WASHINGTON - Older women who take calcium supplements to maintain bone strength may have an increased risk of heart attack, researchers in New Zealand said on Tuesday.

The researchers cautioned that they do not consider their findings the definitive word on the subject, but said the higher heart attack risk they saw merits further study.

"This effect could outweigh any benefits on bone from calcium supplements," researchers led by Ian Reid of the University of Auckland wrote in the British Medical Journal.

Many women take calcium supplements to try to prevent osteoporosis, a condition in which bones become weak and brittle, leading to fractures.

The study involved 1,471 healthy post-menopausal women, average age 74, who already had participated in a study on 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. They were followed for five years.

Heart attacks were more common in the women taking the calcium supplements, with 31 women who took supplements experiencing a heart attack compared to 21 women who got a placebo, the researchers said.

The researchers noted that previous research had suggested that taking calcium supplements might protect against vascular disease by lowering levels of bad cholesterol in the blood.

They said that because calcium supplements raise blood calcium levels, this possibly accelerates the formation of deposits in the arteries that could lead to heart attack.

The new results "are not conclusive but suggest that high calcium intakes might have an adverse effect on vascular health," the researchers wrote.

"In the meantime this potentially detrimental effect should be balanced against the likely benefits of calcium on bone, particularly in elderly women," they wrote.

(Reporting by Will Dunham, Editing by Maggie Fox and Cynthia Osterman)