Among the principal tenets of pediatric nutrition is the expectation that meeting the requirements for essential nutrients by feeding either artificial formulae or human milk results in indistinguishable physiological outcomes. This expectation has not been met. Were this observation confined to functional outcomes related to constituents of human milk not found in formula (e.g. immunological components, hormones and very-long-chain polyunsaturated fatty acids), differences between breast-fed and artificially fed infants would not be surprising. However, differences in growth between breast- and bottle-fed infants who live under favorable conditions were mostly unexpected. Bottle-fed infants demonstrate accelerated growth patterns compared to infants who are breast fed (1, 2). Similarly, differences between the growth of breast-fed infants and established growth references were not anticipated (2–4).