Abstract
BACKGROUND: Recent studies indicate that vitamin D impacts male reproductive function, with deficiency linked to infertility. This review evaluates the effect of vitamin D supplementation on male fertility, focusing on total testosterone, free testosterone, the free androgen index (FAI), follicle-stimulating hormone (FSH), luteinizing hormone (LH), sex-hormone-binding globulin (SHBG), and estradiol. METHODS: We systematically searched Medline, Web of Science, Cochrane Library, and Scopus from their inception until July 2024 for randomized controlled trials (RCTs) involving adult males. The primary focus of these studies was on reproductive hormone parameters, analyzed using a random-effects meta-analysis and weighted mean difference (WMD). Evidence quality was assessed using ROB2 and GRADE. Meta-regression and dose-response analyses were performed. RESULTS: Seventeen studies met the criteria for quantitative analysis. Vitamin D supplementation significantly increased total testosterone levels (WMD 0.38, 95% CI 0.06-0.70, n = 15, I(2) = 67.03). However, it had no significant effect on other hormone parameters: free testosterone (WMD 0.00, 95% CI -0.02-0.03, n = 9, I(2) = 48.12), FSH (WMD -0.02, 95% CI -0.57-0.53, n = 7, I(2) = 48.72), LH (WMD -0.09, 95% CI -0.30-0.12, n = 8, I(2) = 0.00), SHBG (WMD 0.73, 95% CI -1.14-2.61, n = 10, I(2) = 69.05), FAI (WMD -0.92, 95% CI -2.12-0.27, n = 6, I(2) = 0.00), and estradiol (WMD -0.02, 95% CI -2.95-2.92, n = 5, I(2) = 20.63). CONCLUSION: This meta-analysis shows that vitamin D supplementation may increase total testosterone levels in men. However, further well-designed RCTs are needed to determine vitamin D's effects on other reproductive hormone parameters.