Effect of vitamin D supplementation on glycemic control in patients with type 2 diabetes: a systematic review of interventional studies.
BACKGROUND
Diabetes and vitamin D deficiency are global epidemics. Researchers have long been exploring the role of potentially modifiable factors to manage type 2 diabetes. We conducted a systematic review of prospective studies and randomized controlled trials that involved vitamin D supplementation and specifically intended to study glycemic outcomes related to type 2 diabetes.


METHODS
Two authors independently searched Medline and PubMed for longitudinal studies that had assessed the effect of vitamin D supplements on glycemic control, insulin resistance and beta-cell dysfunction in patients with diabetes.


RESULTS
Seventeen randomized control trials and seven longitudinal studies with a minimum follow-up of one month were included. Results of the various short-term studies (follow up ≤ 3 months) suggested that vitamin D supplementation had a positive impact on glycemic control and metabolic parameters such as insulin resistance and beta cell dysfunction. However, the evidence was weak due to the low methodological quality of the studies. There was no significant effect on HbA1c, beta cell function and insulin resistance in the long-term studies (follow up > 3 months). There existed heterogeneity in the methodology of the studies, inclusion criteria, mode of supplementation of vitamin D and the duration of follow up.


CONCLUSIONS
Current evidence based on randomized controlled trials and longitudinal studies do not support the notion that vitamin D supplementation can improve hyperglycemia, beta cell secretion or insulin sensitivity in patients with type 2 diabetes. Large-scale trials with proper study design, optimal vitamin D supplementation and longer follow up need to be conducted.
Division of Endocrinology, Department of Medicine, University of Toronto, Toronto, ON Canada.
Nisha
N
Nigil Haroon
Toronto Western Hospital, University Health Network, Toronto, ON Canada.
Ammepa
A
Anton
Department of Medicine, Wayne State University, Detroit, MI USA.
Jisha
J
John
Endocrinology Unit, Department of Medicine, King George Medical University, Lucknow, India.
Madhukar
M
Mittal
Cholecalciferol; Glycemic control; Insulin sensitivity; Type 2 diabetes; Vitamin D