It depends. In the first study to look into this, 28 lactating women were placed on a controlled diet that delivered 400 mcg of naturally occurring folate, and given a prenatal supplement containing 750 mcg of folic acid (FA)for up to 10 weeks. Breast milk samples were taken at baseline and at study end and analyzed for 5-methyl tetrahydrofolate (5-MTHF), a bioactive form of folate, and FA.  Total folate levels were not impacted, but folic acid level was increased (accounting for 40% of the folate species in breast milk) suggesting its displacement of 5-MTHF.  This is higher than what has been typically reported and this is why this is important.

A significant amount of breast-milk folate is bound to protein folate receptor (FR-⍺). This protein is important for bioavailability of folate to the infant. But here is the potential problem. The protein bindsFA more tightly than 5-MTHF and unfortunately, this may make it less available to the infant, resulting in an unexpected and paradoxical finding that enhancing breast milk with FA may not be optimal for the nursing infant's folate needs.