HONG KONG, Jan 16 (Reuters) - Calcium supplements may increase the risk of heart attack and stroke in healthy postmenopausal women, a study in New Zealand has found.

Calcium supplements are commonly prescribed to postmenopausal women to maintain bone health, and some data suggest it might protect against vascular disease by lowering levels of LDL cholesterol, the so-called "bad cholesterol", in the blood.

But in a five-year study of 1,471 healthy postmenopausal women, researchers at the University of Auckland found that incidences of heart attack and stroke were higher in the group given calcium supplements.

"This effect was more noticeable in those with high compliance with the study drug and seems to be progressive during the 30 to 60 months of the study, consistent with an initial latent period during which vascular damage occurs before event rates increase," they wrote in an article published in the latest issue of the British Medical Journal.

The women recruited had a mean age of 74 and had been postmenopausal for more than five years. Those who were receiving treatment for osteoporosis, already taking calcium supplements, or suffering any major ongoing disease were excluded.

Of the 1,471 participants, 732 were given calcium supplements and 739 given placebos. They were followed up every six months for five years.

Researchers also checked hospital admissions and reviewed all death certificates for study participants to identify any unreported events.

Heart attacks were more common in the calcium group, with 31 women in that group suffering a total of 36 such events, while 21 women on placebo suffered 22 such events.

More strokes were also seen in the calcium group, with 34 women suffering 37 such events, while 25 women on placebo suffered 26 such events.

While the researchers cautioned that the findings were not conclusive, they suggested that high calcium intakes might have an adverse effect on vascular health.

"If confirmed by other studies, this effect could outweigh any beneficial effects of calcium on bone," they wrote.

"In the meantime, this potentially detrimental effect should be balanced against the likely benefits of calcium on bone, particularly in elderly women." (Reporting by Tan Ee Lyn; Editing by Alex Richardson)

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