Conundrum of vitamin D on glucose and fuel homeostasis

Abstract

As an endocrine hormone, vitamin D plays an important role in bone health and calcium homeostasis. Over the past two decades, the non-calcemic effects of vitamin D were extensively examined. Although the effect of vitamin D on beta cell function were known for some time, the effect of vitamin D on glucose and fuel homeostasis has attracted new interest among researchers. Yet, to date, studies remain inconclusive and controversial, in part, due to a lack of understanding of the threshold effects of vitamin D. In this review, a critical examination of interventional trials of vitamin D in prevention of diabetes is provided. Like use of vitamin D for bone loss, the benefits of vitamin D supplementation in diabetes prevention were observed in vitamin D-deficient subjects with serum 25-hydroxyvitamin D < 50 nmol/L (20 ng/mL). The beneficial effect from vitamin D supplementation was not apparent in subjects with serum 25-hydroxyvitamin D > 75 nmol/L (30 ng/mL). Furthermore, no benefit was noted in subjects that achieved serum 25-hydroxyvitamin D > 100 nmol/L (40 ng/mL). Further studies are required to confirm these observations.

Keywords: Vitamin D, Glucose metabolism, Diabetes mellitus, Insulin sensitivity, Beta cell function

Core Tip: Vitamin D deficiency is a well-recognized health issue and contributes to bone loss and calcium dysregulation. Evidence suggests that excess vitamin D is not in and of itself of therapeutic benefit. Available clinical data suggests that vitamin D supplementation appears to limit the development of diabetes in vitamin D deficient subjects. However, no benefit was observed in non-vitamin D deficient subjects. Furthermore, overreplacement of vitamin D is of no beneficial effect and could possibly be harmful.