A new study adds to the growing body of evidence that suggests calcium supplements may do more harm than good.
Women are often urged to load up on calcium supplements as a way of building strong bones and preventing fractures in old age. But research has linked the tablets to an elevated risk of cardiovascular disease including heart attacks and strokes.
The latest study, published this week by the British Medical Journal, comes from the same New Zealand researchers who first raised safety concerns about the popular supplements three years ago.
Earlier work by the team at the University of Auckland looked at women who took calcium on its own. But their initial findings of cardiac risk were greeted with some skepticism because the research did not include women who popped calcium along with vitamin D - one of the most common ways the supplement is consumed.
So in their latest study, the New Zealand researchers sought out data that included women who also took vitamin D. In total, they reviewed 14 previously conducted trials that involved a total of 29,000 participants who were randomly assigned to receive either calcium - with some women also getting vitamin D and some not - or a placebo.
The new results show that "when you add vitamin D it really doesn't make a difference to the analysis," said the lead researcher, Ian Reid. People who took calcium plus vitamin D, or calcium alone, faced the same 25 per cent increased risk of cardiovascular problems compared with those given placebos, he said.
Dr. Reid noted the supplements trigger a sudden rise of the mineral in blood levels. By contrast, calcium found in food leads to a slow, steady rise of calcium in the blood stream. He speculated the sharp spike in calcium could contribute to hardening of the arteries or lead to other blood vessel changes that promote cardiovascular disease.
An editorial accompanying the study cautioned that there still isn't enough data to prove beyond a doubt that calcium supplements are directly responsible for cardiac problems.
Even so, Dr. Reid thinks it's time for the medical community to re-evaluate the routine use of calcium supplements for the prevention of osteoporosis.
"We really have to take this risk seriously," he said. "It is not responsible to just ignore it and hope it will go away because, I think, that is not going to happen. All the evidence is pointing in the same direction."
Some medical experts agree the merits of supplements should be questioned.
Ruth McPherson of the University of Ottawa Heart Institute pointed to previous studies that have shown calcium supplements reduce the risk of bone fractures by only 10 per cent.
If it turns out the tablets really do boost the odds of cardiovascular problems by 25 per cent, then "the risks of calcium supplements clearly outweigh the benefits," Dr. McPherson said.