Calcium supplements—too much of a good thing?

Calcium supplements are often taken to stave off osteoporosis. Unfortunately, there is some evidence that they can increase the risk of heart attacks. This has been difficult to assess because in most studies, some of the participants were already taking their own calcium supplements as well as the ones assigned by the test protocol. For example, one of the largest calcium studies was the Women’s Health Initiative Calcium/Vitamin D Supplementation (WHI CaD) study. This seven-year long study included over 36,000 postmenopausal women who were allocated to either the calcium and vitamin D (1g calcium and 400 IU vitamin D daily) group, or the placebo group. Both placebo and CaD groups included women who were already taking calcium supplements. Needless to say, this makes the conclusions somewhat problematic.

To overcome this confounding detail, Ian Reid and his colleagues from the Universities of Auckland and Aberdeen specifically separated out women who were taking calcium supplements from those who weren’t. To be clear, all the calcium in this study was administered as a supplement. In other words, the four groups of women in the study included those taking no calcium, those who were given a calcium supplement, those who had already been taking their own calcium supplements but were given a placebo, and those who were taking their own personal calcium supplements and were given an additional calcium supplement.


Women with no personal use of calcium who received calcium in the study had a modest increase (6 events/ 1000 women) in cardiovascular events compared to those on placebo. Women who were already taking calcium supplements before the study began showed no such increase.

I find this result a bit puzzling. One possible explanation is that any amount of calcium supplementation will increase the risk of cardiac arrest regardless of dosage. Thus, the women who were already taking calcium supplements did not increase their risk of heart attack when they added more calcium rather than a placebo.


Calcium supplements of any dosage will abruptly increase serum calcium levels, which in turn can lead to more vascular calcification, a common cause of heart problems. However, it's not clear whether that's what's going on here. Like with the vitamin D study I discussed earlier, we need a lot more evidence before we draw any solid conclusions.


That said, I think it's excellent that Reid and his colleagues thought  of personal calcium usage as a possible confounder. I imagine that it's exceedingly difficult to get people to remember, let alone disclose every food or supplement they take during studies, which must affect results. Good for these researchers for attempting to isolate any and all calcium usage for their study.