In reply: Glendenning raises a number of interesting points, which require some clarification.

Are ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3) biologically equivalent?

The statement that ergocalciferol and cholecalciferol are bioequivalent in humans is made by most authoritative textbooks, based mainly on evidence from early studies of the antirachitic efficacy of ergocalciferol and cholecalciferol compounds, and contrasts with reduced efficacy of ergocalciferol in birds and monkeys.1 Recent studies using more precise measurements have raised some doubts as to the absolute equivalency of ergo- and cholecalciferol, but the differences are marginal2 and not universally found.3 There are few recent data on relevant biological endpoints. Serum concentrations of the active hormone, 1,25-dihydroxyvitamin D, were not different after administration of ergo- or cholecalciferol,2 and increases in bone mineral density were greater after ergocalciferol therapy than after cholecalciferol in patients taking anticonvulsants.1 In short, on current evidence, differences in biological activity between ergocalciferol and cholecalciferol are likely to be relatively minor.

Are there problems monitoring therapy?

25-Hydroxyvitamin D values may be used for monitoring treatment. The possibilities of impaired detection of the 25-hydroxy metabolite of ergocalciferol by some assays,2 and perhaps a smaller rise in total 25-hydroxyvitamin D concentrations after low doses of ergocalciferol, should be borne in mind when monitoring therapy.

What is the current recommendation for vitamin D supplementation?

While the availability of larger dose sizes and/or cholecalciferol preparations would be helpful, 800 IU of ergocalciferol and 1 g of calcium for six months was shown to reduce secondary hyperparathyroidism in older patients,1 and 600 IU/day (same for ergo- and cholecalciferol) is the new recommended adequate intake for older patients with limited sun exposure.1