People who regularly take calcium supplements to keep their bones healthy may also be raising their risk for a heart attack, a new study suggests.

Some recent research has suggested that calcium supplements might increase rates of heart attack and cardiovascular events in healthy older women.

So a team of researchers from Britain and the U.S., led by Dr. Ian Reid, professor of medicine at the University of Auckland in New Zealand, analyzed the results of 11 randomized, controlled trials of calcium supplements (without co-administered vitamin D).

The studies involved 12,000 patients, with about half given calcium supplements and the other half placebo pills. They were then tracked for an average of four years.

They found that calcium supplements were associated with about a 30 per cent increased risk of heart attack. They also found smaller, non-significant, increases in the risk of stroke and death.

The findings, published in the British Medical Journal, were consistent across the studies regardless of the age, gender or type of supplement.

The authors estimate that for every 1,000 people taking calcium supplements for five years, there will be:

• 14 extra heart attacks
• 10 more strokes
• 13 more deaths

Previous studies have found no increased cardiovascular risks with higher dietary calcium intake, suggesting that the risks are restricted to supplements.

It's not clear why calcium supplements would increase the risk for heart attacks, but it's possible that the supplements lead to increased calcium in the blood, which may damage blood vessels. It's also possible calcium supplements accelerate the formation of deposits in the arteries that could lead to a heart attack.

The increase in heart attack risk from calcium supplements may be modest, but considering the widespread use of calcium supplements -- which are often taken by seniors on the advice of their physicians -- even a small increase in rates of heart attacks translates into huge effect, warn the authors.

An accompanying editorial by Prof. John Cleland and colleagues suggests the need to reassess the role of calcium supplements in osteoporosis management, especially since the there is only modest evidence that calcium supplements affect bone density and lower fracture risk.