Potential negative cardiovascular effects of calcium supplements

Calcium supplements have been used for decades in the prevention and, as an adjuvant, for treatment of osteoporosis because low calcium intakes are frequent and have negative effects on bone health. There is an abundant literature showing the beneficial effects of an adequate calcium intake on the maintenance of bone mineral density (BMD) in adults, and on the slowing of the loss of BMD in the elderly. There is even some evidence that it has a moderate effect on fracture risk. In other words, the prescription of calcium supplements in the prevention of osteoporosis has its place, in so far as it causes no harm. Although a very high intake of 3–4 g per day is not recommended, there is no proof that such intakes are harmful. Hypercalciuria in kidney stone formers and gastrointestinal intolerance are the only well-known contraindications. Fractional calcium absorption decreases with higher intakes and protects the body from excess intake, at least in part. Indeed, calcium supplementation had no safety restrictions. The negative effects of calcium supplements listed in the recent report of the Institute of Medicine of the US [1] include kidney stones, milk-alkali syndrome and hypercalcemia with its various consequences. But the risk of renal stones is not confirmed [2], that of hypercalcemia is not documented, and as for provoking the rare milk-alkali syndrome, it needs more than just a calcium supplement. If the same strict scientific parameters were applied for assessing the upper tolerable intake level of calcium (or the lowest observed adverse effect level), as for assessing the positive effects of calcium supplements on bone, it would be impossible to define an upper safety limit. New information on the possibility of negative cardiovascular effects puts a cloud in the so far quiet sky of calcium supplementation. In the paper by I. Reid et al. in this issue [3], the two studies by the same authors, which provided this troublesome information, are again discussed as a response to the international reaction and criticism which these two papers provoked [4, 5]. The authors defend very carefully their observation that calcium supplements increase cardiovascular risk and discuss the hypothetical mechanisms. As calcium prescribers, one might be tempted to accept this notion, safe in the knowledge that calcium from nutrients is harmless and therefore preferable. However, patients rarely consume the recommended amount of calcium with their food, and for this reason, we should examine carefully the claim for harmful effects of calcium supplements. Without discussing the methodical aspects of the two studies—the authors of the manuscript in this issue do this extensively—a few considerations allow us to question their practical significance.