Nutrition in the first year of life is crucial to an infant's development and lifelong health. Early nutritional deficits are intrinsically linked to impaired mental and physical growth. Consequently, the risk of developing metabolic and cardiovascular diseases such as obesity, diabetes, and hypertension in childhood and later life is likely determined by early feeding and nutritional status.

Breastfeeding is the biologically superior way to feed an infant. The American Academy of Pediatrics recommends exclusive breastfeeding for the first six months of an infant's life and continued breastfeeding while introducing age-appropriate foods until an infant is 12 months old or older.^[1] This provides the infant optimal nutrition and immunity while supporting growth and development.

Breastfeeding provides many benefits to infants and mothers. For example, research in developing and industrialized nations demonstrates that breastfeeding is associated with reduced risk of acute and chronic illnesses, enhanced cognitive performance, and reduced pain during medical procedures such as a heel-stick or immunization.^{[1] [2]} Women who breastfeed experience faster return to pre-pregnancy weight and reduced risk of certain types of cancer.^[3]

Despite the many benefits of breastfeeding, data indicate that breastfeeding rates in the United States are low. Among children born between 2010 and 2013, approximately 80 percent were breastfed initially, but only 20 percent were breastfed exclusively (receiving no formula or other foods) at six months of age.^[4] Several factors influence whether a woman initiates and continues to breastfeed, including the mother's age, race, ethnicity, education level, socioeconomic status, family structure, and mental status.^[5]

This article provides an overview of the production and composition of breast milk, the benefits of breastfeeding to both infants and mothers, and practical information for dealing with the challenges associated with breastfeeding.

"Emerging evidence suggests that some of the immune and health benefits that infants acquire during breastfeeding may be lifelong." Click To Tweet

Breast milk production and composition

Breast milk production

The human breast undergoes profound changes in size, shape, and function that precede milk production. This process, called lactogenesis, occurs in two distinct stages.

During stage I lactogenesis, which occurs in the last half of pregnancy, breast glandular tissue differentiates, and the alveolar mammary epithelial cells, whose function is to accumulate, produce, package, and export the components of milk, proliferate and mature into lactocytes, the milk-producing cells. Lactocytes sit in a somewhat loose configuration in the breast during pregnancy, but within a few days after childbirth, tight junctions form between the cells, forming an impermeable barrier that prevents milk leakage.^[6] During pregnancy, little milk secretion occurs due to the high progesterone to estrogen ratio, which favors breast growth.^[7] By late pregnancy, however, some women can express colostrum (see below).^{[8] [9]} Stage II lactogenesis occurs in the first four days postpartum. The delivery of the placenta triggers a rapid drop in progesterone and elevated levels of cortisol and prolactin. The breasts swell, and copious milk production begins.^[8]

Maintaining lactation requires both mechanical and hormonal inputs. Nipple stimulation and subsequent milk removal promote the continued release of prolactin and oxytocin. In turn, these hormones drive the ongoing production and release of milk (respectively).^[8]

Colostrum

The first fluid a mother produces is colostrum, a thick, sticky fluid that is commonly yellow, orange, or white. Colostrum is rich in immunomodulatory factors, and its primary role is immunological rather than nutritional. Relative to a mother's later milk, colostrum is low in lactose, potassium, and calcium and high in sodium, chloride, and magnesium.^[10]

Transitional milk

During the first few days to two weeks after childbirth, an upsurge in milk production begins, and tight junction closure occurs between the lactocytes, signaling the production of transitional milk. The lactose content of the milk increases, and the sodium-to-potassium ratio drops. ^[10]

Mature milk

The mother's milk is considered "mature by four to six weeks after childbirth." ^[10] Mature milk expressed during the early part of a single feeding session, called foremilk, is high in lactose and has a watery consistency. The milk expressed during the latter part of the feeding, called hindmilk, is high in fat and has a creamy consistency.^[11]

Breast milk composition

Breast milk is a complex and highly dynamic fluid that changes composition during a single feeding, from day to night, and throughout the lactation period in response to the growing infant's needs.^{[10] [12]}

