Cardiovascular risks of calcium supplements in women
Authors: Graeme Jones, Tania Winzenberg
Journal: BMJ (2008)

Calcium is an important component of bone, and a sufficient intake of calcium is needed for bone homoeostasis. Calcium supplements can reduce the risk of fractures in elderly women who are deficient in calcium and vitamin D, but data on the risk of adverse effects on cardiovascular outcomes have so far been inconclusive. In their accompanying paper, Bolland and colleagues report a preplanned secondary analysis of their randomised controlled trial of calcium supplements in 1471 postmenopausal women. They analysed the effect of calcium supplements on myocardial infarction, stroke, and sudden death.1

Calcium and vitamin D supplements have been shown to reduce the risk of hip fractures in elderly institutionalised women who are deficient in calcium and vitamin D.2 More recent large trials based in the community have been negative, but this may have been the result of poor adherence,3 which is particularly important for calcium to be effective. Benefit has been shown only in analyses restricted to women who adhered to treatment for total fractures,4 hip fractures,5 and forearm fractures.6 A recent meta-analysis suggested an overall 12% decrease in the relative risk of fracture.3 If we assume that the average incidence of fracture in women aged 80-84 years is 4% each year, then the number needed to treat (NNT) for five years to prevent one fracture is 42.7

Calcium supplements have generally been thought not to be harmful. Patients often complain of constipation,6 and the risk of renal calculi is slightly increased.5 Possible positive effects on obesity and cholesterol have implied a protective effect on cardiovascular outcomes. However, calcium based phosphate binders are associated with increased vascular calcification in patients about to undergo dialysis.8 The potential mechanisms of arterial calcification are many and complex,9 but biologically plausible mechanisms support the role of calcium. Under certain stimuli, vascular smooth muscle cells may undergo a phenotypic switch to bone-like cells,9 and in the presence of high amounts of calcium these may be capable of producing vascular calcification.

So what does the analysis by Bolland and colleagues tell us? The data were not totally consistent, but if most weight is placed on the verified events (from medical records alone as well as a search of a national database of hospital admissions), women taking calcium had a significantly higher risk of cardiovascular disease (relative risk 2.12, 95% confidence interval 1.01 to 4.47, P=0.047), especially myocardial infarction (1.49, 0.86 to 2.57, P=0.16). The equivalent risks for stroke were 1.42 (0.83 to 2.43, P=0.21) and 1.37 (0.83 to 2.28, P=0.23), respectively. The survival curves started to diverge after about two years, indicating a slow onset of effect.

When directly comparable incidence rates for myocardial infarction and stroke are used in 80-84 year old women, the number needed to harm (NNH) for five years is 10-17 and 26-28, respectively.10 Both are considerably less than the NNT, which indicates that the risks greatly outweigh the benefits in an elderly population. Indeed, the absolute risk of fracture would have to be four times that of cardiovascular disease for the NNT to be less than the NNH. This would be met only in women with a very high risk of fracture, in whom guidelines recommend more effective treatment anyway. These estimates are subject to wide confidence intervals and they contrast with the absence of increased cardiovascular risk in the women’s health initiative study in younger postmenopausal women, although poor adherence in that study may contribute to this.5

These adverse findings need to be replicated by re-examining the databases of other large trials for cardiovascular endpoints, especially in people with good long term adherence. Until then, the use of calcium supplements as monotherapy in elderly people does not seem to be justified, except possibly in women with very low calcium intakes. Data suggest that it may be safe to use supplements to prevent osteoporosis in younger postmenopausal women. Caution is necessary, however, given the need for long term use to maintain the benefit on bones and the likely time lag between treatment with calcium and adverse cardiovascular events.

The place of calcium as co-therapy with other treatments for osteoporosis—such as bisphosphonates and strontium—is less clear. None of these agents has been shown to be effective without co-administration of calcium and vitamin D. The literature on dialysis provides a rationale for the use of bisphosphonates to prevent arterial calcification,11 so these agents may offset one of the harmful effects of calcium. The recent report of decreased mortality caused by cardiovascular disease starting about 18 months after treatment with zoledronic acid is consistent with this hypothesis.12 Furthermore, these patients are at much higher risk of fracture, so the NNT would be lower. It therefore seems reasonable to continue supplementation in people taking bisphosphonates.