Selenium supplementation effect on glycemic control: A GRADE-assessed systematic review and dose-response meta-analysis of randomized controlled trials

Results from different studies on the effects of selenium supplementation on glycemic control are still debated. To fill this knowledge gap, we investigated the overall effects of selenium supplementation on some glycemic parameters such as fasting blood sugar (FBS), hemoglobinA1c (HbA1c), fasting insulin, quantitative insulin sensitivity check index (QUICKI), and homeostatic model assessment of insulin resistance (HOMA-IR). A comprehensive literature search was conducted from inception to April 2023 on Scopus, Web of Science, PubMed, Google Scholar, and Cochrane databases. All randomized controlled trials (RCTs) which reported an effect of selenium supplementation on glycemic parameters were included. A random-effects model was used to estimate the weighted mean difference (WMD) and 95% CI for each outcome. Between-studies heterogeneity was assessed by the I2 and Cochran's Q test. 20 trials were included in the meta-analysis. Pooled analysis showed that selenium intake significantly reduced fasting insulin (WMD: -3.02 µIu/mL, 95% CI; -5.13, -0.90, P = 0.005) and increased QUICKI levels (WMD: 0.01, 95% CI: 0.01, 0.02, P = 0.005). However, selenium supplementation did not change FBS (WMD: -1.32 mg/dL, 95% CI; -4.02, 1.37, P = 0.332), HbA1c (WMD = 0.05%, 95% CI: -0.19, 0.28, p = 0.701), and HOMA-IR (WMD: -0.82, 95% CI; -2.14, 0.50, P = 0.223). Moreover, we found that there is a non-linear association between selenium supplementation dosage and FBS (P-nonlinearity = 0.008). In conclusion, our study findings indicate some benefits of selenium on fasting insulin, and QUICKI compared with placebo, but elicits no effect on HbA1c, HOMA-IR, and FBS. Further well-designed RCTs with larger samples are necessary to ascertain the effects of selenium supplementation on glycemic control.