The content of breast milk is profoundly influenced by circadian rhythms, the body's 24-hour cycles of biological, hormonal, and behavioral patterns. For example, analysis of human milk samples from more than 75 women taken every three hours during a single 24-hour period demonstrated that the amino acid content of the milk varied according to a circadian rhythm. The study's authors suggested that if an infant is fed expressed breast milk, the milk should be provided at the same time of day that it was expressed to maintain the infant's circadian rhythm.^[13] Other research demonstrated that certain breast milk nucleotides, one of the structural components of DNA and RNA, also showed circadian rhythmicity. Whereas some nucleotides peaked during the day, others peaked at night, suggesting a potential role for these nucleotides as sleep inducers.^[14]

Breast milk is species-specific, containing nutritional and non-nutritional components tailored to serve the infant's varied needs.

Nutritional components of breast milk

Breast milk contains proteins, carbohydrates, fats, vitamins, and minerals to nourish the infant.

Protein

Approximately 415 proteins have been identified in human breast milk.^[15] In addition to supplying essential nutrition, these proteins provide many benefits, such as aiding in the digestion and utilization of the various components in breast milk and supplying antimicrobial and immunomodulatory factors that compensate for deficiencies in the infant's immune system.^{[16] [10]} The total protein content in human breast milk is markedly lower than milk from other species, which corresponds to the slow growth rate of human infants.^[17]

Carbohydrates

Breast milk contains a variety of complex carbohydrates. Lactose is the primary carbohydrate in breast milk, and its concentration remains relatively stable throughout lactation, regardless of maternal diet.^[17] Lactose provides the necessary energy for the infant's brain. The second-largest concentration of carbohydrates in human breast milk are non-nutritive components called human oligosaccharides, described in detail below.

Fats

Fats are the predominant energy source provided in breast milk, supplying roughly half of its calories.^[18] In addition to delivering necessary energy for growth, fats aid in the maturation of the infant's gastrointestinal and central nervous systems and protect from pathogens, including group-B Streptococcus bacteria.^[10]

Approximately 98 percent of the fat in breast milk is in the form of triglycerides, whose properties and structure depend on incorporated fatty acids.^[19] Nearly 200 fatty acids have been identified in human breast milk, with oleic, palmitic, and linoleic acids present in the highest concentrations.^[20] The structural configuration of these fats (which is not always replicated by infant formulas) enhances the fats' absorption in the infant's gut.^{[21] [22]}

The fatty acids in breast milk are encapsulated in fat globules surrounded by a triple-layered structure called a milk fat globule membrane, or MFGM. The MFGM is interspersed with various proteins, enzymes, and cholesterol that exert bioactive properties that confer many of breast milk's antibacterial and anti-inflammatory properties.^[23] Currently, most infant formulas do not contain MFGM; however, a recent clinical trial found that the bovine form of MFGM exerts similar beneficial effects on human infants when included in infant formula. The double-blind, randomized, controlled trial involved 451 healthy, full-term infants who received either regular formula or formula containing MFGM and lactoferrin (an iron-binding protein found in human and bovine milk) at concentrations similar to those in human milk. At the end of the 18-month-long study, the infants who received formula containing MFGM and lactoferrin scored higher on cognitive, language, and motor skills tests than infants who received ordinary formula.^[24] In fact, their scores were similar to those observed in children who were breastfed, suggesting that the addition of MFGM and lactoferrin could narrow the gap in cognitive development commonly observed between formula-fed infants and breastfed infants.^{[24] [25]} Figure 1. shows some of the many factors that influence the composition of milk fat globules in breast milk, which, in turn, impact the infant gut in early life.

