Evidence grows for heart attack risk from calcium supplements

BMJ Group News

What do we know already?

Research that we reported on last July showed a link between calciumsupplements and an increased risk of a heart attack. But the research didn't look at the popular combination supplements that contain vitamin D as well as calcium.

In 2007, a study of more than 36,000 women found that taking vitamin D and calcium didn't seem to increase the risk of heart attacks. But about half of the women in the study were already taking these supplements when the study started, which may have affected the results.

So, researchers have looked again at the study, separating out the women who were already taking supplements. They have also looked at two recently published studies, and produced an updated review of the research that also looks at women taking vitamin D and calcium.

What does the new study say?

The risk of a heart attack or stroke went up for women who started taking vitamin D and calcium at the start of the 2007 study. Among women who were taking supplements already, there was no change in risk. That's exactly what you'd expect if calcium and vitamin D supplements increased the risk of a heart attack.

Taking into account all the new information since their last review, the researchers calculated that taking calcium with or without vitamin D increases the risk of having a heart attack or stroke by a small amount. Over 10 years, about 7 in 100 people not taking supplements had a heart attack or stroke, compared with about 8 in 100 who did take calcium, with or without vitamin D.

Calcium in the diet (from sources such as dairy products and green leafy vegetables) didn't affect the risk of a heart attack or stroke. The researchers say the risk from supplements may be because supplements increase the amount of calcium in the bloodstream quickly, whereas calcium from food is absorbed more slowly.

We know that calcium and vitamin D help to prevent broken bones, so the questions is, do the benefits outweigh the risks? The researchers calculated that, if you treated 1,000 women for five years with calcium (or calcium plus vitamin D), you'd prevent three fractures, but cause an additional six heart attacks or strokes.

How reliable are the findings?

The findings should be fairly reliable. Women who'd made the decision to take supplements before the start of the 2007 study may have been different from ones who didn't, which could have affected the results. But the researchers looked at these differences and said that, if anything, women not taking calcium supplements by choice would have been more likely to have heart attacks or strokes, because they were more overweight. So, any bias shouldn't change the overall direction of the results.