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Vitamin D in type 2 diabetes mellitus

Vitamin D is a very relevant substance at the nutritional level, since it regulates blood levels of calcium and phosphorus, participates in bone mineralization, and interferes with immunological processes. Currently, vitamin D has been suggested to improve glycemic control in patients with type 2 diabetes mellitus (T2DM).1 An investigation by Issa2 conducted in Lebanon in 2017 analyzed the association between vitamin D and T2DM. Results show a possible inverse relationship between vitamin D levels and T2DM, as well as associated complications. Vitamin D replacement therapy is likely to decrease T2DM incidence and improve glycemic control, especially by increasing insulin secretion, decreasing insulin resistance, and decreasing inflammation. Although additional studies are required, vitamin D replacement therapy may be beneficial to prevent and improve some diabetic complications, especially nephropathy and cardiovascular diseases. A review by Wu et al.3 conducted in China in 2017 assessed the efficacy of vitamin D supplementation in reducing glycated hemoglobin A1c (HbA1c) and fasting blood glucose (FBG) levels. Results of some studies reviewed by Wu et al.3 showed that vitamin D supplementation is associated with reduced HbA1c levels but had no influence on FBG levels. However, analysis from other studies suggested that vitamin D supplementation was associated with reduced HbA1c and FBG levels. It was possible to conclude that vitamin D supplementation could be effective in improving glycemic control in non-obese or vitamin D deficient patients with T2DM. Similar results were obtained in the review by Lee et al.4 conducted in the United States in 2017. A study by Upreti et al.5 conducted in India in 2018 investigated the effect of oral vitamin D supplementation on glycemic control in patients with T2DM and hypovitaminosis D by conducting a randomized placebo-controlled trial with 60 patients. Case group showed significant decrease in mean HbA1c and FBG levels. Supplementation to achieve normal levels of vitamin D can be a promising adjuvant therapy for T2DM patients and coexisting hypovitaminosis D. It is worth noting that this study has some limitations, including sample size and statistical analyses, since paired and unpaired t-tests were used rather than covariance analysis. An analysis by Harrison and Sisley6 conducted in the United States in 2018 revealed that vitamin D deficiency is related to T2DM and showed that this may be due to the action of vitamin D in the paraventricular nuclei in the hypothalamus of the brain. A clinical trial by Safarpour et al.7 conducted in Iran in 2018 assessed the effect of vitamin D supplementation on glucose and lipid profiles, blood pressure, and biomarkers of liver and kidney in patients with T2DM. A double-blind randomized clinical trial was conducted with 90 patients with T2DM and serum 25-hydroxy vitamin D levels of less than 30 ng/ml. e