What Makes Breast Milk So Powerful for Health?

Breast milk, often hailed as nature’s perfect food, is a miraculous elixir uniquely tailored to nourish and protect infants during their most vulnerable stages of life. Beyond its role as a source of sustenance, breast milk is teeming with bioactive compounds, immune factors, and beneficial bacteria that contribute to the optimal growth and development of newborns. In this exploration, we delve into the unparalleled health benefits of breast milk, shedding light on its profound impact on infant health and well-being. Join us as we uncover the extraordinary properties of this liquid gold and its essential role in shaping the foundation of a healthy life.

1. Milk Composition

In recent years, there has been a growing recognition of the importance of encouraging breastfeeding to enhance the health, growth, and development of infants. Human breast milk stands out as the ideal feeding choice for newborns, often referred to as the gold standard. It contains the precise balance of nutrients and bioactive compounds necessary to provide complete nutrition for infants while also offering protective benefits from beneficial bacteria that bolster vulnerable immune systems against diseases.

Breast milk is a dynamic biological fluid that adjusts its composition throughout the lactation period to meet the evolving needs of the growing infant. The composition of milk varies between mothers who have delivered full-term babies and those who have delivered prematurely. To promote optimal growth, development, and health, the World Health Organization recommends exclusive breastfeeding for the first six months of an infant’s life worldwide.

1.1. Macro and Micronutrient Composition

Human milk undergoes significant changes in composition as lactation progresses from colostrum to transitional milk to mature milk. Colostrum, the initial fluid produced after childbirth, is present in small amounts during the first two to four days. It differs from mature milk in color, composition, and consistency. While the nutrients in colostrum and mature milk remain similar, their levels fluctuate during lactation. Colostrum is rich in whey proteins and minerals but contains lower levels of lactose, fats, and certain vitamins compared to mature milk. It also has higher levels of chloride, sodium, and magnesium but lower levels of calcium and potassium.

Transitional milk, produced from five days to two weeks postpartum, resembles colostrum and supports the growing infant’s nutritional needs. Mature milk, established around two weeks postpartum, maintains relatively stable composition throughout lactation. It typically contains 3–5% fat, 6.9–7.2% lactose, 0.8–0.9% protein, and 0.2% mineral constituents. Major proteins in human milk include casein, lactoferrin, lactalbumin, lysozyme, secretory immunoglobulin IgA, and serum albumin, with higher protein concentrations in colostrum and early milk.

Lipids are a crucial energy source in human milk, with higher fat concentrations found in hind milk compared to foremilk. Fat composition can be influenced by maternal diet and parity. Lactose, the primary carbohydrate, increases rapidly after colostrum and remains constant throughout lactation.

1.2. Bioactives

Human milk not only supplies essential nutrients for energy but also contains a plethora of bioactive components and immune factors crucial for infant health. These include antibodies, immunoglobulins, lactoferrin, lysozyme, antimicrobial peptides, growth factors, white blood cells, microRNAs, and human milk oligosaccharides (HMOs). These components bolster the developing infant’s immune system and provide defense against pathogens. Colostrum, the initial milk produced after birth, contains higher levels of immunoglobulins, cytokines, and immune cells compared to mature milk. Bioactive compounds originate from various sources, with some secreted by the mammary epithelium and cells in milk, while others are transported across the mammary epithelium from maternal serum via receptor-mediated transport. This review focuses on select bioactive compounds that vary throughout lactation.

  • Human Milk Oligosaccharides

Human milk oligosaccharides (HMOs) are intricate glycans found abundantly in breast milk, with levels ranging from 20–25 ng/L in colostrum to 5–15 ng/L in mature milk. Over 200 HMOs have been identified, each varying in structure and composition throughout lactation. While infants cannot digest HMOs, they serve as the third most prevalent component in human milk, following lactose and lipids. HMOs act as Prebiotic agents, nourishing beneficial microorganisms in the infant gastrointestinal tract (GIT) and promoting their growth. They also modulate intestinal epithelial cell responses, deflect pathogens, and prevent their adhesion to intestinal epithelium.

