In a new analysis from the National Institutes of Health, men who took calcium tablets were more likely to die of heart disease over more than a decade than those who didn't get extra calcium in supplement form.
"The effect of supplemental or dietary calcium on heart disease has always been a bit of an unanswered story," said Howard Sesso, a preventive medicine researcher at Brigham and Women's Hospital and Harvard Medical School in Boston.
"It could be that when you take supplements, maybe you're taking doses that far exceed what you need," he added. But it's still unclear how that might raise cardiovascular risks.
The new findings are based on a study of close to 400,000 middle-aged Americans initiated in 1995 to 1996.
Study volunteers answered questions about their lifestyle, general health and diet, including use of supplements. Researchers then tracked how many of them died, and from what causes, over the next 12 years.
About half of men and more than two-thirds of women said they took calcium supplements or multivitamins containing calcium at the outset.
During the study period, almost 12,000 people - or about three percent - died of cardiovascular disease.
Qian Xiao from the National Cancer Institute and her colleagues found men who took 1,000 milligrams of calcium per day or more were 20 percent more likely to die of heart-related causes than those who passed on calcium supplements. That was after the researchers took into account men's age, race and weight, as well as other measures of diet and lifestyle.
However, there was no link between calcium supplements and heart disease deaths in women. And calcium from food and beverages wasn't tied to heart problems in men or women, the research team wrote Monday in JAMA Internal Medicine.
It's possible that calcium build-up in the arteries and veins may affect cardiovascular risks in some people, Xiao wrote in an email to Reuters Health.
No cause-effect link
But Sesso, who wasn't involved in the new study, told Reuters Health he wasn't sure why, biologically, calcium supplements would be linked to a higher risk of heart problems in men but not women.
The findings don't prove a cause-and-effect link between the supplements and heart problems. It's possible there were certain differences between men who did or didn't take extra calcium that the research team couldn't measure.
"Although we observed an increased risk of death from heart disease in men who reported taking supplements containing calcium, we cannot say for sure that it was a result of using those supplements," Xiao said.
According to Sesso, the study won't change the fact that calcium supplements are typically recommended for reasons not related to heart disease - such as to prevent fracture risk in older adults who don't get enough calcium through food.
Eating a balanced diet and keeping a healthy weight are as or more important than any supplement when it comes to cardiovascular health, Sesso said.
"If you have a good diet in the first place, a supplement might not be adding all that much," he said.