Arachidonic acid (ARA, 20:4n-6) is an n-6 polyunsaturated 20-carbon fatty acid formed by the biosynthesis from linoleic acid (LA, 18:2n-6). This review considers the essential role that ARA plays in infant development. ARA is always present in human milk at a relatively fixed level and is accumulated in tissues throughout the body where it serves several important functions. Without the provision of preformed ARA in human milk or infant formula the growing infant cannot maintain ARA levels from synthetic pathways alone that are sufficient to meet metabolic demand. During late infancy and early childhood the amount of dietary ARA provided by solid foods is low. ARA serves as a precursor to leukotrienes, prostaglandins, and thromboxanes, collectively known as eicosanoids which are important for immunity and immune response. There is strong evidence based on animal and human studies that ARA is critical for infant growth, brain development, and health. These studies also demonstrate the importance of balancing the amounts of ARA and DHA as too much DHA may suppress the benefits provided by ARA. Both ARA and DHA have been added to infant formulas and follow-on formulas for more than two decades. The amounts and ratios of ARA and DHA needed in infant formula are discussed based on an in depth review of the available scientific evidence.

Considerable advances have been made in the field of infant feeding research. The last few decades have witnessed the expansion in the number of studies on the composition and benefits of human milk. The practice of breastfeeding and use of human milk represent today’s reference standards for infant feeding and nutrition. Additional research regarding the benefits of breastfeeding is needed to determine which factors in human milk and in the act of breastfeeding itself, singly or in combination, are most important for producing the beneficial effects on infant growth, body composition, and neurodevelopmental outcome. We examine evidence that breastfeeding confers health benefits and offer suggestions on how best to interpret the data and present it to the public. We also describe some examples of well-designed infant nutrition studies that provide useful and clinically meaningful data regarding infant feeding, growth, and development. Because not all mothers choose to breastfeed or can breastfeed, other appropriate feeding options should be subjected to critical review to help establish how infant formula and bottle feeding can confer benefits similar to those of human milk and the act of breastfeeding. We conclude with the overarching point that the goal of infant feeding research is to promote optimal infant growth and development. Since parents/families may take different paths to feeding their infants, it is fundamental that health professionals understand how best to interpret research studies and their findings to support optimal infant growth and development.