Calcium supplements can almost double the risk of a heart attack, according to new research, and should be "taken with caution" and only for medical reasons, such as to prevent bone thinning. The study contradicts the commonly held belief that consuming extra calcium can help prevent heart disease or a stroke.
The findings are based on a study of the calcium intake of 23,980 men and women in Heidelberg, Germany, who were aged 35-64 when they joined a local cohort of the European Prospective Investigation into Cancer and Nutrition (EPIC) study in 1994-98. Researchers checked participants' health for 11 years afterwards, during which time 354 of them had a heart attack and 260 a stroke and there were 267 associated deaths. They also tracked how much calcium they consumed from any source.
They found that people who used calcium supplements regularly were 86% more likely to suffer a heart attack than those who did not.
This constituted "a statistically significantly increased myocardial infarction [heart attack] risk in comparison with non-users of any supplements", say the four co-authors led by Professor Sabine Rohrmann, from Zurich University's institute of social and preventative medicine.
Those who obtained their calcium exclusively from supplements were 2.7 times more likely than non-users to experience a heart attack, they say in their research paper, published in the medical journal Heart.
"Increasing calcium intake from diet might not confer significant cardiovascular benefits, while calcium supplements, which might raise MI risk, should be taken with caution," they conclude.
In an editorial in the journal two professors from Auckland University warn that the safety of calcium supplements "is now coming under increasing scrutiny". Previous research has linked them to kidney stones and gut and abdominal symptoms, they say.
"Calcium supplements have been widely embraced by doctors and the public on the grounds that they are a natural and therefore safe way of preventing osteoporotic fractures. 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 say.
"We have good evidence that calcium is good for bones. Calcium is important for other organs, including the heart," said Dr Kevin Fox, the Royal College of Physicians' spokesman on cardiology, who added that the study's findings should be "treated cautiously".
But he added: "The message is that if you have a medical need to take calcium supplements to protect your bones, you should do so and there is good evidence to support this. If you have no medical need, then just stick to a healthy mixed diet and don't take unnecessary supplements."
Natasha Stewart, a senior cardiac nurse with the British Heart Foundation (BHF), said supplements did not necessarily lead to heart attacks.
"This research indicates that there may be an increased risk of having a heart attack for people who take calcium supplements. However, this does not mean that these supplements cause heart attacks," she said.
Further research was now needed "to determine whether potential risks of the supplements outweigh the benefits calcium can give sufferers of conditions such as osteoporosis. If you have been prescribed calcium supplements, you should still keep taking your medication, but speak to your doctor if you have any concerns", she said.
Dr Claire Bowring, of the National Osteoporosis Society, also urged cautious use of supplements. "This study further highlights the need for care when considering taking calcium supplements. If you get all of the calcium that you need from your diet then a supplement will not be necessary. Boosting calcium beyond recommended levels has no extra benefit for bones.
"Supplementation may be warranted if you are unable to get enough calcium in your diet, but it needs to be done with consideration", she said.
Anyone with a heart condition or who is at risk of a heart attack should talk to their doctor before deciding whether or not to use calcium supplements, she added.
