Taking a calcium pill is part of the daily health regimen of millions of Americans age 40 and older, particularly women. For years, doctors (and advertisers) have told us that the supplement will ward off osteoporosis by keeping our bones dense. And dense bones — or so the theory goes, and it’s only ever been a theory — would protect our hip, wrist and other bones from fractures, including the spinal fractures that lead to the dreaded “dowager’s hump.”

The theory has always had its critics, but recent research is helping their voices — finally — to be heard. For although calcium supplements can create denser bones, their effect on the outcome that matters — reducing fractures — is marginal, at best. In fact, some research suggests that the supplements may actually increase fractures.

Researchers have also been reporting (first in 2006, and then in 2008) that calcium supplements may be harmful to the heart. To investigate this troubling matter further, a team of researchers from Australia, Britain and the United States decided to go back and analyze 11 previously conducted randomized controlled trials of calcium supplements, this time combing the data specifically for heart attacks and other “cardiovascular events.”

The results of this meta-analysis were reported online Thursday in the British Medical Journal. And they’re not reassuring for supplement users. The researchers found that taking calcium supplements was associated with a 31 percent increased risk of heart attack (and a smaller, non-significant increased risk of stroke and premature death).

The increased risk was independent of a person’s age, gender or the type of calcium supplement taken. Furthermore, the mean duration of the 11 studies was four years, so the increased risk of heart attacks began appearing relatively soon after the studies’ participants began taking the calcium supplements.

Here are the specific numbers: Of the 11,921 people in the 11 randomized controlled trials analyzed, 166 people in the calcium group and 130 people in the placebo group had a heart attack.

That difference — 36 people — may seem small, but as the authors of the study point out, “the widespread use of calcium supplement means that even a small increase in incidence of cardiovascular disease could translate into a large burden of disease in the population.”

What might cause the supplements to damage the cardiovascular system in ways that lead to heart attacks? That remains unknown. The researchers suspect, however, that the high levels of calcium that the supplements unleash into the blood may contribute to the formation of plaque in the blood vessels.

By the way, other studies have found no increased risk to the cardiovascular system from eating calcium-rich foods — perhaps because the body absorbs the calcium from food more slowly.

This meta-analysis did not look at studies that combined calcium and vitamin D, but as an accompanying BMJ editorial points out, “no conclusive data are available to show that current doses of vitamin D supplements with or without calcium supplements reduce the rates of fracture.” Indeed, the editorial adds, studies that have shown any positive benefit from the combined supplements have serious reporting bias.

Calcium supplements “seem to be unnecessary in adults with an adequate diet,” the editorial concludes. “Given the uncertain benefits of calcium supplements, any level of [heart attack] risk is unwarranted.”