Calcium supplements, taken by millions of elderly people and post-menopausal women to prevent bone thinning, may double the risk of having a heart attack, a study has found.
Researchers warned that the pills should be “taken with caution”, and experts commenting on the findings questioned their safety.
Previous studies linked higher calcium intake with a reduction of heart disease risk factors such as high blood pressure, obesity and Type 2 diabetes.
But the new research from Germany points to a vital difference between dietary calcium from sources such as milk, cheese, greens and kale, and supplements.
Researchers analysed data on 23,980 German men and women aged 35 to 64 taking part in a study called the European Prospective Investigation into Cancer and Nutrition.
Participants whose diets included a moderate intake of calcium - around 820 milligrams (mg) daily - from all sources had a 31% lower heart attack risk than those with the lowest intake.
But no significant benefit was seen when calcium intakes rose to more than 1,100 mg per day.
The picture changed for the worse when the scientists focused on supplements. People taking supplements that included calcium were 86% more likely to suffer a heart attack than those taking no supplements.
For participants who only used calcium supplements, heart attack risk more than doubled.
The researchers led by Professor Sabine Rohrmann, from the University of Zurich, Switzerland, wrote in the online edition of the journal Heart: “In conclusion, this study suggests that increasing dietary calcium intake might not confer significant cardiovascular benefits, while calcium supplements, which might raise MI (myocardial infarction, or heart attack) risk, should be taken with caution.”
Natasha Stewart, the British Heart Foundation’s senior cardiac nurse, said: “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.
“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 have been prescribed calcium supplements, you should still keep taking your medication, but speak to your doctor if you have any concerns.”
- Li K, et al. Associations of dietary calcium intake and calcium supplementation with myocardial infarction and stroke risk and overall cardiovascular mortality in the Heidelberg cohort of the European Prospective Investigation into Cancer and Nutrition study (EPIC-Heidelberg). BMJ 2012; Advance online publication.