Omega-3 fatty acids

Omega-3 fatty acids play critical roles in infant brain development. Maternal intake of omega-3 fatty acids is associated with larger brain volumes in breastfed or mostly breastfed infants. For example, imaging studies of 92 one-month-old full-term infants who were breastfed exclusively or most of the time found that infants of women who consumed higher quantities of omega-3 fatty acids had greater brain volume in specific regions of the frontal cortex and corpus callosum – areas of the brain involved in consciousness, communication, memory, attention, and integration of motor, sensory, and cognitive performance between the brain's hemispheres, respectively.^[26]

Docosahexaenoic acid

Docosahexaenoic acid, or DHA, is the most abundant omega-3 fatty acid in a newborn's brain and influences mental and psychomotor development.^[27] The quantity of DHA in breast milk is strongly influenced by the mother's diet and lifestyle. For example, when lactating women took a dietary supplement containing 400 milligrams of DHA, their breast milk contained 123 percent more DHA than the breast milk of women who took a placebo.^[28] Furthermore, infants whose mothers took the DHA supplement exhibited significantly lower plasma omega-6 to omega-3 ratios than infants whose mothers took a placebo.^[28] A lower omega-6 to omega-3 fatty acid ratio is more desirable in reducing the risk of many chronic diseases.^[29] It is noteworthy that smoking cigarettes reduces omega-3 fatty acid (especially DHA) uptake into breast milk.^[30]

Vitamins & minerals

Breast milk's vitamin and mineral content varies according to maternal diet and tissue stores. If a mother's nutritional status is compromised, her milk may be inadequate to supply sufficient quantities of essential micronutrients to her infant, and supplementation (for the mother, infant, or both) may be necessary.

However, some nutrients are low in breast milk regardless of maternal status. For example, vitamin K, which is essential for normal blood clotting, is very low in breast milk. Consequently, the American Academy of Pediatrics recommends that all newborn infants receive an injection of vitamin K to prevent hemorrhage.^[3]

Iron is an essential nutrient that plays a key role in infant growth.^[31] Breast milk is astonishingly low in iron, which might seem counterintuitive to infant survival. However, microbes require iron for their growth, as well. To prevent infection, the body restricts microbes' access to iron.^[32] Within hours of birth, a dramatic drop in an infant's serum iron levels occurs, reducing the infant's risk of developing neonatal sepsis, a generalized, life-threatening bacterial infection that commonly occurs within the first days (up to one month) of life.^{[33] [32] [34]} By the age of six months, however, iron deficiency anemia may manifest, so introducing iron-rich complementary foods is recommended.^[3]

In addition, breast milk contains negligible amounts of vitamin D and is typically considered inadequate to prevent vitamin D deficiency in exclusively breastfed infants. However, recent studies demonstrate that daily supplementation with high-dose vitamin D (6400 IU) in healthy lactating women can increase the vitamin D concentration of breast milk to a level that supplies adequate vitamin D intake for the breastfeeding infant.^[35] Similarly, a study involving lactating women who were vitamin D deficient found that supplementation with high dose vitamin D (4000 IU) daily met the needs of the infant while also addressing the mother's deficiency.^[36]

Maternal cigarette smoking impairs the uptake of some nutrients into breast milk. For example, evidence indicates that iodine concentrations in the breast milk of women who smoke are roughly half those of non-smoking women's milk.^[37] Iodine is critical for healthy thyroid function. Rodent studies have found that maternal nicotine exposure impairs thyroid function in offspring and promotes resistance to the hormone leptin, which is associated with obesity.^[38]

Similarly, levels of antioxidant vitamin C and vitamin E are reduced in smokers' breast milk, promoting a pro-oxidative state in the infant. This finding was demonstrated in a study that compared levels of ethane, a marker of oxidative stress, in the exhaled air of infants. The breastfed infants of women who smoked exhaled seven times more ethane than the infants of women who did not smoke.^[39]

Non-nutritional components of breast milk

Prebiotics and probiotics

Establishment of the healthy infant gut begins at birth when the infant is exposed to the microbial milieu of the mother's vaginal canal. In fact, infants delivered by Caesarean section have a markedly different microbiota than infants delivered vaginally.^[40] Further microbial exposure occurs at the mother's breast from her skin and milk.^[41] In turn, immune cells in breast tissue sample bacteria from the infant's mouth. The mother's immune system is also exposed to the infant's developing microbiota through contact with bodily fluids. Through repeated exposures, the mother and infant form a dyad of microbial exchange, with the infant's immune system benefitting from the mother's mature microbiota and the mother's immune system responding to the infant's needs.^[42] See Figure 2. below.

