High-dose calcium supplements may damage the cardiovascular system

Women need to be cautious and knowledgeable regarding high-dose calcium supplements as the safety and efficacy of calcium supplements in preventing bone loss is now suspect.  In addition, according to some studies, high dose calcium supplements may damage the cardiovascular system.  Learning the pros and cons of calcium supplementation is important, however, the wisest choice for all women is to make sure their diet contains adequate calcium.

A steady stream of research has questioned the safety of high-dose calcium supplements for the cardiovascular system, in addition to questioning their value for preventing osteoporosis.  This has huge public health implications, as 52 percent of American women take calcium supplements.¹

The most recent study took data from the Multi-Ethnic Study of Atherosclerosis (MESA). At the start of the study, participants answered questions on their diet, medications, and supplements, and their coronary artery calcium was measured. Ten years later, coronary calcium was measured again.

Participants were broken into five groups (quintiles) based on their typical total daily calcium intake (food plus supplements).  In the highest quintile of total calcium intake, the risk of coronary artery calcification at 10 years was reduced by 27 percent, suggesting that calcium is protective.  However, the use of calcium supplements was also associated with an increased risk (12-22 percent) of coronary artery calcification, suggesting that dietary calcium and supplemental calcium may have contradictory effects.

Each quintile was then split into users and non-users of calcium supplements.  Out of those 10 groups, the greatest risk of coronary artery calcification was in supplement users in quintile 1, meaning the group that had the lowest food calcium intake but took calcium supplements.² This study suggests that dietary calcium is protective, and supplemental calcium may not be and may even be harmful.

The question is if calcium supplements are harmful, but food calcium isn’t, why?

The primary difference seems to be the dose.  Many calcium supplements contain 1000 mg or more, which is much more than would be found in a typical meal.

Blood calcium is very tightly regulated, always kept in a narrow range.  However, after a large dose (such as a high-dose calcium supplement), the usual regulatory mechanisms are temporarily overwhelmed, there is a transient increase in blood calcium,³ and this could be a problem. Observational studies suggest that elevations in blood calcium contribute to carotid atherosclerotic plaque thickness, calcified plaque, and cardiovascular events.^{4, 5}  This is presumably because elevated blood calcium promotes blood coagulation and deposits of calcium (calcification) in the arteries.

Calcification stiffens the arteries, increases the vulnerability of atherosclerotic plaques, and increases cardiovascular risk.^{6, 7}  Only high dose supplemental calcium can raise blood calcium significantly. 

The question then is, are we sacrificing our bones to save our arteries?

Are women more prone to osteoporosis if they don’t take high-dose calcium supplements?  The research says no.  Calcium deficiency can cause osteoporosis, however the evidence suggests that calcium supplementation is helpful only when someone is vitamin D-deficient.  Vitamin D adequacy allows for increased absorption of calcium when intake is low.  The risk of fracture due to low calcium intake is increased if vitamin D intake is also low.⁸

When calcium and vitamin D were supplemented together, vitamin D plus a smaller amount of calcium (less than 1000 mg) better prevented fractures than vitamin D plus larger amounts of calcium (1000 mg or more).⁹   High-dose calcium supplementation was even found to increase hip fracture risk in a meta-analysis of 4 clinical trials.¹⁰  The take-home message from these studies is that high-dose calcium is not necessary for protecting bones, and may even make things worse, but vitamin D adequacy is important.

Studies don’t agree leaving cause for concern

Not every study has detected a harmful effect of calcium supplements.  Two meta-analyses pooling many calcium supplementation trials determined that regularly using calcium supplements (1000 mg or more calcium/day in most studies) was associated with a 20-30 percent increase in the risk of heart attack and/or stroke.^{11, 12}  However, two other meta-analyses did not did not find any increase or decrease in the risk of cardiovascular events in the groups taking calcium supplements compared to control groups.^13-15

The major difference between the meta-analyses coming to different conclusions was the data included from one trial in particular, the Women’s Health Initiative.  The Women’s Health Initiative allowed women to continue taking their personal calcium supplements during the trial.  Some argue that this could have masked the risk associated with calcium supplements.^{12, 16}  Others argue that the data is still conflicting, and the evidence doesn’t support a beneficial or harmful effect of calcium supplements, yet.^{17, 18}

My conclusion: I advocate a cautious, conservative approach to calcium supplementation

Despite this disagreement, and in light of the new study on arterial calcification, there is enough evidence to be cautious about high-dose calcium supplements.  The wise focus should still be on obtaining adequate calcium primarily with food, especially since high-dose calcium supplements provide minimal (if any) benefit for osteoporosis prevention.

For many people, green vegetables, beans, nuts and seeds most likely provide enough calcium without the need for supplements (see table).  Certain groups – such as pregnant, nursing, and postmenopausal women, as well as those with osteopenia or osteoporosis and those with small appetite for green vegetables – may want to consider adding a small dose of supplemental calcium, spaced throughout the day to assure optimal calcium intake, along with their Vitamin D3 supplements.  For these groups, I recommend using lower dose, whole food calcium supplements (whole powdered seaweed, for example) to more closely replicate getting calcium from foods. 

See the Vitamin Advisor for more information on calcium supplementation.

Plant food sources of calcium¹⁹