Vitamin D plays a crucial role in bone and mineral metabolism
and has been increasingly linked to extra-skeletal diseases,
including diabetes mellitus and its complications [1–6]. Identification
of vitamin D receptor (VDR) expression in almost
all human cells and the fact that VDR activation regulates
hundreds of genes is regarded as a good rationale to evaluate
the involvement of vitamin D in various chronic diseases like
cardiovascular diseases [7–11, 12•]. Although the knowledge
on vitamin D has rapidly increased over the last few years,
clinicians are nowadays frequently confronted by divergent
recommendations regarding the clinical use of vitamin D
[12•]. We therefore aimed to present an up-to-date overview
on the role of vitamin D in the development of insulin resistance
and type 2 diabetes. After a brief introduction on vitamin
D metabolism we summarize mechanistic and observational
studies. The main focus of our review is on recent randomized
controlled trials (RCTs), which evaluated vitamin D effects on
parameters of glucose metabolism and incident diabetes. After
some general considerations regarding vitamin D treatment
we present our conclusions.