Selenocysteine is the 21st amino acid, incorporated into selenoproteins, which play essential roles in antioxidant defense and redox regulation. Scientific evidence supports a connection between selenium status (and thus selenocysteine-containing selenoproteins) and male reproductive health, particularly sperm function. Selenium is crucial for the activity of glutathione peroxidase 4 (GPx4), a selenoenzyme highly expressed in the testes, where it protects developing sperm from oxidative damage and is essential for sperm maturation and motility. Several studies have shown that selenium deficiency can impair spermatogenesis, reduce sperm motility, and increase the risk of male infertility. Supplementation with selenium (typically as selenomethionine or selenite, but metabolized in the body to selenocysteine-containing proteins) has been associated in some studies with improvements in sperm parameters and fertility outcomes, especially in men with low baseline selenium status.

However, the evidence is not uniformly strong: many studies are small, have methodological limitations, or show modest effects. The use of selenocysteine itself as a supplement is rare; typically, selenium is given in forms that the body converts to selenocysteine. While the biological rationale is clear and there is some supportive data, large, high-quality trials are lacking, and the benefit may be most relevant in populations with selenium deficiency. Overall, scientific evidence exists for the role of selenium (and by extension, selenocysteine) in male reproductive health, but it is moderate rather than definitive.