The measurement of vitamin D has proven to be a tricky business. The development of laboratory testing and validation has been fraught with problems. It seems that the best test we have is to measure 25- OH vitamin D, this metabolite in the blood does correlate well to exposure of vitamin D. The real question , however , is whether this marker correlates with the biological effect of vitamin D which we are studying.

The association of low vitamin D blood levels with an increase in Type 2 diabetes and is one such conundrum. We do not know why these things are apparently linked or if the association with diabetes is actually due to an effect of vitamin D at all. There is a need for further research before these issues can be clarified.