These early microbial exchanges set the stage for short-term and long-term disease protection. For example, infants with lower levels of beneficial gut bacteria are three times more likely to develop allergies by the age of two years than those with higher levels.^[43] Breastfed Infants have less diversity in their gut microbiome than formula-fed infants, but the overall composition is more beneficial.^[44] Gut bacteria composition is associated with reduced negative emotions and fear reactivity in infants.^[45] Furthermore, greater quantities of lactic acid-producing gut bacteria are associated with positive emotions in infants.^[45]

Critical to establishing a healthy gut are the prebiotic and probiotic components of breast milk.

Prebiotic factors

Some of the more unique components of breast milk are complex, indigestible sugars called human milk oligosaccharides, or HMOs. More than 200 HMOs have been identified, and they are the third most abundant factor in breast milk after fat and lactose, averaging 20 to 25 grams per liter in colostrum and five to 20 grams per liter in mature milk.^{[46] [47] [48]} The quantity and composition of the HMOs in breast milk are genetically determined and differ slightly between women.^[49]

The primary role of HMOs is to serve as prebiotics – compounds that induce the growth or activity of beneficial bacteria – in the infant gut. In turn, these beneficial bacteria produce short-chain fatty acids and other substances that prevent colonization of pathogenic bacteria in the gut.^[50]

"The primary role of HMOs is to serve as prebiotics – compounds that induce the growth of beneficial bacteria – in the infant gut. In turn, these beneficial bacteria produce short-chain fatty acids and other substances that prevent colonization of pathogenic bacteria in the gut." Click To Tweet

But HMOs exhibit unusual characteristics that set them apart from most prebiotics. For example, HMOs serve as "decoys" to protect the infant from gut infections. For pathogenic bacteria to cause infection, they must first target and bind to specific carbohydrates found on the surface of cells that line the gut. However, HMOs' overall structure and shape mimic that of the bacterial targets. The pathogens bind to the HMOs instead, foiling their ability to establish themselves in the gut.^[10]

Another interesting quality of HMOs is their capacity to break down biofilms that group-B Streptococcus bacteria create to protect themselves from antimicrobials and antibiotics.^[51] Furthermore, they enhance some antibiotics' activity by increasing the membrane permeability of pathogenic bacteria.^[52]

Learn more about HMOs in this clip featuring Drs. Erica and Justin Sonnenburg.

Probiotic bacteria

Contrary to the belief that human milk is a sterile solution, breast milk is teeming with hundreds of types of bacteria. This highly diverse and complex community of microbes contains primarily strains of bacteria from the Streptococcus, Staphylococcus, Serratia and Corynebacteria genera. They arrive in the mother's milk via various sources, including the mother's skin, retrograde flow from the infant's saliva, and potentially other means.^{[41] [53]} Exposure to this rich bacterial community via breast milk may contribute to the differences observed in gut microbial populations between breastfed and formula-fed infants and provides a rationale for the inclusion of probiotics in infant formulas.^{[54] [55]}

Harmful substances present in breastmilk

Many harmful substances can transfer from the mother's blood and accumulate in her breast milk. For example, heavy metal concentrations are elevated in breast milk.^[56] Cadmium, a toxic metal that impairs the metabolism of nutrients essential to infant development, including selenium, zinc, copper, and magnesium, poses a particular threat because it is also a carcinogen. Cadmium levels in the transitional milk of women who smoke are approximately four times higher than the milk of women who don't smoke.^[57]

Also of concern are drugs, most of which can be taken up into breast milk via passive diffusion. A drug's size, chemical characteristics, and factors that alter maternal drug metabolism influence the degree to which it passes into the milk. An infant's capacity to metabolize drugs is markedly lower than an adult's, and most adverse events related to maternal drug use occur in infants under two months of age. Drugs that should not be taken during breastfeeding include anticancer drugs, lithium, oral retinoids, iodine, amiodarone, and gold salts.^[58]

Evidence indicates that some drugs, such as "social drugs," including alcohol, nicotine, caffeine, and cannabis, carry some risk to the infant.

