Calcium Supplement Side Effects: Higher Heart Attack Risk?

UPDATE: Turns out that a 2008 study by Dr Ian Reid, Professor of Medicine and Endocrinology of the University of Auckland found similar findings on calcium supplement side effects on the heart. Here’s what he says in an interview with Norman Swan of the Australian Broadcasting Corporation’s Health Report:

Norman Swan: What did you find?

Ian Reid: All of the major studies showed a 20% to 30% increase in the risk of heart attack and when you pulled all those data together overall there was a 30% increase in the risk of heart attack. There was a similar adverse trend in relation to strokes but that was not statistically significant. And what we can calculate is that if 1,000 people are treated with calcium for five years there would be 14 more heart attacks in the calcium group, 10 more strokes and 13 more deaths balanced against 26 fewer fractures. So it appears that use of calcium is associated with 37 more adverse events than 26 fewer fracture events. So we think this really says that the balance is negative.

Norman Swan
: There are a couple of things that you do as a cross-check, just as a reality check. One is more calcium — more heart attacks, was there a dose effect?

Ian Reid: We haven’t got a large number of different doses so that is not something that we can easily address from this data base.

Norman Swan: The other reality check is is there a plausible biological mechanism whereby calcium could be having this effect?

Ian Reid: We think there is, for instance in patients with kidney failure who are on dialysis and in patients who have milder degrees of kidney failure not requiring dialysis it’s quite clear from clinical trials that the use of calcium supplements increases the risk of either dying or accelerates the rate of vascular calcification and vascular damage. The second clinical cross-check is that we know when people take calcium supplements as opposed to taking food calcium that their blood calcium levels rise over the following few hours and often wind up setting very near the upper end of the normal range for blood calcium. And there’s quite a lot of epidemiological data showing that if you just look at normal populations and compare cardiovascular event rates and vessel wall thickness and vessel wall damage and mortality in relation to base line blood calcium levels, people who habitually sit with blood calcium levels near the top of the normal range are more likely to have heart attacks, more likely to have damaged arterial walls and more likely to die than other people in that population who sit in the lower part of the normal range. So that gives some credibility to this finding.

If you start doing studies in the laboratory then there are studies that demonstrate that if you expose the cells from blood vessels to higher concentrations of calcium you see more deposition of calcium into the walls and that’s thought to be one of the mechanisms whereby heart attacks are caused.

Norman Swan: Yet diets that are high in calcium don’t seem to cause a problem?

Ian Reid: No they don’t and our explanation for that, or our hypothesis related to that is that when you take a meal that’s rich in calcium your blood calcium level hardly changes at all and we believe that that’s because the protein and the fat that are part of that meal slow the rate of absorption. So a meal very gradually releases its calcium into the circulation whereas when you take a calcium supplement you get an abrupt change in blood calcium level which lasts for quite a few hours.

Norman Swan: And what happens when you take nutraceuticals, you know milk that has been fortified with calcium which is effectively you are drinking a calcium supplement, does that affect your calcium levels that much?

Ian Reid: You get some change in calcium but I mean there are some studies that have been done with calcium fortified milk and the change in blood calcium that they produce is about a fifth of what you get from taking the same amount of calcium in a tablet form.

Norman Swan: And how sure are you that these people actually had heart attacks, were they well diagnosed?

Ian Reid: Well yes they were, the majority of the events have been adjudicated and we find exactly the same trends in the adjudicated data bases as we do in those where we were not able to adjudicate them.

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Calcium Supplement Side Effects: Higher Heart Attack Risk? posted 11 April 2015. Last update on 15 October 2015.