Postmenopausal women who take calcium supplements to strengthen bones may have an increased risk of heart attack, stroke and sudden death, according to a new study from researchers in New Zealand.

"Calcium supplementation in healthy postmenopausal women is associated with upward trends in cardiovascular event rates," reads the study, published Tuesday in the British Medical Journal.

Overall, 1,471 healthy older women -- with an average age of 74 -- participated in the five-year study led by Ian Reid of the University of Auckland.

All of the women were previous participants in a study measuring the effects of calcium on bone density and fracture rates. Of them, 732 were given a daily calcium supplement and 739 were given a placebo.

Researchers confirmed 36 heart attacks in 31 women who took the calcium supplements, compared with 22 heart attacks in 21 women who took the placebo.

Stroke and sudden death rates were also higher in the supplement group; however, researchers cautioned these results were not conclusive.

The study suggests that calcium supplements accelerate the formation of deposits in the arteries that could lead to a heart attack because the supplement increases blood calcium levels.

Reid recommends that women over the age of 70, who are already at an increased risk of coronary heart disease compared to younger women, should be cautioned against taking calcium supplements.

However, Dr. Reinhold Vieth, a University of Toronto professor and director of the bone and mineral laboratory at Mount Sinai Hospital, cautioned that the study was unusual because it dealt with a form of less commonly used calcium.

"There are all sorts of kinds of calcium. You can have the calcium that's found in milk, the less expensive calcium carbonate, or the more expensive calcium citrate," Vieth told CTV's Canada AM on Wednesday.

Vieth said previous calcium studies showing no effect on vascular disease used calcium carbonate while this study used calcium citrate -- a calcium supplement that is less commonly used.

The study noted that previous research had suggested calcium supplements actually decreased the risk of heart disease by lowering levels of "bad" cholesterol in the blood.

The study also suggests that any risk in heart attack should be weighed against "the likely benefits of calcium on the bone, particularly in elderly women."

Researchers concluded that while their results are not the definitive word in calcium analysis, their findings merit more study.

Vieth contends that postmenopausal women should simply take vitamin D to counteract any potential risk of heart attack associated with calcium supplements.

"Take the straightforward calcium carbonate, incorporate vitamin D with it, because that is associated with protection against cardiovascular disease," he said.