Alcohol

Alcohol readily transfers into breast milk; however, the amount an infant is exposed to is low due to the infant's high body water content.^[59] However, infants detoxify alcohol less efficiently than adults do, so high maternal alcohol intake could have negative effects on a breastfed infant, including altered sleep patterns, decreased milk intake, weight gain, alcohol-induced hypoglycemia, and impaired motor development.^[60] Alcohol is metabolized at a constant rate (referred to as zero-order kinetics), so drinking water or disposing of expressed milk ("pumping and dumping") does not hasten clearance.^[60]

Nicotine from smoking

Nicotine, a potent stimulant derived from the nightshade family of plants, transfers into breast milk via passive diffusion and may harm infant health. Evidence suggests that nicotine concentrations in the breast milk of women who smoke are three times higher than maternal plasma levels. An infant's capacity to eliminate nicotine is three to four times less than an adult's.^[61] Furthermore, nicotine impairs lactation by altering maternal prolactin levels, the principal hormone involved in milk production.^[56]

A study involving 15 mother-infant pairs investigated the effects of maternal smoking on infant sleep and activity patterns and found that infants slept approximately 30 minutes less when fed immediately after the mother smoked than when fed after the mother abstained from smoking.^[62] Multiple studies indicate that infants of women who smoke cigarettes are more susceptible to respiratory infections and colic and typically exhibit poor respiratory function after breastfeeding.^[60]

Breast milk concentrations of nicotine obtained from nicotine patches are approximately 70 percent lower than those obtained from smoking, indicating that smoking cessation strategies that rely on nicotine patches may be a viable option for nursing women who smoke. However, findings from a rodent study indicate that even low levels of nicotine exposure during pregnancy or early in life can increase the risk of sudden infant death syndrome, or SIDS, by inhibiting the neonate's ability to autoresuscitate – the capacity to recover from prolonged periods of apnea, widely considered a causal factor in SIDS.^[63]

Caffeine

Caffeine is found in various foods, beverages, and other products, such as gum. The amount of caffeine in breast milk varies according to differences in maternal caffeine metabolism and is typically low – approximately 1 percent of maternal blood concentrations, which usually peak one or two hours after ingestion. Caffeine metabolism is very poor in infants, so infants of women who consume extremely high quantities of caffeine (750 milligrams per day or more, about 6 to 8 cups of coffee) could potentially achieve toxic concentrations of caffeine from breast milk.

Cannabis

Cannabis is the collective term for the psychoactive components of the cannabis (marijuana) plant. An analysis of breast milk samples from 50 lactating women who reported using marijuana found that 63 percent of the samples had detectable levels of delta-9-tetrahydrocannabinol (THC), the primary psychoactive ingredient in marijuana, and these levels were present up to six days after the last reported use.^[64] A study that assessed the effects of maternal marijuana use on infant development found that marijuana exposure via breast milk during the first month of an infant's life was associated with decreased infant motor development at the age of 12 months.^[65]

Benefits of breastfeeding to infants and mothers

Breast milk provides a compensatory immune system for a growing infant

Breast milk's immune-enhancing benefits are particularly important to the growing infant. Although experts recommend exclusive breastfeeding for at least six months, children breastfed for any length of time are 64 percent less likely to develop nonspecific gastrointestinal tract infections, an effect that lasts as long as two months after the cessation of breastfeeding.^[3] Similarly, infants breastfed for at least three months have a lower risk of developing atopic dermatitis (eczema).^[66] Meta-analyses of randomized controlled trials, prospective studies, and case-controlled studies demonstrate that infants exclusively breastfed for more than four months are 72 percent less likely to be hospitalized due to lower respiratory tract infections in the first year of life compared to infants who were formula-fed.^[67] Interestingly, breastfeeding demonstrates a dose-dependent effect on the risk of developing two types of leukemia. Infants breastfed for six months or longer had a 20 percent lower risk of developing acute lymphocytic leukemia and a 15 percent lower risk of developing acute myeloid leukemia.^[66]

