Infancy is a time of dramatic change and development. In the first year of life, babies triple their body weight and develop from tiny bundles whose daily activities involve eating, sleeping, and creating dirty diapers to toddlers well on their way to walking, talking, and feeding themselves. The first 1,000 days of life, beginning at conception and continuing through toddlerhood, also represent the most active period of brain development in the lifespan, laying the foundation and establishing neural networks to support cognitive, motor, and social-emotional skills throughout life. All of these critical developmental processes are supported through sensitive caregiving, a safe home environment, and of course, a healthy diet. 1 Nutrient requirements on a per-kilogram body weight basis are higher during infancy than in any other stage in the lifespan.
At birth and continuing through the first 4 to 6 months of life, breast milk, infant formula, or some combination of the two should be the sole source of nutrition for infants. This is because young infants’ gastrointestinal tracts aren’t yet ready to process more complex foods, and they lack the oral motor skills to swallow solid foods safely. Breast milk is uniquely adapted to meet the nutrient needs of young infants, and infant formula is also designed and regulated to ensure that it is safe and provides adequate nutrition. Any other substitute, including cow’s milk, goat’s milk, plant-based beverages such as soy milk, homemade infant formula, or watered down formula, should be avoided. These do not provide the right balance of nutrients to meet infants’ requirements and can cause serious problems such as damage to the intestines or kidneys. 2
The World Health Organization and the American Academy of Pediatrics, as well as many other health organizations, recommend that infants be exclusively breastfed (only receiving breast milk, with no formula or other foods) for about the first 6 months of life. Breast milk is considered the optimal source of nutrition for infants, but it also contains many other bioactive molecules, including immunoglobulins (or antibodies), hormones, enzymes, growth factors, and protective proteins. 3 In addition, breast milk contains special carbohydrates, called human milk oligosaccharides, that are indigestible to infants (they lack the enzymes to break them down) but help to establish a healthy gut microbiome by serving as a food source for friendly bacteria and binding up harmful bacteria. 4 Breastfeeding is beneficial to babies in many ways; it reduces a baby’s risk of gastrointestinal, respiratory, and ear infections, and it may also protect babies from sudden infant death syndrome (SIDS), eczema, and asthma. It is also associated with a small increase in IQ and a reduced risk of obesity later in childhood. In addition, breastfeeding benefits the health of mothers; it’s associated with a reduced risk of hypertension, type 2 diabetes, and breast and ovarian cancer. 3 However, it’s important to note that most of the data on benefits of breastfeeding come from observational studies, and these studies have many confounding factors. Women who breastfeed tend to have higher incomes, be more educated, be older in age, and are more likely to be white compared with those who don’t breastfeed. This means breastfed babies are often born with more advantages beyond how they are fed, so it can be difficult to separate correlation from causation in studies of infant feeding outcomes. 5 Despite the challenges in breastfeeding research, breastfeeding has many well-established benefits, and most mothers intend to breastfeed. In 2017, 84% of babies born in the U.S. were breastfed at birth, but just 58% were still breastfeeding at 6 months of age, and 26% were exclusively breastfed through 6 months. 6 These statistics represent the many challenges women face with breastfeeding. Although breastfeeding is natural, it doesn’t always come naturally or easily. Breastfeeding challenges can often be addressed with the help of an experienced professional such as a lactation consultant, but this support is not easy for all women to access. Women may also find it hard to establish and continue breastfeeding if they have to return to work just a few weeks or months postpartum, a scenario common in the U.S., the only developed country that has no national policy for paid parental leave. 7,8 There are also a few circumstances in which babies should not or can not be breastfed. In the U.S., it’s recommended that women infected with HIV do not breastfeed, because the virus can pass to infants in milk. Infants born with rare metabolic disorders may not be able to metabolize breast milk and need to receive special formulas. There are also some medications which, if taken by the mother, are not safe for breastfeeding infants. In these cases, mothers may be advised to formula-feed. 3 Many women find breastfeeding enjoyable and a wonderful way to bond with their baby. For others, breastfeeding can be a struggle for one or more of the reasons discussed above, and that struggle can overshadow a mother’s relationship with her baby. For all of these reasons, the choice to breastfeed or formula-feed, or to feed some of both, is complex and individual. Parents should be supported whatever their decision, and they can be confident that infant formula is a safe, nutritionally-adequate option.
