Vitamin B9, specifically in its active form 5-Methyltetrahydrofolate (5-MTHF), has scientific support for its use in supporting fertility, particularly in women. Folate (Vitamin B9) is essential for DNA synthesis, cell division, and methylation, all of which are crucial during gamete (egg and sperm) formation and early embryonic development. Deficiency in folate has been linked to infertility, increased risk of miscarriage, and neural tube defects. Supplementation with folate is standard practice for women planning pregnancy and those undergoing fertility treatments.

5-MTHF is the predominant form of folate in circulation and is preferred for supplementation, especially in individuals with genetic variants (such as MTHFR polymorphisms) that impair the conversion of folic acid to its active form. Several clinical and observational studies suggest that adequate folate status improves ovulation, egg quality, implantation, and reduces the risk of miscarriage. Some studies also indicate that folate supplementation can improve sperm quality in men.

However, while folate is clearly important for reproductive health, direct evidence from large randomized controlled trials specifically linking 5-MTHF supplementation (as opposed to general folic acid) to improved fertility outcomes is still limited. Most evidence is extrapolated from studies on folic acid and folate status in general. Therefore, the evidence is moderate (3/5), with a strong biological rationale and supportive clinical data, but more targeted research is needed.