The American Academy of Pediatrics (AAP) seeks to support the optimal physical, mental, and social health and well-being of all infants and children. ¹ Largely because of its health benefits, the preferred method of feeding for achieving these goals for almost all young infants is exclusive breastfeeding for about the first 6 months and after complementary foods are introduced for at least the first 2 years after birth and beyond, as long as mutually desired by mother and child.² The growth pattern of breastfed infants defines normal growth in infancy. Prior to the advent of safe drinking water, refrigeration, techniques for food preservation, and curd-reducing milk technologies, breastfeeding by mother or wet nurse was necessary for infant survival, although the previously common practice of wet nursing was associated with its own medical and social liabilities.³

In the 1940s, approximately 65% of infants were being breastfed, but by 1958, with safer infant foods, improved hygiene, and changed attitudes toward breastfeeding and maternal adaptations to modern life, the percentage of 7-day-old infants who were breastfed had decreased to just 25%. The breastfeeding rate remained at that level for more than a decade.^4,5 Despite major swings in breastfeeding initiation and duration, infant mortality in the United States decreased rapidly throughout the 20th century.⁶

Medical professional, government, and lay group enthusiasm for, and promotion of, breastfeeding since the early 1970s has been associated with increased breastfeeding initiation, exclusivity, and duration.⁷ The most recent Centers for Disease Control and Prevention (CDC) Breastfeeding Report Card (2020 data) cites the US breastfeeding initiation rate at 84%, exclusive breastfeeding for 3 months at 47%, breastfeeding at 6 months at 59% and any breastfeeding at 1 year of age at 35%.⁸ Breastfeeding rates at 3, 6, and 12 months were initially rising during the first half of the last decade but have plateaued since 2016 and we have seen some decline. According to the CDC report, 46.3% of infants were exclusively breastfeeding in 2018 at 3 months of age, down from 47.5% in 2016. From 2016 to 2018, there were increases in the percentage of infants supplemented with infant formula by 2 days, 3 months, and 6 months of age.⁹ Women in southeastern states, those of lower socioeconomic status, those younger than 20 years, those who are employed, and those receiving benefits from the Special Supplemental Nutrition Program for Women, Infants, and Children (WIC) are less likely to breastfeed.^10,11,12

With their advocacy for breastfeeding, children's health care professionals and other advocates for children's health have been variably successful in enacting and enforcing constraints on the marketing of infant formula directly to mothers, and even to health care professionals. The AAP expressed its disapproval of direct advertising of infant formula to the general public in 1989 and reaffirmed that stance in 1993.¹³ Such advertising runs counter to the World Health Organization (WHO) International Code of Marketing of Breastmilk Substitutes policy, of which the United States is not a signatory.¹⁴ Despite these efforts, a 2020 WHO report found several formula companies were in violation of the 1981 WHO code,¹⁵ and there have been calls for the AAP to reassess its relationship with formula manufacturers. ¹⁶ Another development that has reduced some forms of promotion of infant formula in the United States is the Baby-Friendly Hospital Initiative of the WHO and United Nations International Children's Emergency Fund (UNICEF). Attaining certification as a Baby-Friendly Hospital requires documentation of hospital practices that are believed to support breastfeeding, including a requirement that hospitals not distribute infant formula "discharge packs" and do not accept free formula samples for hospital use.¹⁷ In the United States, only 2.9% of births occurred in Baby-Friendly Hospitals in 2007; however, by 2020 this rose to 28.9%.¹⁸ Initiation of breastfeeding is higher in Baby-Friendly Hospitals, and use of discharge packs has been decreasing.^19,20

Not all infants are exclusively breastfed for the first 6 months. For those infants, the AAP recommends use of an iron-fortified infant formula as the best and safest alternative for the first year after birth.²¹ At some point during their first year, most infants in the United States receive formula.²² Given the widespread use of commercial infant formula and its advantages for infants relative to cow milk and other human milk alternatives,²³ it is important for pediatric health care providers to have a practical understanding of its use and the nutrition that it provides.^24,25 This chapter reviews the development, composition, and feeding of infant formula for term infants.

As with human milk in breastfed infants, formula makes up a progressively smaller percentage of energy and nutrient consumption in the second 6 months after birth. Complementary foods may be introduced at about 6 months of age on the basis of developmental readiness (eg, oromotor coordination, head control) and nutritional needs of the growing infant. After 6 months, in addition to solid foods, either breastfeeding or iron-fortified infant formula should be used for the remainder of the first year after birth rather than feeding cow milk. The composition of cow milk does not match the nutritional requirements of infants, particularly the lack of iron, which contributes to iron-deficiency anemia.²⁶ Avoidance of cow milk until 1 year of age reduces the risk of inadequate intakes of other nutrients including zinc, vitamin E, essential fatty acids, and long-chain polyunsaturated fatty acids. This also prevents excessive intakes of protein and electrolytes. There is no role for fruit juice in the diet in the first year after birth.²⁷ The 9th edition of the US Dietary Guidelines includes a chapter on infants and children under 2 years.²⁸ The recommendations there are consistent with the AAP advice on infant feeding. More technical information on infant feeding is available on the CDC website.²⁹ Of note is that each of these sources recommend introduction of potentially allergenic foods early in life, generally by about 6 months of when foods are first added to the diet, and strict avoidance of honey for the first year because of the risk of botulism.