Emerging evidence suggests that some of the immune and health benefits infants acquire during breastfeeding may be life-long. For example, breastfeeding appears to confer long-term protection to the gut. Infants breastfed are 31 percent less likely to develop childhood inflammatory bowel disease and 52 percent less likely to develop celiac disease if breastfed at the time of gluten exposure.^{[68] [69]} Recent animal studies demonstrate that females exposed to infection before pregnancy may pass on lifelong immunity to their offspring via the transfer of immune cells in their milk even after nursing stops.^[70]

The immunity conferred in breast milk has relevance for COVID-19. A small, prospective cohort study investigated whether antibodies to SARS-CoV-2, the virus that causes COVID-19, are present in the breast milk of women vaccinated against the disease. The study involved six lactating women who planned to receive both doses of the Pfizer or Moderna mRNA vaccines. A comparison of breast milk samples before and after the women received their vaccines indicated that SARS-CoV-2-specific IgG and IgA antibodies appeared in the women's milk seven days after the first vaccine dose, suggesting that maternal vaccination may protect COVID-19 in infants.

An infant's immune system is the last of the biological systems to develop, taking months or even years to match an adult's capacity for defense.^[71] Breast milk contains a multitude of components that work together synergistically to supply a compensatory immune "system," including antimicrobial agents (lactoferrin, secretory IgA, lactalbumin, lysozyme, and monolaurin); anti-inflammatory agents (interleukin [IL]-10, lactoferrin, and lysozyme); immunomodulatory factors (memory T cells, IL-4, IL-10, IL-12, colony-stimulating factor 3, tumor necrosis factor-alpha, interferon-gamma); and leukocytes, among others.^[72] Some of these components have demonstrated the capacity to prevent the translocation of infectious pathogens across the gastrointestinal tract, kill Streptococcus pneumoniae and Mycobacterium tuberculosis, inhibit the growth of Escherichia coli, protect against rotavirus, and interfere with postpartum transmission of human immunodeficiency virus, or HIV.^{[73] [74] [75] [76] [77] [78]}

These varied components confer both passive and active immunity. A critical element of the infant's active immunity is provided by maternal leukocytes – living white blood cells that protect from infection in the infant's gut and other tissues.^[79] Breast milk leukocytes differ from their counterparts in blood, exhibiting unique, highly specialized immune function profiles.^[80] Concentrations of leukocytes are highest in colostrum and taper off in transitional milk, eventually reaching a baseline level in mature milk.^[79] This level remains fairly constant throughout lactation unless the infant, mother, or both develop an infection, when levels may increase up to 94 percent above baseline.^[79]

Many other components in breast milk have been identified and are too numerous to list here, including a vast array of growth factors, immunomodulatory factors, and cell signaling proteins.^[72]

Breastfeeding enhances infant brain development

Breastfeeding profoundly affects an infant's brain, greatly influencing intellectual development. A meta-analysis of 17 reviews linking breastfeeding and intelligence found that children and adolescents who breastfed as infants scored nearly three points higher on intelligence tests than those not breastfed, even after considering maternal intelligence.^[81] Reinforcing these findings are the data from randomized controlled trials. For example, a large trial that followed up on more than 13,000 infants from 13 hospitals found that total and exclusive breastfeeding led to improved performance on intelligence tests at the age of six, with breastfed children averaging roughly 7.5 points higher on the tests than children not breastfed.^[82]

Other research has found that by the age of two years, babies exclusively breastfed for at least three months had enhanced development in key parts of their brains, compared to children fed formula exclusively or fed a combination of formula and breast milk. The breastfed babies' white matter growth was 20 to 30 percent greater, especially in brain regions associated with language, emotional regulation, and cognition.^[83]

The beneficial effects of breast milk on infant brain development may be particularly relevant for preterm infants (born before 37 weeks' gestation), who have a significantly higher risk for white matter injury during birth.^[84] Long-term studies and data obtained from randomized feeding trials indicate that verbal intelligence is higher and white matter and total brain volumes are greater in eight-year-olds and adolescents who received breast milk as infants in the NICU. These findings were particularly robust in male infants.^{[85] [86]}

Breast milk may confer cardioprotective effects in preterm infants

Preterm birth is associated with an increased risk of cardiovascular complications, including small ventricle size, heart muscle hypertrophy, and poor blood pressure control. The long-term outcomes of these complications are often manifested in adulthood as an increased risk of ischemic heart disease, hypertension, and perturbed metabolism, as evidenced by insulin resistance, glucose intolerance, and abnormal lipid profiles.^[87][88]

