What role does B12 play in fertility?

Vitamin B12 is essential for cell division, DNA synthesis, and homocysteine metabolism—all processes relevant to fertility. Observational studies link B12 deficiency with both female and male subfertility, partly through elevated homocysteine, which may impair oocyte quality and sperm function. However, the evidence is largely associative; robust randomized controlled trials demonstrating a causal benefit of B12 supplementation for fertility outcomes are lacking. B12 is often studied together with folate, making it difficult to isolate its independent effect. Maintaining adequate B12 levels (e.g., through diet or standard supplements) is prudent, but megadosing is not supported by evidence. Caveat: Direct-to-consumer genetic tests for B12-related variants (e.g., FUT2) often overstate their impact on fertility. For individuals with unexplained infertility, a blood test for B12 and homocysteine is more informative than a DNA test alone.