WEDNESDAY, May 23, 2012 (Health.com) — Calcium supplements, widely taken by older people to prevent bone fractures, may be doing more harm than good, a large new study suggests.
Researchers tracked nearly 25,000 European adults for 11 years, and found that people who reported regularly taking calcium supplements were more than twice as likely to have a heart attack as those who didn't use any supplements.
Only the use of calcium supplements, and not overall calcium intake, was associated with an increased risk of heart attack. In fact, people who consumed higher amounts of calcium from foods, such as milk and other dairy, tended to have a lower risk of heart attacks than people who consumed less.
The new study, which appears in the journal Heart, adds to the preliminary yet growing evidence that calcium supplements may harm the heart. Since 2010, two separate studies that re-analyzed existing data have reported a similar link between calcium supplements and heart attacks.
"Calcium is an important mineral," says Sabine Rohrmann, Ph.D., the senior author of the new study and an epidemiologist at the University of Zurich, in Switzerland. "However, we probably do not need mega-doses of calcium, and the current recommendation"—1,000 to 1,200 milligrams for most U.S. adults—"can be met by a balanced diet that includes (low-fat) milk and dairy products."
Older people who don't tolerate dairy products well could try mineral water or calcium-rich vegetables, such as certain types of cabbage, Rohrmann adds.
Why is calcium from food any different than calcium from supplements? Experts aren't entirely sure, but one factor may be that supplements tend to release their calcium in a single flood rather than a slow, steady stream.
Calcium, along with cholesterol, is one of the substances that contributes to the hardening and narrowing of arteries (atherosclerosis). For reasons that remain unclear, when calcium levels spike suddenly, the calcium appears more likely to end up in plaques that line artery walls, a major culprit in heart attacks.
"People who take supplemental calcium have a surge in [blood] calcium levels, and we know that surge increases calcium uptake in atherosclerotic plaques," says Christopher Cove, M.D., assistant director of the cardiac catheterization lab at the University of Rochester Medical Center, in Rochester, N.Y.
On the other hand, Cove says, "Calcium from your everyday diet is digested slowly, and it's also not the high concentration you get from tablets."
For now, though, there are more questions than answers. The new study, known as an observational study, is the first of its kind to look at the link between calcium supplements and heart attacks, and it has several important blind spots that will need to be explored in future research.
Notably, only about half of the supplement users in the study itemized which vitamins and minerals they took, so the authors likely missed some people who took calcium supplements, potentially skewing the results. And only about 1.5% of the study participants had a heart attack during the study, limiting the power of the findings.
In addition, as in all observation studies, unknown factors may have contributed to heart attacks. Although the authors controlled for a wide range of health measures and behaviors, it's possible that calcium-supplement users were more vulnerable to heart attacks for reasons that had nothing to do with the supplements.
Even these tentative findings could lead doctors to reconsider the use of calcium supplements, however.
Many postmenopausal women take calcium supplements to preserve their bone health as they age, but as an editorial accompanying the study suggests, the effectiveness of calcium supplements in preventing bone fractures appears to be so modest that it may not outweigh even a potential, still-unproven heart risk.
"Recommending calcium supplements may not be the right thing to do," Cove says.