium supplements.
The authors emphasise that mortality was not increased with customary levels of calcium intake.
The study, reported in the BMJ, included 61,433 women, born between 1914 and 1948, from a Swedish mammography cohort, established in 1987-1990.
The researchers used data from food frequency and lifestyle questionnaires, completed by all the women at baseline and by 38,984 women in 1997, to establish their levels of calcium intake.
Using Swedish registry data, the researchers established that 11,944 (17%) women died during the median follow up of 19 years and 1,094,880 person-years at risk. Of these women, 3862 died of CVD, 1932 of ischemic heart disease, and 1100 of stroke.
Analysis showed that the highest rates of death from all causes and CVD, but not from stroke, were among women with the highest dietary calcium intake, above 1400 mg/day. Compared with dietary intakes of 600-1000 mg/day, calcium intake above this level was associated with 1.40, 1.49, and 2.14 times the risk for death from any cause, CVD, and ischemic heart disease, respectively.
These risk increases were seen after multivariable adjustment, and the authors note that a shift from lower to higher risk with the multivariable model was mainly the result of adjusting for use of calcium containing supplements.
A quarter of the study population in 1997 were taking calcium supplements; among these women, a high dietary calcium intake exceeding 1400 mg/day was associated with an even greater excess mortality risk.
Indeed, in a separate analysis from the date of the second survey, stratified according to no use or type of calcium supplement use, women with the highest dietary calcium intake who took calcium tablets (500 mg per tablet) had a 2.57-fold increased risk for all cause mortality compared with those with a dietary intake of 600-999 mg/day, and those who took any type of calcium-containing supplement had a 1.51-fold increased risk, whereas the risk was not significant among non-users of calcium supplements.
"Thus among women with a high dietary intake of calcium, the addition of calcium supplements increased the risk of death in a dose dependent fashion," writes the team.
"Our present data together with previous observations suggest that for the prevention of fractures in elderly people and simultaneous avoidance of possible serious adverse events related to a high calcium intake (such as higher risk of hip fracture, cardiovascular disease, renal stone, and, as observed in the current study, mortality) emphasis should be placed on people with a low intake of calcium rather than increasing the intake of those already consuming satisfactory amounts," Karl Michaëlsson (Uppsala University, Sweden) and team conclude.
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