To the Editor The study by Xiao et al1 demonstrating increased cardiovascular risk from dietary calcium supplementation highlights an important but underappreciated principle: the nontransitivity of clinical inference. In formal logic, if A implies B and B implies C, then it follows that A implies C. However, if calcium deficiency is associated with poor health outcomes and calcium supplementation can raise calcium levels, it does not necessarily follow that taking calcium supplements benefits health, intuitive as it may seem. As this study shows, intuitively appealing inferences can be not only false but even harmful, and when large populations are affected, the harm can be substantial.
Colgrove R. Dietary and Supplemental Calcium Intake and Mortality. JAMA Intern Med. 2013;173(19):1840. doi:10.1001/jamainternmed.2013.9267
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