In a recent publication in the journal BMC Endocrine Disorders, researchers conducted a systematic review and meta-analysis of randomized, controlled trials utilizing PubMed/Medline, Web of Science, Scopus, Embase, Cochrane Central Register of Controlled Trials (CENTRAL), and NIH’s Clinical Trials Registry from the start of each database up to January 1, 2021.

The researchers included 46 randomized, controlled trials (RCTs) comprised of 2,164 intervention subjects and 2,149 placebo controls.

With regard to the effect of vitamin D supplementation on fasting plasma glucose (FPG), there were 35 studies that included a total of 3,528 participants. Assessments for evaluating changes in FPG found a significant decrease in the intervention group after vitamin D supplementation reporting (weighted mean difference [WMD]: –5.02 mg/dL (–0.28 mmol/L); 95%CI: –6.75 to –3.28 (–0.37 to –0.18 mmol/L), P <.00), suggesting an inverse effect of vitamin D.

A total of 42 studies involving 4,098 participants assessed the effect of vitamin D supplementation on hemoglobin A1c (HbA1c). In the pooled analyses for HbA1c, a statistically significant variation between the intervention and control group was reported as a (WMD (95%CI): –0.20% (–0.29, –0.11); P <.001), also indicating an inverse effect of vitamin D on HbA1c.

A total of 19 studies that included 1,272 individuals assessed the impact of supplementation with vitamin D on homeostasis model assessment insulin resistance (HOMA-IR). The findings reported a statistically meaningful variation between the intervention and control group with (WMD [95%CI]: –0.42 (–0.76, –0.07); P = .019) reported, and it also revealed an inverse effect of vitamin D on HOMA-IR. Moreover, subgroup assessments showed that HOMA-IR levels diminished considerably in many subgroups including study populations from Asia, those with deficient serum vitamin D status, oral supplementation type, duration of 12 or fewer weeks, and vitamin D dose of more than 2,000 international units.

The authors indicated that the subgroup assessments revealed the most efficacy in a higher dose and short intervention period and in subjects with deficient vitamin D status.

The authors wrote, “In the current systematic review and meta-analysis of RCTs, we found that vitamin D supplementation interventions resulted in a reduction in FPG, HbA1c, and HOMA-IR for individuals with type 2 diabetes. However, there was evidence of substantial heterogeneity. The heterogeneity appeared to be partially explained by diversity between study populations for serum vitamin D status, supplementation characteristics (dose and duration), BMIs, and ethnicity.”

Based on their findings, the authors concluded that the use of vitamin D supplementation may be beneficial in patients with type 2 diabetes who are deficient in vitamin D, and the clinical benefits were more apparent when vitamin D was given in large doses and for a short period of time.

The authors also found that their results aligned with findings from previous studies regarding the possible benefits of vitamin D supplementation in conjunction with the use of other antidiabetic drugs to enhance glycemic control and to avoid diabetes-related complications.

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