The calcium supplements that older women often take to keep their bones strong may also be putting them at risk for heart attacks and other cardiovascular events, more research is finding.

The link described in a new report in this week's British Medical Journal adds to the mounting evidence that the heart risks posed by calcium supplements might outweigh any benefits they offer in reducing bone fractures.

This study looked at the largest group yet, using data from the long-running Women's Health Initiative (WHI) study, which was a major research study on the most common causes of death and disability among postmenopausal women.

That initial WHI study included 36,000 women showed no increased risk for heart disease among those who received 1,000 milligrams of calcium and 400 international units (IU) of vitamin D per day, compared with those who were randomly assigned to receive a placebo.

But some of these women had already been taking their own calcium supplements. Prof. Ian Reid, a professor of medicine and endocrinology at the University of Auckland wondered whether that could have masked the initial findings. So Reid and colleagues focused on a subgroup of more than 16,700 women with an average age of 63 who were not taking calcium pills at the start of the study.

Reid's analysis found that those who began the combo supplements as part of the study were at an increased risk of cardiovascular events, especially heart attack, but also stroke and heart disease. The risk was boosted by 13 to 22 per cent, depending on the heart event.

Yet among the women who were already taking calcium supplements at the start of the trial, the combined calcium and vitamin D supplements did not raise their cardiovascular risk.

Reid's team suspects the sudden spike in blood calcium levels that occurs when starting a supplement is responsible for the increased risk.

"Even though these increases are relatively small, we think they're significant, firstly, because a lot of people around the world take calcium and vitamin D, and secondly because what the data in these databases suggest is that we're probably going to cause more heart-related events than we prevent fractures," Reid tells CTV News.

Put another way, Reid calculates that if 100 people were to begin taking calcium supplements with or without vitamin D for five years, it would cause as many as six more heart-related events while preventing only three fractures.

"So the balance doesn't actually make sense," he says.

The authors suspect that the abrupt change in blood levels of  calcium may be what causes the heart attacks and other heart events. As well, high blood calcium levels over the longer term are linked to calcification (hardening) of the arteries.

When the team did further analyses, adding data from 13 other trials involving 29,000 women in total, they also found consistent increases in the risk of heart attack and stroke associated with taking calcium supplements -- with or without vitamin D.

"This data and the previous meta-analyses suggest that calcium supplementation may increase cardiovascular events," Dr. Rick Adachi, an osteoporosis specialist at McMaster University, told CTV News. "And that while a little bit of supplementation may be OK, high doses and over-supplementation are not."

Reid believes taht the calcium from food would not cause the same problems. Unlike the calcium in supplements, the calcium in food is absorbed slowly over many hours, so that blood calcium levels hardly change at all, Reid says.

The findings lead Reid and his team to conclude that there needs to be reassessment of the use of calcium supplements in older women.

"What we're suggesting is that we should move away from using supplements and instead, we should be encouraging people to get their calcium from their diet," he says.

"We should rely on our diet to source our calcium. And for people who are at a high risk of fracture, they should be relying on medicines to decrease their fracture risk, not taking supplements," he said.

But in an editorial accompanying the study, Profs. Bo Abrahamsen of the Gentofte Hospital in Copenhagen, Denmark, and Opinder Sohota of Queen's Medical Centre in Nottingham, England, argue there is still too little information to know for sure whether calcium boosts heart risks. They say still more research is needed.

"Clearly further studies are needed and the debate remains ongoing," they write.

With a report from CTV medical specialist Avis Favaro and producer Elizabeth St. Philip