Findings of the study were published in the European Heart Journal.
It showed that taking 1,000mg of calcium over 12 months would lead to significantly higher number of CVD-related cases, as compared to taking calcium and vitamin D together.
However, the incidence of CVD events did not differ between individuals taking only calcium supplements and those who did not take calcium supplements.
The decade-long retrospective cohort study looked at 514,866 South Koreans aged 40 to 79 and were suffering from osteoporosis.
The mean age of the study population was 59.9 ± 8.8 years and 87.9 per cent of them were females.
The subjects were categorised into three groups: 1) those who took only calcium supplements (567 individuals) 2) those who took both calcium and vitamin D (10,730 individuals), and 3) those who did not take calcium or vitamin D (11,297 individuals).
The mean calcium dose in the two groups taking calcium supplements was 537.8mg daily, and the mean duration of calcium supplementation was 22.6 months.
Results showed that the number of CVD events was 57 in the calcium only group – which is equivalent to a hazard ratio of 1.54.
However, the number of CVD events in the group taking both calcium and vitamin D was significantly lower, with a hazard ratio of 1.12.
The incidence of non-fatal myocardial infarction (MI) was also lower in the group taking calcium with vitamin D group with a hazard ratio of 1.13, while that of the calcium only group was 1.89.
There was no significant differences between more serious cases of cardiovascular deaths, where the hazard ratio for the calcium only group and calcium and vitamin D group was 0.94 and 0.96 respectively.
“Calcium supplementation did not increase the risk of composite CVD event.
“The risks of non-fatal MI, ischaemic stroke, and death due to CVD causes were not different between the [calcium only and control] groups,” said the researchers.
“[However], taking calcium supplements alone without vitamin D supplements was associated with an increased risk of CVD, especially non-fatal MI, even among populations with low calcium intakes, such as the Korean population.
“Therefore, our study confirms the cardiovascular safety of calcium supplements with vitamin D, but excess calcium supplements without vitamin D are associated with a potential risk of CVD,” they added.
The adverse effect of calcium supplementation on the cardiovascular system has been a concern ever since a secondary analysis of the Auckland Calcium Study reported an unexpected warning signal of a higher risk of MI that came with calcium supplementation.
There was a dose and duration dependent effect on calcium only supplementation on the risk of CVD events.
In patients taking only calcium supplement, 138 took at least or more than 1,000mg of calcium daily and there were 26 events of CVD cases in this group.
Another 146 and 265 took less than 500mg and 500mg to 1,000mg calcium respectively and reported 16 and 15 events respectively.
“Therefore, when calcium supplementation alone is used, the elemental calcium dose should be restricted up to 1,000mg/day for cardiovascular safety,” said the researchers, adding that calcium supplements containing mainly calcium salts could increase serum calcium acutely.
The researchers have pointed out a number of reasons on why calcium supplementation alone could increase CVD risk.
First, serum calcium levels will rapidly increase after taking calcium supplements and this leads to accelerated vascular calcification.
Second, calcium plays an important role in blood clot coagulation and progressive arterial calcification in patients receiving calcium supplementation have been reported in the past.
On the other hand, vitamin D could protect heart vessel walls against atherosclerosis by increasing the expression of anti-inflammatory cytokines, decreasing the expression of pro-inflammatory cytokines, and down-regulating plaque-destabilising enzymes.
In fact, vitamin D deficiency is a well-known risk factor for CVD, said the researchers.
Source: European Heart Journal
Cardiovascular risks associated with calcium supplementation in patients with osteoporosis: a nationwide cohort study
Authors: Kyoung Jin Kim et al