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Sharonne Hayes, MD, the director of the Women's Heart Clinic at the Mayo Clinic, in Rochester, Minn., says that in the past she did not discourage any of her patients from taking calcium supplements. Now, she says, she will try to determine if her patients have a calcium deficiency before making a recommendation either way.
"Up until this point, there was no evidence of harm," says Dr. Hayes, who was not involved in the new research. "Now there needs to be a good reason to use [supplements]."
In the study, researchers combined data from 11 clinical trials in which patients were randomly assigned to receive calcium supplements or placebo pills. In all, the trials included nearly 12,000 people who were followed for an average of about four years. Most were women, and the average age was 72.
About 2.7% of the participants taking calcium had heart attacks during the trials, compared with 2.2% of those taking a placebo. This translated into an increased risk of between 27% and 31%, depending on how the researchers analyzed the data.
A surprising finding
Dr. Baron was surprised by the findings. "Calcium supplements have been widely used for quite a while," he says. "Other studies have suggested, if anything, that [calcium] might have a protective effect."
The findings do not imply that people should reduce the amount of calcium in their diet, Dr. Hayes stresses. Calcium is found in vegetables, fortified cereal, and dairy products such as milk and yogurt, which are an important source of vitamin D in addition to calcium.
"Calcium isn't bad," she says.
John Cleland, MD, a cardiologist at Hull York Medical School, in the U.K., says that calcium supplements are helpful for "very few" patients.
It's unclear how calcium supplements might increase heart attack risk. They may contribute to the hardening of arteries (atherosclerosis) by increasing calcium levels in the blood, or they may cause changes to blood flow. Hormone responses caused by calcium may also be involved, the study notes.
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