KEY POINTS:
Large doses of calcium supplements, which just five years ago were thought to lower the risk of heart disease, have now been found in a new study to increase the risk.
Calcium supplements are widely used to prevent bones from weakening. Over the age of 60, half of women and nearly a third of men will suffer a fracture caused by the bone-thinning condition osteoporosis.
The Auckland University study, presented to an American conference, found that 36 of those taking calcium supplements had a heart attack, compared with 22 from the trial's placebo arm.
Some women died from cardiac-related illnesses during the trial, but none of the deaths was thought to have been caused by the calcium supplements.
The 1500 postmenopausal women in the trial were randomly assigned to take the 1g supplement or a placebo pill daily for five years, the Herald on Sunday reported.
Professor Ian Reid told the paper that while the study did not necessarily prove that all calcium supplements caused heart attacks, he was worried by the trend. Smaller studies elsewhere had produced similar results.
People with heart disease developed a build-up of calcium in their heart vessels. Taking extra calcium could speed that up.
He has written to the trial participants, advising those with heart problems or poor kidney function to seek another way of preventing osteoporosis.
"If you are elderly and you have heart disease, you should probably be looking after your bones in other ways than with high calcium intakes."
For others, he suggested reducing the dose to 500mg a day.
In 2002, Professor Reid's group published findings that calcium improved cholesterol balance and he speculated they could therefore reduce the risk of heart disease and stroke.
Doctors interviewed by the Herald emphasised that patients' individual risks of heart disease needed to be weighed against the benefits their bones received from the supplements.
"There are other ways of looking after your bones," said Professor Geoffrey Horne, a Wellington orthopaedic surgeon and formerly a scientific adviser to Osteoporosis New Zealand.
"It's all a matter of balancing risks. If you take some of the pharmacologically active drugs, they too have risks ... Every time you put something in your mouth you are taking a risk."
Heart Foundation medical director Dr Norman Sharpe said it was an important study and people taking calcium supplements should discuss them with their doctor.
"There will be people who are at very high risk of osteoporosis where the treatment is well justified because the benefits will outweigh any risks and there will be those at relatively low risk where calcium supplementation is going to provide little benefit and will still carry this sort of risk."
The principal technical adviser of drugs regulator Medsafe, Dr Stewart Jessamine, said he looked forward to reading the study once it was published.
HEALTH FACTS
* Calcium is an important component of a healthy diet.
* A deficit can affect bone and tooth formation. Too much can cause kidney problems.
* Calcium is found in dairy products, some nuts and seeds, oranges and seaweed.
