Jan.15 2008 -- Calcium supplements, generally thought to preserve both bone and heart health, may boost the risk of heart disease in healthy postmenopausal women, according to New Zealand researchers.
"Loading with high doses of calcium reduces bone loss but at a cost in heart health that is not justified," says researcher Ian Reid, MD, professor of medicine and endocrinology at the University of Auckland.
Calcium, Heart Attack Study Details
Reid and his colleagues followed 1,471 healthy postmenopausal women, ages 55 and above, assigning half to get a daily calcium supplement of 1,000 milligrams and half to placebo pills. The average age in both groups was 74.
Calcium supplements are typically prescribed to women after menopause to preserve bone health, and some studies suggest it might also protect heart health by improving the ratio of good cholesterol to bad cholesterol.
The New Zealand researchers initially conducted the study to look at the effect of calcium on bone health, says Reid, who has received research support from calcium supplement manufacturers. This study is what is known as a secondary analysis. Researchers evaluated the women's calcium intake from diet and examined them every six months for five years, looking for reports of heart attack, stroke, or sudden death.
The women in the supplement group got 861 milligrams of calcium from diet per day, on average, boosting their total daily intake to 1,861. The placebo group averaged about 853 milligrams of calcium daily from their diet.
Calcium, Heart Attack Results
To obtain a more complete picture, the researchers also looked for events not reported at the visits by checking hospital admissions and reviewing death certificates of those who had died.
Heart attacks were more common in the calcium group, with 31 women on supplements having 36 heart attacks compared to 21 women on placebo having 22 heart attacks during the follow-up period.
The risk of a heart attack was about 1.5 times greater for those in the supplement group, but the link did not reach statistical significance.
Considered together, strokes, heart attack, or sudden death were more common in those on supplements than on placebo, but the differences -- when taken as a whole -- were statistically only of borderline significance, Reid's team found.
The researchers took into account such factors as cigarette smoking, high cholesterol, and blood pressure problems.
Calcium, Heart Attack: What's the Mechanism?
Reid cautions that the findings must be replicated and plans to do more research on the proposed link.
But he speculates that the calcium supplements may elevate blood calcium levels and possibly speed calcification in blood vessels, which is known to predict the rates of vascular problems such as heart attack.
Second Opinion: Calcium, Heart Attacks
The link between calcium supplements and heart attack suggested by the New Zealand team "seems implausible," says Robert P. Heaney, MD, John A. Creighton University professor at Creighton University in Omaha, Neb., and a long-time researcher of calcium's effect on health.
Typically, Heaney tells WebMD, "Extra calcium doesn't build up in your arteries. The body regulates the blood concentration of calcium.'' Only in people who have lost the ability to regulate calcium levels could the blood concentration of calcium increase, he says, and this condition is rare.
Calcium and Heart Health Advice
Women should keep taking the recommended amounts of calcium, Heaney says. "Postmenopausal women should be getting 1,500 milligrams [a day] through diet and supplements," he says.
The levels recommended by the Institute of Medicine are a bit lower: 1,200 milligrams of calcium for men and women ages 51 and older, and 1,000 milligrams for those 19 to 50.
"Even if it turns out this [proposed link between calcium supplements and heart attacks] is true and replicated [with further research] you have to weigh that against fracture protection," Heaney says of calcium supplements.
Reid disagrees, suggesting women over the age of 70 and some others should rethink calcium supplements.
"It is likely that this is primarily a problem for elderly women because they are more likely than younger subjects to have prevalent coronary heart disease," he tells WebMD. "Therefore it seems wise to advise against [high amounts of] calcium supplementation in those over the age of 70 years and in those known to have coronary heart disease. Aiming at a total calcium intake of approximately 1 gram [1,000 milligrams] a day [equivalent to four servings of dairy products] seems sensible in these subjects."
For instance, a woman who took in 500 milligrams of calcium from foods should take no more than 500 milligrams in supplements daily, he says.
Younger women can continue supplementing without worry, he says. "At present, there is no evidence of adverse cardiovascular effects of calcium supplementation in younger women, so the conventional use of calcium supplements seems reasonable in these subjects."