Evaluation of the effects of vitamin D deficiency and cigarette smoking on insulin resistance in type 2 diabetes mellitus: A meta-analysis of randomized controlled trials

There are contradictory findings regarding the effects of vitamin D supplementation and cigarette smoking on glucose metabolism in individuals with type 2 diabetes mellitus (T2DM). Consequently, this meta-analysis focused on the association between vitamin D interventions and smoking cessation on glycemic control in T2DM patients. This study adhered to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Cochrane Library, EMBASE and PubMed databases were used for a language-inclusive literature search until November 2022. The primary outcomes of this meta-analysis were changes in glycated hemoglobin (HbA1c) level, vitamin D concentration and body mass index (BMI) values. This meta-analysis included 14 randomized controlled trials (RCTs) with a total of 23,289 individuals with T2DM. Nine RCTs were related to vitamin D supplementation interventions, and 5 RCTs were related to smoking cessation interventions. The studies on vitamin D supplementation showed a substantial change in the intervention group, with a risk ratio (RR) of 0.72 (95% confidence interval (95% CI): 0.58, 0.88; p = 0.001) and an odds ratio (OR) of 0.52 (95% CI: 0.34, 0.78; p = 0.002); high heterogeneity was observed (I2 ≥ 95%). Similarly, the smoking cessation studies showed a substantial change in the intervention group, with a RR of 0.92 (95% CI: 0.86, 0.99; p = 0.04) and an OR of 0.86 (95% CI: 0.74, 0.99; p = 0.04); high heterogeneity was observed (I2 = 87%). In conclusion, both vitamin D supplementation and smoking cessation are associated with moderate BMI decline and an improvement of insulin sensitivity in people with T2DM.