Response to Costelloe et al.

In their Letter to the Editor, Costelloe et al. draw attention to the fact that the Roche electrochemiluminescence immunoassay (ECLIA) 25-hydroxyvitamin D assay is highly reproducible with a low bias against LC-MS and therefore suitable for high-throughput measurement, but with the major drawback of not detecting 25-hydroxyvitamin D_2. The predominant natural source of vitamin D in humans is vitamin D₃ and nutritional intake of vitamin D₂ does not contribute significantly to the human vitamin D reservoir. ^1–3 In a recent analysis of 153 randomly selected samples with total 25-hydroxyvitamin D concentration <60 nmol/L, we found only six to have detectable 25-hydroxyvitamin D₂ as determined by liquid chromatography-tandem mass spectrometry (LC-MS/MS) (unpublished observations). These concentrations were negligible (<8 nmol/L), confirming the data on 291 cases, which were representative for the German population, from our paper. ⁴ There is no vitamin D fortification of dietary products in Germany and in the USA fortification is primarily with vitamin D₃, in quantities which are too low to have a substantial beneficial effect. ⁵ Furthermore, in Germany, vitamin D as a supplement is available over-the-counter or the internet only in the form of vitamin D₃ and in the USA, as low content multivitamin (vitamin D₂ or D₃) or higher content preparations as vitamin D₃. ⁵ Unlike the situation in the UK and the USA, vitamin D₂ as a prescription supplement is not available in Germany. As a result we can assume that vitamin D₂ supplementation or prescription plays no role in Germany.