AUCKLAND, New Zealand—Calcium supplements may increase the risk of heart attack and stroke in healthy postmenopausal women, according to a new study in the British Medical Journal.¹

"The present data do not permit definitive conclusions to be reached in this regard, but do flag cardiac health as an area of concern in relation to calcium use and mandate that this is assessed carefully in future studies of calcium supplementation," conclude study authors led by Mark J. Bolland, MBChB, FRACP, a research fellow at the University of Auckland in New Zealand. "In the meantime this potentially detrimental effect should be balanced against the likely benefits of calcium on bone, particularly in elderly women."

While preliminary, the association between calcium and heart risk is "not necessarily surprising," the study authors note. Calcium supplements raise serum calcium levels, which could accelerate vascular calcification.

Twice as many MIs in women who took calcium supplements

In the new study of 1471 postmenopausal women with a mean age of 74, a total of 732 women were randomized to receive 1 g of elemental calcium daily as citrate and 739 were randomized to receive an identical placebo. The report is a preplanned secondary analysis of a randomized controlled trial looking at the effects over 5 years of calcium on bone density and fracture in healthy postmenopausal women.

There were 45 myocardial infarctions (MIs) among 31 women taking calcium, compared with 19 MIs among 14 women taking placebo (P = .01), the study showed. In addition, the composite endpoint of MI, stroke, or sudden death was also more common in the calcium group. There were 101 such events in 69 women taking calcium, compared with 54 such events in 42 women taking placebo (P = .008).

MI remained more common in the calcium group after adjudication. Specifically, there were 24 events in 21 women taking calcium and 10 events in 10 women taking placebo (RR 2.12). For the composite endpoint, there were 61 events in 51 women in the calcium group and 36 events in 35 women in the placebo group after adjudication (RR 1.47).  

The researchers then reviewed hospital records and death certificates to look for any unreported events. They found that MI was still more common in the calcium group, but there was some attenuation of the calcium effect: 36 MIs in 31 women taking calcium versus 22 MIs in 21 women on placebo. Their analysis showed the relative risk of MI was 1.49, the composite endpoint was 1.21. "The present study does not unequivocally show an adverse cardiovascular effect of calcium, but suggests that this matter needs to be considered carefully before calcium supplementation can be broadly advocated," they study authors conclude.

Findings may change how doctors think about calcium supplements

MSK Report editorial board member Nancy E. Lane, MD, director of the Aging Center at the University of California at Davis, said that the new study will likely have an effect on her practice. Going forward, "I will be sure my patients receive only 1500 mg a day of calcium, which is the combination of what they get in their diet plus supplements, and I probably will only supplement 750 mg a day if calcium metabolism appears to be adequate."

Patients with poor renal function should receive even less, Dr. Lane said, echoing comments made by the researchers in their conclusion. "Individuals with poor renal function may have more trouble with calcium supplementation and so this group should have reduced supplements or no supplements."

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