Data Inconsistent on Calcium Supplement Benefits, Risks

A range of trials has failed to resolve whether calcium supplementation significantly decreases the risk of fracture or increases the risk of cardiovascular events.

Consistent evidence is lacking to determine whether calcium supplements are beneficial or whether they may be associated with adverse cardiovascular events, according to a review published in the October 17, 2013, issue of the New England Journal of Medicine.

The current recommended dietary intake of calcium is 1000 mg per day for women younger than 50 and men aged 19 to 70 and 1200 mg for women older than 50 and men older than 70. (The recommended upper intake level is 2500 mg per day for those younger than 50 and 2000 mg per day for those older than 50.) Despite the existence of these guidelines, patients and clinicians may often be confused by conflicting advice on calcium intake and whether calcium supplements should be taken to meet one’s daily needs. To clarify what is known about calcium and calcium supplementation, the review summarizes findings from past studies on calcium intake, fracture risk, and safety concerns associated with calcium supplements.

Cross-sectional surveys have shown that 43% of American adults, and almost 70% of postmenopausal women, regularly take calcium supplements. Nonetheless, most adults do not consume the recommended amount of calcium. A population-based study found that dietary intake of elemental calcium averaged 900 mg to 1200 mg in men and 750 mg to 850 mg in women in the United States.

Observational studies indicate that when daily calcium intake is lower than 700 mg to 800 mg, the risk for bone loss and fracture increases. Therefore, many studies have assessed the effect of calcium supplementation on fracture risk. However, most of these studies include vitamin D rather than isolating the effect of calcium supplementation and do not specifically recruit patients with low calcium intake. One such study, the Women’s Health Initiative (WHI) Calcium/Vitamin D Supplementation trial, did not find a significant reduction in hip or any other fractures in women taking 1000 mg of elemental calcium and 400 IU of vitamin D supplements per day compared with those receiving placebo.

However, 2 meta-analyses did show a small but significant decrease in fracture risk associated with calcium supplements. One analysis of 16 trials of calcium and vitamin D supplements found that the supplements were associated with a 12% reduction in risk for any fracture. However, the benefits were significant only among institutionalized patients and not among community-dwelling patients. A second analysis of 9 trials of calcium supplements alone found that supplementation was associated with a 10% overall reduction in fracture risk. But another analysis of 3 trials involving calcium supplements alone confusingly contradicts these results, finding that supplementation was linked with a 50% increase in hip fracture risk.

Evidence regarding a potential association between calcium supplements and cardiovascular events is just as varied. A meta-analysis of 15 trials found an increased risk of heart attack among those assigned to take calcium supplements alone. A second analysis conducted by the same researchers that included trials of calcium plus vitamin D supplementation showed similar results. However, an analysis conducted in 2010 that included all participants from the WHI trial found no significant relationship between calcium plus vitamin D supplements and cardiovascular events.

In addition, 2 large cohort studies found an association between calcium supplements and increased risk of cardiovascular events, while a Canadian prospective cohort study and the extended-follow up to the WHI trial did not find a significant association.

The author of the current review, Douglas C. Bauer, MD, concludes that more research is needed to determine whether calcium supplements may increase cardiovascular risk and to determine differences in the effect of dietary calcium and that of supplemental calcium. Until then, he suggests that patients with calcium deficiencies consume more dairy products and other calcium-rich foods before resorting to supplements.