You're trying to do your bones a favor when you pop your daily calcium pill. And doctors who recommend the pills are trying to do patients a favor too. What then, to make of a suggested link between daily calcium supplements and a slightly increased risk of heart attacks?
The findings, announced in July and noted in last week's Institute of Medicine report on vitamin D and calcium, caused concern among patients and some doctors alike. But many physicians say that the research needs to be confirmed before people start tossing away their calcium pills.
Calcium supplements have long been recommended by doctors to ensure that patients are getting adequate calcium in order to ward off osteoporosis (for adults older than 50, the recommended level is 1,200 milligrams, obtained either from diet or supplements). For people who have osteoporosis, calcium supplements are prescribed in addition to prescription bone-building medications, the first line of treatment. Calcium supplements have been shown to slightly reduce the risk of fracture.
The study, in the British Medical Journal, looked at 15 published clinical trials of calcium supplements. In their analysis of 11 of the trials (involving nearly 12,000 adults with an average age of just older than 40), the scientists observed 166 cases of heart attack among those who took 500 mg. or more of calcium daily and 130 cases among those who took a placebo.
In five of the studies, comprising 8,151 adults, more detailed patient information was available. In this analysis, 143 participants who took calcium had a heart attack, compared with 111 who took a placebo.
Overall, the heightened heart risk was about 30%. Study trials in which vitamin D was also given to patients getting calcium — a common combination — were excluded from the analysis.
The authors, from the University of Auckland in New Zealand, the University of Aberdeen in Scotland, and Dartmouth Medical School, said the findings warranted a reassessment of the use of calcium supplements for adults with or without osteoporosis.
The topic comes up "fairly frequently" with patients, says Dr. Sundeep Khosla of the Mayo Clinic
in Rochester, Minn., president of the American Society for Bone and Mineral Research.
But, he adds, bone doctors have questioned the study in large part because of its exclusion of trials in which vitamin D was taken along with calcium.
Vitamin D at issue
In practice, says Khosla, doctors almost always recommend that calcium supplements be taken with vitamin D, which helps calcium be absorbed by the body. Thus, he and other bone doctors question the finding's clinical relevance.
The exclusion also meant that several highly regarded clinical trials never made it into the analysis, Khosla says, including a few with conflicting findings.
The Women's Health Initiative, funded by the National Institutes of Health, conducted one of the largest of these trials. In it, researchers looked at the effects of calcium and vitamin D supplements on more than 36,000 healthy women who were 50 to 79 years old at the start of the study.
Participants who took 1,000 mg. of calcium and 400 international units (the daily recommended dose) of vitamin D had no increased risk of heart attack after seven to 11 years.
Another study, published July in the Journal of Bone and Mineral Research, followed 1,460 healthy women who were an average age of 75. After 41/2 years of follow-up, heart attack rates for those who took calcium were the same as those who took a placebo.
Some doctors also expressed skepticism over the accuracy of the data about heart attacks in the British Medical Journal study.
Since the goal of the trials in the analysis was to look at bone health, most didn't collect information about participants' underlying risk factors for heart disease — such as smoking or diabetes. Some doctors also questioned whether the heart attack data were accurately collected for all participants.
The doubt expressed by some bone doctors and nutrition experts doesn't surprise Dr. John A. Baron, a co-author of the study and professor of medicine at Dartmouth Medical School. He says that the finding is uncomfortable because it calls for a change in thinking.
"It doesn't fit easily into their previous frame of reference," he says.
Despite their uncertainty, some bone doctors say they'll probably make small changes in their recommendations.
Khosla, for one, says he'll watch to ensure that patients don't go too far over the daily recommended dosage (especially in the case of patients whose bodies can't efficiently process excess calcium, such as those with kidney problems or the elderly).
"Prior to this study, I wasn't as concerned about the upper limit of calcium intake," he says. Now, "if they are already getting 800 mg. or so in their diet, then we only need a 400 mg. supplement."
But though most experts agree that the results bear further investigation, both bone and heart doctors believe that the findings aren't enough for patients to stop taking calcium altogether.
"I think it is a useful study because it has raised an issue of potential concern," said Alice Lichtenstein, director of the Cardiovascular Nutrition Laboratory at Tufts University. "But any type of change as far as decreasing risk of cardiovascular disease would be premature."