Studies have shown that Bifidobacterium spp., beneficial microbes, thrive on HMOs in the infant gut, while restraining potentially harmful bacteria. Breast-fed infants generally have a higher abundance of beneficial Bifidobacterium spp. compared to formula-fed infants due to the absence of HMOs in infant formula. To mimic HMO benefits, non-HMO prebiotics like fructooligosaccharide (FOS) and galactooligosaccharide (GOS) are added to formula, encouraging a Bifidobacterium spp.-dominated gut microbiome in both infants and adults, as suggested by several studies.

  • MicroRNA

MicroRNAs (miRNAs) are small RNA molecules that regulate gene expression and are found in various human body fluids, including blood, breast milk, urine, and saliva. In breast milk, miRNAs are abundant and are believed to contribute to infant development. They can be packaged into exosomes, membrane vesicles containing various miRNAs, making them stable even under low pH conditions. MiRNAs in breast milk mainly originate from the mammary gland and are found in lipid and cell fractions, with higher concentrations than in skim milk.

Comparisons between mothers of full-term and preterm infants have revealed differences in miRNA expression profiles, suggesting potential implications for infant health. Breast milk contains a higher proportion of miRNAs than infant formula and plays a crucial role in the infant’s immune system development. Studies have shown elevated expression levels of immune-related miRNAs, such as miR-155, during the first six months of lactation, supporting innate immune system regulation and B- and T-Cell maturation.

  • Other Bioactive Compounds
  • Lactoferrin : Lactoferrin, the second most abundant protein in human milk, acts as an iron-binding glycoprotein with various immune functions. It exhibits antimicrobial and anti-infectious properties, often preventing neonatal sepsis, diarrhea, and necrotizing enterocolitis in preterm infants. Colostrum contains high levels of lactoferrin (7 ng/L), which gradually decrease to 2–4g/L in mature milk.
  • Specialized growth factors : Human milk also contains specialized growth factors like epidermal growth factor, aiding intestinal mucosa healing, and insulin-like growth factors (IGF) 1 and 2, promoting tissue growth. Neuronal growth factors in milk assist peristalsis, among other developmental enhancements.
  • Immunoglobulins : Secretory IgA (SIgA) and SIgG are the predominant immunoglobulins in milk, offering vital immune protection to newborns. SIgA, the main protective antibody in breast milk, reaches concentrations up to 12 mg/mL in colostrum, preventing pathogen adherence to epithelial cell surfaces and neutralizing toxins.

1.3. Preterm Milk Composition

Preterm births often result in infants with underdeveloped immune systems, making them more vulnerable to conditions like necrotizing enterocolitis and long-term health issues. Breast milk is recommended as the primary feeding option for preterm infants to support their growth and immune development. However, very preterm or low birth weight infants may require fortified breast milk to ensure adequate intake of nutrients like energy, protein, and micronutrients.

There are notable differences in the composition of preterm and full-term breast milk. Preterm milk typically contains higher levels of protein, fat, and various bioactive molecules compared to full-term milk. Initially, preterm milk is rich in protein, fats, sodium, and free amino acids, though these levels decrease in the first few weeks postpartum. While mineral and trace element content is similar between preterm and full-term milk, preterm milk generally contains lower calcium levels that do not rise over time. Preterm milk also tends to have higher levels of copper and zinc. Additionally, preterm milk exhibits elevated concentrations of bioactive and immune factors, including epidermal growth factor, SIgA, and HMOs. During the early days of lactation, preterm milk contains higher levels of lysozyme and lactoferrin compared to full-term milk.

2. Breast Milk Microbiome

Breast milk, once thought to be sterile, is now recognized as a vital source of microbes for infants, comprising its unique microbiome with beneficial, commensal, and potentially probiotic bacteria. Research indicates that breastfed infants ingest approximately 8 x 105 bacteria daily from breast milk, making it the second major source of microbes after the birth canal for vaginally born babies.

Initially, it was believed that the milk microbiome resulted from bacterial contamination from maternal skin and the infant’s mouth. However, various factors influence the milk microbiome :

  • Maternal diet ;
  • Genetics ;
  • Health ;
  • Demographics ;
  • Environment, and
  • Mode of delivery.

