['Many lactating women in North America are exposed to high synthetic folic acid intakes because of food fortification and vitamin supplement use. Few data exist on the potential long-term effect of high folic acid intakes on milk folate concentrations, whereas no data are available on the effect of supplemental [6S]-5-methyltetrahydrofolate ([6S]-5-methylTHF).', 'The aim of the present study was to investigate the effect of 3 treatments (placebo, folic acid, and [6S]-5-methylTHF) on milk folate and folate-binding protein (FBP) concentrations and to determine whether unmetabolized folic acid is present in milk.', 'In this 16-wk randomized, placebo-controlled intervention, 69 lactating women were randomly assigned to receive [6S]-5-methylTHF (416 microg/d, 906 nmol/d) or a placebo, or were assigned to receive folic acid (400 microg/d, 906 nmol/d) within 1 wk postpartum. Total milk folate, FBP, and unmetabolized folic acid concentrations were measured at 16 wk.', 'Unmetabolized folic acid was detected in 96% of milk samples tested representing approximately 8% of total milk folate concentrations. Total milk folate, FBP, and the proportion of unmetabolized milk folic acid did not differ between treatments; however, FBP concentrations were significantly lower than those published before mandatory folic acid fortification of the food supply.'] Are unmetabolized folic acid and total folate concentrations in breast milk unaffected by low-dose folate supplements?