Vitamin D daily short-term supplementation does not affect glycemic outcomes of patients with type 2 diabetes

There is currently insufficient evidence of a beneficial effect to recommend vitamin D supplementation for optimizing glycemic status in patients with type 2 diabetes mellitus (T2DM). Taking into consideration the significant extra-skeletal effect of vitamin D on pancreatic β-cell function and insulin secretion and the large number of scientific evidence supporting the inverse association between vitamin D status and hyperglycemia, this review article aims to examine whether vitamin D supplementation therapies are beneficial to patients with T2DM considering specific factors through randomized controlled trials (RCTs). EBSCOhost and Medline databases were searched from the beginning of 2009 until the end of 2014 for RCTs in patients with T2DM. Parameters, such as baseline vitamin D levels, frequency/dosage of supplementation, length of the study and type of supplementation, were independently assessed, based on their effect on glycemic status. Although all different types of supplementation were safe and effective in the achievement of vitamin D sufficiency in a dose-dependent way, the impact on glycemic status was different. 14 RCTs were included with daily supplementations ranging from 400-11.200 IU/daily, 40.000-50.000 IU/weekly and 100.000-300.000 IU/intramuscularly or once given, for a period from 8 to 24 weeks. Daily supplementation of vitamin D (up to 11.200 IU) showed no effect, whereas combined supplementation, with calcium (≥300 mg), and with vitamin D doses similar to the RDA, showed positive effects. Additionally, high weekly doses of vitamin D (40.000-50.000 IU) were effective on glycemic outcomes but available data are limited.