Healthy women who take calcium to protect their bones could potentially do more harm than good, a study suggested.
Previous research has shown that calcium may protect against vascular disease by improving levels of good cholesterol in the blood.
But now researchers from the University of Auckland have found the supplements could actually increase the chances of suffering an attack.
Their study, published online in the British Medical Journal (BMJ), involved 1,471 healthy postmenopausal women aged 55 and over, and with an average age of 74.
Women were included in the study if they had been postmenopausal for more than five years and had a life expectancy of more than five years.
Women already receiving treatment for osteoporosis or taking calcium supplements were excluded from the study.
All the participants were then split into two groups, with one group given 1g daily of calcium supplements and the other a dummy pill (placebo).
Those given calcium were asked to take two tablets (each containing 200mg calcium) before breakfast and three in the evening.
The intake of calcium from all the women's diets was also noted.
Both groups were followed up every six months for five years.
The study found that heart attacks were more commonly reported in the calcium group and the occurrence of any three vascular events (heart attack, stroke or sudden death) was also more common.
The experts then checked hospital records and death certificates for the women to identify any heart attacks or vascular events that could have been unreported.
With these added events, heart attacks still remained more common in the calcium group (36 events in 31 women versus 22 events in 21 women in the placebo group).
Rates for heart attack, stroke or sudden death were also increased in this group (76 events in 60 women versus 54 events in 50 women in the placebo group), although these were of borderline significance.
The authors concluded," Healthy older women randomised to calcium supplementation showed increased rates of myocardial infarction (heart attack).
"This effect could outweigh any benefits on bone from calcium supplements."
However, Judy O'Sullivan, cardiac nurse at the British Heart Foundation, said further studies were needed.
She said, "This small study has identified a potential increase in the risk of heart attacks and strokes in postmenopausal women taking calcium supplements.
"However, more rigorous research is needed before any firm conclusions are drawn as previous studies have shown calcium supplements reduce the risk of heart attacks by improving the levels of protective cholesterol, HDL.
"Anyone who has been advised by their doctor to take calcium supplements to protect their bones should not stop doing so in light of this study alone without medical advice."
Pamela Mason, nutritionist and spokeswoman for the Health Supplements Information Service (HSIS), said the study had several limitations.
The HSIS is funded by Boots, Bayer, Perrigo, Seven Seas and Wyeth Consumer Healthcare.
She said the study had a high drop-out rate, with only 50% of the calcium supplemented group and 40% of the placebo group completing five years of the research period.
She added that it was a small study, saying, "Though 1,471 women were recruited, this is actually a small number from which to measure the large number of cardiovascular endpoints included in the study (i.e., heart attack, angina, chest pain, stroke, transient ischaemic attack, sudden death, and death as well as an endpoint combining three of the above, namely heart attack, stroke and sudden death)."
She continued, "Calcium is an essential mineral, vital for bone health and nerve and muscle function.
"The results of this study certainly do not suggest that people should lower their calcium intakes below the RDA (Recommended Daily Amount).
"Indeed, because of the importance of calcium it remains imperative for people to achieve the RDA.
"Given that significant numbers of the U.K. population are not achieving this through their diet, a calcium supplement is beneficial."
A spokeswoman for the National Osteoporosis Society said, "This is an interesting study, but more research is needed to confirm whether calcium supplementation in older people increases risk of heart attacks.
"People at high risk of fracture, and particularly older, frailer people whose dietary intake of calcium is low, may benefit from calcium and Vitamin D supplementation to help maintain strong bones and reduce fracture risk."
From the January 21, 2008, Prepared Foods e-Flash