Much of the data regarding the cardioprotective effects of breast milk in preterm infants comes from follow-up studies of a cohort of more than 900 preterm infants born in the United Kingdom. The infants in the studies received either donor breast milk or a nutrient-enriched preterm formula exclusively until their body weight reached 2000 grams. In one of the follow-up studies, a subset of the cohort was assessed for cardiovascular health via MRI when they reached adulthood (23 to 28 years of age).^[89] The MRI findings indicated that those fed breast milk exclusively as preterm infants had larger ventricle size and stroke volume than those fed formula. A separate follow-up study found that the infants who received breast milk exclusively had lower diastolic and systolic blood pressure at the ages of 13 to 16 years.^[90] These findings suggest that breast milk mitigates some cardiovascular-related complications associated with preterm birth.

Breastfeeding protects against cancer and facilitates postpartum weight loss

Lower risk of breast and ovarian cancer

A review of 47 epidemiological studies found that for every 12 months of breastfeeding, the risk of developing breast cancer decreased by 4.3 percent.^[91] Findings from a case-control study of more than 400 women with ovarian cancer suggest that even short-term breastfeeding reduces the risk of developing ovarian cancer.^[92]

The mechanisms behind the protective effect on the breast are poorly understood. Still, some evidence suggests they may be related to involution, a process that occurs when lactation ends due to weaning or when a woman is unable or chooses not to breastfeed after giving birth. During involution, extensive cell death and tissue remodeling occur in the breasts as they return to the pre-pregnancy state. Various immune and inflammatory processes related to breast involution are considered tumorigenic.^[93] Slowly weaning the infant (such as would occur when complementary foods are gradually introduced) may moderate the harmful effects of these processes.^[94]

Return to pre-pregnancy weight

Weight gain is a normal, healthy part of pregnancy due to the additional weight of the fetus, placenta, amniotic fluid, uterine and breast tissue, blood and plasma volume, and fat stores. The fat stores accumulated during pregnancy support breastfeeding.

In 2009, the Institute of Medicine issued new guidelines for weight gain during pregnancy.^[95] These guidelines consider the growing problem of overweight and obesity among women in the United States. A woman who is at a healthy pre-pregnancy weight should gain 25 to 35 pounds to support her growing infant. Recommendations for healthy weight gain are shown in the table below.

Losing weight after pregnancy is difficult for many women, but those who breastfeed are more likely to return to their pre-pregnancy weight. They typically do so sooner – within the first three to six months after delivery – than women who do not breastfeed.^[96]

Challenges of breastfeeding

The first few days and weeks of breastfeeding present many challenges to new mothers, ranging from sore nipples and plugged ducts to a lack of family support and concerns about physical image and sexual intimacy. Working with a lactation consultant during the first few weeks of breastfeeding may be helpful, especially for mothers of preterm infants.^[97]

If a woman's milk is insufficient to meet the needs of her infant or if she has a health concern that prevents her from breastfeeding, donor milk may be an option for healthy, full-term babies. Donor milk is insufficient to meet all the needs of preterm infants, however.^[98]

Research indicates that exposure to cigarette smoke influences breastfeeding duration. A study of more than 1,200 mother-infant pairs found that women exposed to household second-hand smoke were 30 percent more likely to stop breastfeeding early than women in non-smoking households.^[99]

Conclusion

Breastfeeding provides many benefits to both infants and mothers, including intellectual and emotional development in the infant and protection against cancer and overweight in the mother. Breast milk is a complex, dynamic fluid containing many nutritional and non-nutritional components that positively influence an infant's physical and emotional development. Breast milk also provides a mechanism for transferring substances or drugs that the mother may inhale, ingest, absorb, or ingest to the infant's benefit (or possible detriment). Although breastfeeding presents challenges to some women, support is available.

Related episodes and clips

• The Biology of Breast Milk
• Human milk oligosaccharides: breast milk sugars that promote a healthy infant gut