Cesarean section births, for example, are associated with higher bacterial concentrations, particularly Streptococcus spp., and lower Bifidobacterium spp. levels compared to vaginal deliveries during early lactation.

Despite variations, 18 bacterial families are shared among mothers. Antibiotic therapy during pregnancy and lactation reduces Bifidobacterium and Lactobacillus spp. levels in breast milk. Maternal BMI and weight gain during pregnancy affect milk microbiome diversity, with lower diversity in colostrum and one-month milk samples from obese women. Geographical differences also influence milk microbiome profiles across regions.

Vertical transfer of bacterial species from mother to infant occurs, as evidenced by the presence of viable Bifidobacterium and Lactobacillus species in both maternal milk and infant stool. Studies tracking the milk microbiome after full-term and preterm births highlight differences in Bifidobacterium spp. levels, with lower levels in preterm milk throughout lactation. Additionally, the microbial composition of preterm milk changes after infant latching, indicating oral cavity-related bacteria like Streptococcus and Rothia.

3. Origins of Milk Microbiome

Breast milk’s microbiota origin remains unclear but is widely acknowledged. Traditional beliefs attributed the milk microbiome to maternal skin contamination during suckling, with similarities noted between adult skin and milk microbiomes, particularly Staphylococcus and Corynebacterium. Studies also suggest that infant oral cavity bacteria may influence breast milk microbiota through retrograde milk flow during suckling or breast pump expression.

Research indicates that saliva from the infant’s mouth may flow back into the mammary gland during retrograde milk flow, potentially stimulating an immunomodulatory response in the mother. This response could increase leukocyte and antibody production in breast milk, providing immunological protection to the infant, especially during infection.

Forthemore, the discovery of anaerobic species associated with gut environments, unable to exist in aerobic settings, has led to interest in the complexity of breast milk bacteria origins. These findings suggest that live bacteria from the maternal gut may travel through an endogenous route to the mammary gland via an entero-mammary pathway. This pathway involves intricate interactions among epithelial cells, immune cells, and bacteria. Evidence supporting the entero-mammary pathway includes the presence of bacterial communities in colostrum collected before the first infant suckling.

4. Benefits of Breast Feeding

Breast milk’s bacterial communities profoundly impact infant health by shaping the gut microbiota in early life. Breastfeeding directly exposes newborns to breast milk microbiota and indirectly influences bacterial growth and metabolism through maternal milk factors and bioactives.

Breastfeeding reduces the risk of death and various diseases in infancy and offers lasting health benefits into adulthood. It protects against gastrointestinal and respiratory tract infections, necrotizing enterocolitis, and sudden infant death syndrome. Breastfed infants also have lower risks of chronic conditions like allergies, asthma, diabetes, and obesity, along with improved cognitive development.

Human milk microbiota plays immediate and long-term roles in preventing bacterial infections in breastfed infants. It produces antimicrobial compounds, competes with pathogenic bacteria, prevents their adhesion to intestinal walls, and enhances mucin production. Lactobacillus species in breast milk have been shown to prevent the adhesion of pathogenic bacteria like Shigella spp., Salmonella spp., and Escherichia coli.

For preterm infants, breast milk is essential despite factors disrupting gut bacterial community development such as gestational age, birth weight, delivery mode, antibiotic usage, and feeding regimen. Preterm infants, especially those with very low birth weights, have underdeveloped gut microbiomes compared to full-term infants. Studies suggest that preterm infants fed with mother’s own milk have a more gradual acquisition of diversity compared to formula-fed infants, with formula-fed infants showing higher levels of Escherichia and Clostridium in their gut microbiota.


Human breast milk is highly regarded as the optimum mode of feeding for newborns due to its ability to provide complete nutrition and capacity to confer health factors to the infant. Until recently, breast milk was considered to be a sterile fluid, however, it has now been established that breast milk has its own microbiota, which provides numerous health benefits, many of which can contribute to infant health and development.