The development of modern infant formulas became possible with advances in knowledge and practice of chemistry, nutritional analysis of foods, nutritional requirements, and food preservation technologies that began in earnest in the 19th century. Initially, the goal for developing microbiologically safe, shelf-stable infant feeding products was to provide energy and protein, and only later, to better match the macro- and micronutrient composition of human milk. Early efforts focused largely on preserving and modifying cow milk and grains to make them more suitable for feeding infants. Both shelf-stable concentrated milk products and milk and carbohydrate powders were commercialized. By 1919, a detailed report was published related to a feeding mixture, in many ways close to modern infant formulas, that was well tolerated and supported infant growth.³⁰ Until the middle of the 20th century, infant formulas were mostly made at home, largely from canned milk products, corn syrup, and powdered sugars with guidance from physicians on their preparation. Dietary vitamin sources, especially orange juice for ascorbic acid and cod liver oil for vitamin D, were fed separately or incorporated into formula ingredients and reduced the high prevalence of scurvy and rickets. Nearly complete infant formulas were available by the 1920s, and most were milk or milk/whey concentrate based.³¹ The initial use of milk fat gave way to the use of vegetable oils as the lipid components, which were better absorbed and eliminated the foul smell of spit-up containing butyric acid from partially digested milk fat.^32,33

The goal of nutritionally imitating human milk was gradually shown to be quantitatively possible but not qualitatively feasible. Most commercial infant formulas are based on cow milk, and although the amounts of macronutrients delivered can be quantitatively modified by food technology to approach those of human milk, the biochemical composition does not match that of human milk. For example, the whey-casein ratios of cow milk and human milk are different (and the ratio changes over the period of lactation in human milk), and the protein and amino acid compositions of the whey and casein protein fractions differ. The human milk fatty acid profile is unlike that of butterfat or vegetable oils usually used in formulas. Human milk, with its low content of iron and vitamins D and K, is a problematic model for a complete infant formula. For some nutrients, bioavailability and utilization also differ between infant formulas and human milk. In addition, there are largely nonnutritional constituents of human milk, including hormones, growth factors, antibodies, immune modulating factors, enzymes, human milk oligosaccharides and leukocytes, with functional properties that cannot be readily incorporated into infant formula. As a result, the human milk composition model for infant formula has broadened to include the important functional perspective of imitating the growth, development, physiology, and health of breastfed infants.³⁴

The first US infant formulas were manufactured as powders. These were followed by liquid concentrates and, ultimately, ready-to-feed formulas. All 3 forms remain marketed today for use by healthy infants. In addition, "exempt" infant formulas are manufactured for use by infants with medical conditions that benefit from a modified formula and are typically exempted from the guidelines for one or more ingredients, as designated by the Infant Formula Act (IFA) of 1980 and its amendments.

The evolution of infant formula regulations has contributed to the safety of formulas sold in the United States (see Chapter 46: Federal Regulation of Foods and Infant Formulas, Including Addition of New Ingredients: Food Additives and Substances Generally Regarded as Safe for formula ingredient requirements and additional regulatory information). Federal regulation of infant feeding mixtures dates to 1941, when initial composition and labeling requirements were first adopted as an amendment to the Food and Drug Act. Expert groups have provided guidance to the US Food and Drug Administration (FDA) regarding the ingredients and composition of US infant formula.^35,36,37 The AAP Committee on Nutrition developed recommendations for minimum nutrient levels for complete infant formulas in 1967, and these were largely adopted by the FDA in 1971, and at the FDA's request, these were updated in 1983.⁷ Micronutrient deficiencies (eg, ascorbic acid, vitamin D, thiamine, pyridoxine, and chloride) continued to be reported sporadically in the United States with specific (mostly milk-free) formulas before implementation of the IFA with its provisions that ensure the nutritional composition of formulas.^38,39 Lower and upper limits for selenium were most recently added by FDA in 2016. Infant formula manufacturers must document bacteriologic safety and nutrient content within the ranges set forth in the IFA and keep detailed records of each batch of infant formula. The labeling of infant formulas must follow a specific format and includes a list of ingredients and nutrient content. As of 2023, there were only 7 companies registered with the FDA to produce infant formula in the United States. All US infant formulas must be manufactured according to Good Manufacturing Practices, and all production facilities are inspected at least annually by the FDA, although the COVID-19 pandemic affected the frequency of these inspections. The agency is authorized to initiate a mandatory recall if it determines that an adulterated or misbranded infant formula presents a risk to human health. In 2022, suspected contamination of powdered infant formula led to formula recalls and infant formula product shortages. When this occurs, it is important never to resort to homemade infant formulas per AAP guidance,⁴⁰ and safeguards are needed to maintain national supplies. Guidance for parents and health care professionals to navigate the formula shortage were made available on websites managed by the White House, the Department of Health and Human Services (FDA and CDC), USDA (WIC), the AAP, and the North American Society for Pediatric Gastroenterology, Hepatology and Nutrition. The FDA permitted temporary import of vetted foreign manufacturers not registered with FDA.⁴¹

