Folic acid and Vitamin B12 are two neglected micronutrients: little is understood about the effects of moderate deficiencies and their interaction with other micronutrients. Dr. Lindsay Allen takes a look at how folic acid and B12 supplementation have affected health outcomes during the perinatal period and the consequences of deficiencies in infants and children. She begins with flour fortification programs worldwide, showing how folic acid fortification has reduced the incidence of neural tube defects. Despite clear benefits, there is some controversy over the safe levels of folic acid and possible adverse effects, such as increased risk of colorectal cancer. Next, Allen examines Vitamin B12. Deficiencies in this vitamin are most prevalent in parts of the world with low intake of animal source foods. Studies in infants with poor Vitamin B12 status show severe and irreversible consequences on development. Using the first Vitamin B12 assay developed for breast milk, Allen and her team reveal that poor maternal B12 status results in low concentrations of this vitamin in breast milk. Furthermore, supplements are ineffective when given in lactation. Allen’s findings demonstrate the importance of early intervention in maternal nutrition status in order to confer health benefits to the infant.