Editor’s Note: This blog post originally appeared on the Amway Answers blog.
Last month a study published by the British Medical Journal stated that women who had never taken a calcium supplement had a greater chance of a heart attack after taking calcium as part of a seven-year clinical trial.
However, women who had taken calcium supplements before the study began had a lesser chance of a heart attack, and a lesser chance of dying from any cause after taking calcium as part of the trial, suggesting that calcium conferred greater protection the longer it was taken.
What was this study?
The Bolland et al. study wasn’t a randomized trial, but an after-the-fact look at the results of almost 40,000 post-menopausal women from a previous study, the seven-year Women’s Health Initiative Calcium and Vitamin D study.
What are the reactions?
Typical of initial mass media mentions, Woman’s Day stated online categorically that “post-menopausal women who took calcium supplements had a 13- 22% increased risk of heart attacks” – which doesn’t tell the whole story.
On the flip side, the Council for Responsible Nutrition (CRN) has called the study flawed, and called for a balanced view. Other mass media channels also have taken a moderate tack, for example the Los Angeles Times on April 20th:
“Calcium supplements appear to slightly raise the risk of heart attack, a new analysis suggests. But the data, from postmenopausal women who took supplements over seven years, are far from conclusive. So don’t throw out the multivitamins just yet – or those calcium supplements that many women take for bone health.”
An accompanying editorial in the BMJ itself called the Bolland study flawed, and also noted the findings of “extended survival” in the group of women who were calcium users.
The chart below summarizes the effects of taking calcium (compared to placebo) in women who had either never supplemented with calcium before the study started, or had supplemented with calcium at some point in their lives before the study started. The asterisk * over the first red bar on the left shows the worrisome results: women who had never taken a calcium supplement before the study showed a (statistically significant) 16% increase in the risk of heart attack after taking calcium as part of the study. There wasn’t any effect on the risk of a death from any cause (first green bar).
By contrast, the double asterisks ** over the second set of bars shows a significant 12% and 16% reduction in risk of heart attack and all-cause mortality respectively, in women who had been or were calcium users prior to the study’s initiation.
As John Hathcock, Ph.D., of CRN has said of calcium and vitamin D supplements, from this study “it appears the more you take, the better off you may be.”
Why the confusing findings?
Such a mix of results is par for the course in these kinds of studies which reanalyze huge datasets from old clinical trials. The study authors speculate that the higher risk of heart attacks in women who had never supplemented with calcium could have been because of a sharp rise in blood calcium levels; which does give rise to the notion that first-time post-menopausal calcium users should be instructed to “start low, go slow.”
But, as the authors acknowledge in another part of their paper (page 8), calcium acutely increases serum concentration by only a modest amount. There is yet no clear hypothesis for how these results could have been obtained. On the other hand there are plenty of possible statistical explanations. In typical prospective clinical trials, subjects are randomized evenly to comparison groups. Potential confounding factors that could muddy interpretation are, as far as feasible, controlled for. This type of retrospective study could only make after-the-fact comparisons. Were women who had never in their lives taken a calcium supplement different in other ways (apart from calcium supplement use) than women who had taken calcium supplements or were doing so before the study? And could such a difference account for their negative reaction to calcium supplementation? The dataset does not allow the teasing out of causal factors; in clinical terms, these findings are associations, “A with B”, that do not necessarily reflect real-world, causal, “A leads to B” relationships.
What Should I Do?
As the BMJ Editors as well as the CRN have recognized, this study has produced more questions than answers. The BMJ Editors conclude with a reassurance that several large studies have shown that calcium with or without vitamin D, in combination with medical treatment of osteoporosis, reduced risk of mortality in men by 28% and in women by 38%.
Another reasonable conclusion, one which is in line with the entirety of the literature on the long term benefits of calcium with or without vitamin D, is that long term supplementation with calcium is a good idea.
Daily dietary calcium requirements range from 800 mg to 1500 mg depending on life stage. As Dr. Robert P. Heaney, a bone expert, has pointed out, most women don’t get enough calcium, so the time to start supplementing if you’re not getting enough from your diet is now.