The researchers say calcium supplements, which are often recommended to the elderly and women after the menopause to protect against bone thinning, should be "taken with caution". Some experts are saying we should wait for further research to corroborate these findings before acting on this advice, and people who take supplements who are concerned should talk to their doctor.
The findings also appear to go against previous studies that have shown a higher calcium intake is linked to a lower risk of a number of conditions that predispose to heart disease and stoke, namely high blood pressure, obesity, and type 2 diabetes.
The study is based on data collected on participants who were aged 35 to 64 years old between 1994 and 1998 when they joined one of the German arms of the EPIC (European Prospective Investigation into Cancer and Nutrition) study in Heidelberg.
At the start of the study the participants filled in questionnaires that assessed their diet for the previous 12 months, and they also answered questions about regular intake of vitamin and mineral supplements.
Their health was followed for an average of 11 years afterwards. During this time, the group experienced 354 heart attacks and 260 strokes, and 267 participants died of related causes.
When they analyzed the results, the researchers ranked them according to levels of calcium intake, and examined them from various points of view, such as calcium intake including supplements, and the effect of supplements alone. They also adjusted them to remove as far as possible effects from other known influencing factors.
They found participants with a moderate intake of calcium from all sources (820mg a day, including supplements) had a 31% lower risk of heart attack than the ones in the bottom 25% of calcium intake.
But those who calcium intake from all sources, including supplements, was higher than 1,100mg a day did not have a significantly lower risk of heart attack than the bottom intake group.
Plus, the researchers found no link between raised or lowered risk of stroke for any level of calcium intake from all sources, which they say reflects the findings of other studies.
However, when they looked at the results for supplements only, they found the participants who took calcium supplements on a regular basis had an 86% higher risk of having a heart attack than those who did not take any supplements at all.
And for participants who only took calcium supplements (that is no other vitamins or mineral supplements), this figure shot up: they had more than double the risk of having a heart attack compared with those who took no supplements at all.
The researchers conclude that their findings suggest:
"... increasing calcium intake from diet might not confer significant cardiovascular benefits, while calcium supplements, which might raise [heart attack] risk, should be taken with caution."
The study has drawn a mixed response, with some saying it adds to those that are questioning the safety of calcium supplements, and others pointing to the study's limitations and calling for further studies to corroborate the findings.
In an editorial in the same issue of the journal, Professors Ian Reid and Mark Bolland from the Faculty of Medical and Health Science at the University of Auckland in New Zealand, write that calcium supplements are "now coming under increasing scrutiny".
They refer to studies that link them to kidney stones and problems with the gut and abdominal symptoms. They also point out that while there is evidence that they reduce levels of cardiovascular risk factors, this does not actually translate into reduction in heart attacks and strokes.
Another point they make is that women who take calcium supplements to protect against bone thinning are already healthier than the women who do not, and the effect is modest anyway, around 10% only.
They say dietary calcium is helpful because it is taken in small amounts that are absorbed throughout the day, whereas supplements tend to be taken all in one go, causing blood calcium levels to spike above normal, and it is this that causes harm, they suggest.
"It is now becoming clear that taking this micronutrient in one or two daily [doses] is not natural, in that it does not reproduce the same metabolic effects as calcium in food," they write, suggesting we should discourage taking supplements to boost calcium intake.
"We should return to seeing calcium as an important component of a balanced diet, and not as a low cost panacea to the universal problem of postmenopausal bone loss," they conclude.
Natasha Stewart, Senior Cardiac Nurse with the British Heart Foundation, said while this study suggests there may be an increased risk of having a heart attack for people who take calcium supplements, it doesn't mean the supplements cause heart attacks. She told the press:
"Further research is needed to shed light on the relationship between calcium supplements and heart health. We need to determine whether potential risks of the supplements outweigh the benefits calcium can give sufferers of conditions such as osteoporosis."
"If you've been prescribed calcium supplements, you should still keep taking your medication, but speak to your doctor if you have any concerns," she added.
This study was published in the journal Heart.
The supplement industry expresses a similar view. Dr Taylor C. Wallace is Senior Director for Scientific & Regulatory Affairs at The Council for Responsible Nutrition (CRN) in the US. He said in a statement issued from Washington on Wednesday that "the study itself is not reason enough to discount the important benefits of calcium".
He urges consumers with questions, and their doctors, to consider some of the study's limitations, for instance the fact it was not originally designed to measure cardiovascular events, so confounding factors were not distributed equally through the groups. As an example he points to the fact the calcium supplement group had more people with high cholesterol and more long-term smokers.
Wallace said consumers like the elderly who are at greater risk of falls and fractures due to weak bones, need calcium, and removing it from their diet could increase their risk of these problems.
"Our advice is for consumers to be aware of how much calcium they get from their diet, supplement with calcium if needed, and check with their doctor or other healthcare practitioner to determine their own personal needs," he urged.