The first milk, called colostrum, is produced immediately after birth and continues for the first two to five days after the arrival of the baby. Colostrum is yellowish in color, thicker than mature breast milk, and produced in small quantities. It is low in fat and easily digestible, yet rich in protein, fat-soluble vitamins, and minerals. Colostrum is also a concentrated source of immunoglobulins that pass from the mother to the baby and provide immune protection to the newborn. The stomach capacity of newborns is small, so they only consume a teaspoon or two of colostrum per feeding in the first few days of life, but they need to feed often. Frequent, on-demand feeding (whenever the baby is hungry, not on a schedule) helps to promote full milk production. 5 After a couple of days, colostrum is replaced by transitional milk, which is produced in much greater volume and lasts through 7 to 14 days postpartum. Compared with colostrum, transitional milk has more fat and lactose, and less protein and immunoglobulins, and it is also more calorie-dense. Finally, women begin to produce mature milk and will continue to make this type of milk through the end of lactation. Mature milk contains about 87% water, 4% fat, 1% protein, and 7% lactose. Together, these meet infants’ macronutrient and caloric requirements. 9 However, there is variability even within mature milk. In a given feeding, the milk secreted at the beginning of the feeding (called foremilk) is thinner and higher in lactose than the milk at the end of the feeding (called hindmilk). The higher levels of fat in the hindmilk helps to ensure the baby’s energy needs are met. 9 Milk can also vary from day to night; nighttime milk is higher in fat and the sleep-promoting hormone melatonin. 10 As breastfeeding continues beyond 6 or 7 months, the levels of some vitamins and minerals begin to decline. This is around the time that babies begin to eat some solid foods, so foods can help to fill in nutritional gaps, while breast milk continues to be an important source of nutrients. 5 Breast milk provides enough of all of the micronutrients that young infants need with two main exceptions: vitamin K and vitamin D. For this reason, newborns should receive a vitamin K shot soon after birth; otherwise, vitamin K deficiency can lead to serious bleeding disorders such as hemorrhage (see Unit 9). The American Academy of Pediatrics also recommends that breastfed newborns be given a vitamin D supplement beginning in the first few days of life and continuing until they are weaned to formula or cow’s milk, both of which are fortified with adequate vitamin D. (Cow’s milk can be given beginning at 12 months of age.) 11 Breast milk is also low in iron, although what little is there is absorbed very efficiently. Newborns are born with a certain amount of iron absorbed from their mothers during pregnancy, and they utilize this stored iron—in addition to that provided in breast milk—to meet their iron requirement in early infancy. However, stored iron is depleted by around 4 months of age, so the American Academy of Pediatrics recommends that exclusively breastfed infants begin taking an iron supplement at this age and continuing until they are eating substantial amounts of iron-rich solid foods, such as meat or iron-fortified cereals. Iron deficiency remains a significant problem, with 11% of 1-year-olds in the U.S. estimated to be iron-deficient. 11 Iron is essential for brain development, and iron deficiency may cause lasting developmental deficits. 12
Breastfeeding mothers have nutrient requirements similar to women in the third trimester of pregnancy. After all, they’re continuing to provide nutrients for their babies through their milk, and milk production requires energy, macronutrients, micronutrients, and water. Breastfeeding women have a remarkable ability to make enough milk to meet their babies’ nutrient needs even without an optimal diet, but eating well supports maternal health and energy levels during this demanding time. Breastfeeding increases energy requirements by about 450 to 500 calories per day, part of which can be supplied by using adipose stored in pregnancy and part of which should be supplied from greater caloric intake. In general, breastfeeding mothers can eat a wide variety of foods, and they don’t usually need to avoid or restrict any specific foods. However, as in pregnancy, they should continue to avoid high-mercury fish. Consuming low-mercury fish 2 to 3 times per week provides the omega-3 fatty acids DHA and EPA, which pass into breast milk and support brain and eye development for the breastfeeding infant. 