It's a controversial topic that's been debated by scientists for nearly a decade: Are calcium supplements bad for your heart?
While findings from previous randomized controlled trials have been inconsistent, evidence from some have raised concern that excess calcium from supplements may increase the risk of heart attack and stroke among women.
Now, a new study from Johns Hopkins University School of Medicine in Baltimore and colleagues adds to the argument that, yes, there is reason for caution.
And perhaps for men, too.
For the study, published this month in the Journal of the American Heart Association, researchers followed 2,742 multiethnic men and women, aged 45 to 84, for 10 years to determine if calcium – from diet or supplements – was linked to coronary artery calcification (CAC), an early sign of heart disease.
CAC is a buildup of calcium in fatty plaques in the heart's artery walls.
It's measured using a special X-ray test called computed tomography (CT scan). The test can show whether you're at risk for heart attack before other symptoms occur.
At the onset of the study, researchers assessed participants' diets over the previous year as well as their use of calcium supplements. At the beginning of the study, and again after 10 years, their CAC was measured using a CT scan.
The findings: The risk of developing calcified coronary arteries over 10 years was 22 per cent higher in adults who took calcium supplements than those who did not.
That was true after taking into account daily calorie intake, body weight, exercise, smoking, alcohol intake and other risk factors for heart disease.
The highest risk for plaque buildup was found among supplement users who consumed the least calcium from their diet.
Conversely, the lowest risk was observed in people who didn't take supplements but consumed the most calcium from their diet.
It's thought that a high intake of calcium in a single dose from supplements can cause a transient elevated blood-calcium level which, in turn, can lead to calcium depositing in artery walls. Over time, calcified fatty plaques can harden and narrow blood vessels, hindering blood flow to the heart.
Excess blood calcium may also influence inflammation, insulin activity and body-weight regulation, other factors that could accelerate hardening of arteries.
It's thought that dietary calcium is metabolized differently than calcium in supplements. Unlike high-dose supplements, calcium in foods is absorbed into the bloodstream in smaller amounts throughout the day.
Strengths and shortcomings
A key strength of the study is that it measured changes in coronary artery calcification over time, at the start of the study and 10 years later.
Previous research looked at calcium artery scores only once, at the end of the study period. It's impossible to know whether participants had calcified coronary arteries to begin with.
The study isn't without limitations, though. It was observational in nature and, as such, it does not prove that calcium supplements are dangerous for heart health.
The study also relied on participants to recall their past food intake and supplement use, which can result in error.
It's also possible that people who eat calcium-rich foods, versus those who rely on calcium supplements, eat more vegetables, nuts and fish, a dietary pattern which delivers cardiovascular benefits.
Why we need calcium
Calcium is essential to health. It's required for muscle function, nerve transmission and hormone secretion. And it's vital to support the structure of bones and teeth.
Consuming adequate calcium is also thought to help reduce the risk of precancerous colon polyps, maintain healthy blood pressure, prevent pregnancy-induced high blood pressure (pre-eclampsia) and ease symptoms of premenstrual syndrome.
So you need calcium. The question is, then, how should you get it?
What about my supplement?
If you use calcium supplements, you're probably wondering what to do.
The decision to continue or stop taking your supplement requires a conversation with your doctor to weigh the likelihood of risk against the benefits of supplementation.
Consider the reason you're taking a calcium supplement. The evidence is weak, at best, that calcium supplements prevent bone fractures in adults who do not have osteoporosis, including men and pre- and postmenopausal women.
If you have osteoporosis or you're at high fracture risk (e.g., you have low bone mass), according to Osteoporosis Canada there's no evidence that taking calcium supplements alone prevents fractures.
In addition to potential heart risks, research has linked calcium supplements to kidney problems, constipation and bloating.
Discuss with your dietitian or doctor your calcium intake from supplements, diet and medications (e.g., calcium-containing antacids). It's possible you're consuming more calcium than you realize – and more than you need.
With potential for harm and inconclusive benefits to bone health, it's prudent to be cautious about supplementing with calcium, particularly so when studies have consistently found no increased risk of heart problems from consuming calcium from foods.
Meet your daily calcium requirement from dietary sources, preferably.
If you need to rely on a calcium supplement to do so (and some people will), choose one that's made from calcium citrate rather than calcium carbonate.
Calcium-citrate supplements provide a lower dose of the mineral, typically 250 to 350 mg per tablet, roughly the amount found in one dairy serving. Calcium-carbonate supplements, widely available and less expensive, contain 500 to 600 mg of calcium per tablet.
To me, it makes sense to consume a smaller dose of supplemental calcium at one time.
Calcium citrate is also much less likely to cause bloating and constipation than calcium carbonate.
If you need to take more than one calcium pill, don't take more than one at a time.
Spread your calcium intake out over the course of the day.
Leslie Beck, a registered dietitian, is based at the Medisys clinic in Toronto.