Low-calcium diet

Our hospital laboratory customarily reports all concentration ratios with the same units for both numerator and denominator (i.e., moles per mole [mol/mol] or micromoles per micromole [μmol/μmol]), and this was the case for both the results and the normative data for our study.¹ However, as Robert Heaney rightly points out, these values were inadvertently mislabelled and reported with units of micromoles per mole. Nonetheless, because the numbers for both the reported results and the reference values are correct (with units of moles per mole), neither the results, their interpretation nor our conclusions are affected by this error.

The study by Kuhnlein and colleagues² does indeed report 21 (standard deviation 400) mg as the calcium intake derived from the traditional portion of a mixed diet. During manuscript revision, this figure was accidentally substituted for the estimated total daily calcium intake, which by extrapolation to a fully traditional diet is on the order of 123 mg/day; this remains profoundly low compared with the recommended daily intake of 900 mg. In any case, as Heaney notes, the reported standard deviation precludes placing too great an emphasis on the precise numeric value. Hence, neither 20 mg nor 120 mg should be regarded as more than a round number illustrating the magnitude of the discrepancy, and neither the results nor the conclusions inferred from them are materially affected by reference to the extrapolated value. Moreover, given this uncertainty and the absence of any reports of bone mineral density for a population using a traditional diet alone, it may be premature to speculate as to the sufficiency of bone mineralization under these circumstances. Further studies in this area are clearly warranted.

Elizabeth Sellers Department of Pediatrics and Child Health University of Manitoba Winnipeg, Man. Atul Sharma Celia Rodd Department of Pediatrics McGill University Montréal, Que.