There is an ongoing discussion if 25-OH-D3 serum values can be used to diagnose vitamin D insufficiency. At least for rickets outcome there are now quantitatitive data that allow a comparison of clinical symptoms, radiological findings, cholecalciferol and alkaline phosphatase levels. I have rearranged the values of a new paper into the following figure

The first figure in the circle gives the spring rate, the second the fall rate where the outcome of the highlighted circles is shown by the arrow.
It is a vexing picture – X-ray and AP percentages are quite stable, X-ray high and AP low; symptoms and 25-OH-D3 highly variable. What is the true rickets prevalence?
scienceblog:doi:10.1111/j.1442-200X.2007.02343.x

Addendum

A single dose of vitamin D may already enhances immunity to mycobacteria: scienceblog:doi:10.1164/rccm.200701-007OC