Evidence check: Selen in the context of fertility

Selenium is an essential trace mineral that, as part of selenoproteins (e.g., glutathione peroxidases), plays a key role in antioxidant defense and thyroid metabolism – both factors that can influence fertility. In men, selenium is important for sperm production and motility; in women, it may support ovarian function and early pregnancy maintenance. Observational studies suggest that low selenium levels are associated with reduced fertility outcomes, but randomized controlled trials are limited and show mixed results. The evidence is moderate at best: plausible mechanistic pathways exist, but effect sizes are small and often confounded by other nutrients (e.g., zinc, iodine). Importantly, selenium supplementation is only beneficial if a deficiency exists; excess selenium can be toxic (selenosis) and may even impair fertility. Genetic variants in selenoprotein genes (e.g., SEPP1, GPX1) can modulate selenium metabolism, but routine testing is not clinically recommended. A blood test (serum or whole blood selenium) is more informative than a DNA test. Overall, selenium is a relevant but not decisive factor in fertility. Evidence grade: human-moderate to mechanistic. Caution: do not exceed the tolerable upper intake level (400 µg/day for adults).