All standard infant formulas manufactured under the IFA meet the energy and nutrient requirements for exclusive feeding of healthy term infants during the first 4 to 6 months after birth. If human milk or formula intake is adequate, healthy infants younger than 6 months of age do not need additional water, except when the environmental temperature is extremely high. When formula feeding is used, bottles should be offered ad libitum, the goal being to allow the infant to regulate intake to meet his or her energy needs. The infant should not be encouraged to empty a bottle when fed infant formula.⁴² After the 1 or 2 weeks, typical intakes of human milk or formula will be 140 to 200 mL/kg per day for the first 3 months after birth to a maximum volume of about 800 to 1000 mL/day. This intake provides approximately 90 to 135 kcal/kg of body weight per day and should result in an initial weight gain of 25 to 30 g/day. Between 3 and 6 months of age, weight gain decreases to 15 to 20 g/day, and between 6 and 12 months of age, weight gain decreases to 10 to 15 g/day.

The late Dr Samuel Fomon described breastfeeding as an evolutionary compromise between the nutritional needs of the infant and the nutritional needs of the mother.⁴³ This controversial concept implies that human milk is unlikely to provide much nutritional surfeit, and that infants are likely to consume more than what is provided by breastfeeding, given the opportunity. Increasing milk volume or milk nutrient density in early life above that usually consumed by breastfed infants can increase growth rates beyond that typical of exclusively breastfed infants. Both of these concepts have been demonstrated in clinical studies. When feeding was compared in infants receiving human milk by bottle versus by breast, infants feeding by bottle consumed greater quantities of human milk and experienced greater weight gain.⁴⁴ Increasing the nutrient density of a formula also accelerates growth in early infancy.^45,46 A study of infants who were fed formula with a slightly reduced caloric density relative to usual infant formulas reported that infants fed the formula did not grow faster than a reference group of breastfed infants.⁴⁷ Thus, to the extent that achieving the growth rate and pattern of exclusively breastfed infants is an important goal of formula feeding, one not universally embraced,⁴³ there are both compositional and behavioral challenges to achieving this goal.

Growth of infants in the United States, including those who are formula fed, approximates, but is not identical to, that of breastfed infants as depicted in the 2006 WHO growth charts, which are based on anthropometric measurements from a study of infants who are mostly breastfed.⁴⁸ In 2010, the CDC and AAP recommended the use of the WHO growth charts for the first 24 months after birth.⁴⁸ Infants in the United States tend to grow a little faster than the international reference data after the first few months after birth.⁴⁹ It is worth noting that milk sources other than human milk consumed by infants in the largely international WHO growth study did not include infant formula for many of the study sites.⁵⁰

In the United States, the WIC program was adopted as a permanent national nutrition program in 1975 to combat poverty and malnutrition that had been well documented. The WIC program continues to have substantial influence and impact on infant feeding practices in the United States. More than half of US infant formula composition is the powdered and concentrated liquid formulas sold through state WIC programs. As of 2020, almost 1.7 million infants in the United States were enrolled in the WIC program.⁵¹ WIC participants receive monthly vouchers for free infant formula that are redeemed in food stores to meet most, but not necessarily all, of an infant's formula usage. Additional formula usually needs to be purchased. Concerns have been raised that food-insecure households may "stretch" infant formula, with the potential for the feeding of diluted formula.⁵²

Breastfeeding rates are lower among WIC participants, consistent with the demographic profile of this population and some WIC practices, if not policies, that promote formula feeding.¹⁶ Attempts to promote more breastfeeding have been made, in part, by more generous WIC food packages being provided to mothers who breastfeed relative to those feeding infant formula.⁵³

Before WIC mandated iron fortification of at least 10 mg/L in infant formula, iron deficiency in the first year after birth and iron deficiency anemia in the second year after birth were common in the United States.^7,38 Nutritional calculations of infant iron requirements and clinical experience from feeding iron-fortified formula to high-risk populations had both indicated that a milk-based formula providing about 12 mg/L could prevent iron deficiency and iron-deficiency anemia.^54,55 After WIC mandated that formulas provided through its program contain at least 10 mg of iron/L and began providing infant formula through the first year after birth, both iron deficiency and iron-deficiency anemia decreased dramatically.⁵⁶ More recently, European standards and recommendations have supported lower iron intake concentrations, and EFSA standards do not allow more than 8 mg/L of iron. As such, in 2022, WIC lowered its standard to 6 mg/L of iron for WIC-approved formulas. At present, it is not certain whether this lower standard will be maintained long-term.