Studies about vitamin D and male fertility:
Vitamin D Treatment Improves Levels of Sexual Hormones, Metabolic Parameters and Erectile Function in Middle-Aged Vitamin D Deficient Men (2017)
Conclusion: In a 12-month study of 102 vitamin D-deficient middle-aged men, monthly high-dose vitamin D therapy raised their testosterone levels, improved erectile function, and reduced body fat.
Vitamin D Deficiency and Low Ionized Calcium Are Linked with Semen Quality and Sex Steroid Levels in Infertile Men (2016)
Conclusion: In a prospective study of 1248 Danish men, those with adequate vitamin D levels had slightly higher testosterone levels and much higher sperm counts compared to those with deficient vitamin D levels.
Conclusion: 12 months of vitamin D therapy revealed to be effective for improving testosterone production in rats and for reversing testicular damage from hyperglycemia-induced inflammation.
Conclusion: This study of fertility in Chinamen found that fertile men had slightly higher vitamin D levels compared to infertile men.
Association of Vitamin D Status with Semen Quality and Reproductive Hormones in Iranian Subfertile Men (2016)
Conclusion: In a study of 278 subfertile Iranian men, a positive correlation was found between vitamin D levels and sperm motility.
Note: This is a detailed, technical article covering the role of vitamin D in male and female fertility. Did you know that even sperm have vitamin D receptors?