13 There is no need to avoid common food allergens, such as peanut or dairy; avoiding these foods while breastfeeding has not been shown to reduce babies’ chances of developing food allergies. 14 Sometimes women find that certain foods, such as garlic or spicy foods, are associated with fussiness or gas in their infants, and they may experiment with avoiding those foods. However, most infants don’t have a problem tolerating these foods. In fact, flavors from the mother’s diet pass into breast milk, so researchers hypothesize that when mothers eat a wide variety of foods and flavors while breastfeeding, their children grow up to be more adventurous eaters. 15 In addition to consuming a nutrient-dense diet, obstetricians sometimes recommend that breastfeeding mothers continue taking a prenatal supplement to ensure that their micronutrient needs are met. Breastfeeding mothers should also be sure to drink plenty of fluids to support milk production. 13 Similar to pregnancy, substances consumed by the breastfeeding mother can pass to the infant in her milk. However, how much passes into the milk depends on the type of substance and the timing of consumption relative to breastfeeding. For example, it’s considered safe for a breastfeeding mother to have an alcoholic drink so long as she waits at least two hours before breastfeeding, because by that point, most of the alcohol will have cleared her bloodstream and will not pass into her milk. Caffeine lasts longer in the blood but is considered safe in moderation, with a limit of about 300 milligrams per day. 13 Cannabinoids, the chemicals found in cannabis, are fat-soluble and remain in a mother’s bloodstream and body tissues for much longer. Tetrahydrocannabinol (THC), the main psychoactive chemical in cannabis, has been detected in breast milk as much as 6 weeks after a mother used cannabis. 16 Although it’s not clear how these chemicals affect babies when consumed in breast milk, medical organizations agree it’s best to avoid using cannabis while breastfeeding. The National Institutes of Health’s LactMed database is a great resource for information about the safety of drugs, medications, and supplements during breastfeeding.
To a certain extent, the nutrient composition of infant formula is modeled after that of human milk, and although it by no means duplicates breast milk, it is a safe and effective substitute. Formula is made from ingredients such as cow’s milk, soy, vegetable oils, and corn syrup. These may be combined in ways to mimic the overall macronutrient composition of breast milk, but the content of individual amino acids, fatty acids, and sugars can vary somewhat. In addition, some of these nutrients are less digestible, so to compensate, formulas tend to have higher levels of some nutrients, such as protein, compared with breast milk. Formula contains more of some micronutrients, such as iron, vitamin D, and vitamin K, so deficiencies of these vitamins and minerals are more common in breastfed infants if they don’t receive appropriate supplementation or solid foods when the time comes. 17 Infant formula also does not contain most of the bioactive molecules found in breast milk, although formula companies are beginning to add versions of some of these molecules. For example, many formulas now include some type of indigestible sugar molecule intended to act as a prebiotic to feed healthy gut bacteria, similar to human milk oligosaccharides. At this point, however, the evidence that such ingredients are beneficial to infants is not very convincing. However, novel ingredients like these are often used as marketing tools, with labels touting vague structure-function claims such as “brain-boosting” and “immune-supporting.” Consumers should know that there is often little evidence that “designer ingredients” in infant formula make them healthier for babies. All infant formulas are required by law to be safe and meet the nutrient requirements of infants, and in most cases, basic store brand formulas cost less and are just as good as other products on the shelf. It may be true that infant formula can’t replicate the complexity of breast milk, but it has a very strong track record of safety, and infants can grow and thrive with formula-feeding. 17
