Where's the vitamin D?
Others argue that the recent analysis is all but irrelevant, since it excluded patients also taking vitamin D, which may protect the heart by helping the body absorb calcium, and which is nearly always prescribed along with calcium supplements. "The field has moved on from the calcium-alone issue to the combination of calcium and vitamin D, because they work in concert to protect bones," says Bess Dawson-Hughes, M.D., director of the USDA Nutrition Center at Tufts University in Boston.
The Women's Health Initiative, a national study involving 36,000 participants, looked at women taking both calcium and vitamin D, and in 2006 reported no overall cardiovascular effects from the supplements.
The New Zealand analysis, however, is not the first to hint that calcium supplements might raise cardiovascular risk. It follows clinical-trial results published two years ago by the same investigators. That analysis was meant to examine possible cardiovascular benefits of the supplements in postmenopausal women. Instead, it uncovered a trend toward more cardiovascular problems among those taking calcium.
And Reid says that at an October meeting of the American Society of Bone and Mineral Research, he will present another analysis of studies on calcium with vitamin D—an analysis heavily dominated by a re-examination of data from the Women's Health Initiative—that also points to heightened heart attack risk.
What are the benefits?
Despite the widespread use of calcium supplements for bone health, there remains some debate about just how useful they are in preventing fractures.
A 2006 report on data from the Women's Health Initiative found no statistically significant reduction in fractures among healthy postmenopausal women taking calcium and vitamin D, although women over age 60 who closely followed their supplement regimens did suffer fewer hip breaks. Meanwhile, this 2006 report suggested the pills might promote kidney stones, which can be quite painful.
An editorial accompanying the recent analysis in the British Medical Journal argued that while calcium supplements may increase bone density, they haven't been proved to actually prevent breaks. "Given the uncertain benefits of calcium supplements," wrote cardiologist John Cleland, M.D., of the University of Hull in England, "any level of risk is unwarranted."
But the editorial cites a 2007 analysis that looked at 17 trials and found a 12 percent reduction in fractures of all types among those on calcium (with and without vitamin D). "Considering there are 1.5 million fractures due to osteoporosis in this country per year, that's not an inconsequential number," says Stuart Weinerman, M.D., chief of the Division of Endocrinology, Metabolism and Diabetes at North Shore-Long Island Jewish Health System.
On the other hand, says Weinerman, even the potential risk of kidney stones, to say nothing of cardiovascular risk, argues for greater caution against the overuse of calcium pills. Though the average American diet is deficient in calcium, he says he often sees patients whose eating habits, combined with excessive supplementation, give them more calcium than they need (about 1,200 mg daily for people 50 and above, according to Institute of Medicine guidelines). "That doesn't make any sense."
Dawson-Hughes agrees. "I'm fairly conservative," she says, "and my view is, if there's not even a purported benefit of going above 1,200 mg, and there's any conceivable risk, why do it?"
Katharine Greider lives in New York and writes about health and medicine.