What is in breast milk?

Breast milk is much more than a source of food. It is a complex, biologically active fluid containing water, fats, carbohydrates, proteins, vitamins, minerals and a range of components involved in immune protection, digestion and development. Its composition changes during the early days after birth, throughout individual feeds and across the entire period of lactation (Ballard and Morrow, 2013).

These changes are a normal part of lactation. Colostrum, produced in the first days after birth, differs from mature milk in its composition and concentration of many components. As lactation progresses, breast milk continues to provide energy and nutrients alongside antibodies, enzymes, hormones and other biologically active substances. Its composition also varies between individuals and according to the stage of lactation (Ballard and Morrow, 2013; Dror and Allen, 2018).

The main components of breast milk

Water

Breast milk is approximately 87% water, making it an important source of hydration as well as nutrition. For a healthy baby who is exclusively breastfed, breast milk provides the fluid needed during the first six months of life, including in warm weather. Additional water is not routinely required during this period (WHO, 2023).

Carbohydrates

Lactose is the principal carbohydrate in breast milk. It provides energy and contributes to the absorption of certain minerals. Breast milk also contains human milk oligosaccharides (HMOs), a diverse group of complex carbohydrates that are not primarily digested by the infant. Instead, many HMOs help shape the developing gut microbiome and can act as decoy receptors for certain pathogens, contributing to the protection of the infant's intestinal environment (Bode, 2012).

The composition of HMOs varies between individuals, partly because of genetic differences. This is one example of how breast milk contains components that do more than provide calories.

Fats

Fat is an important source of energy for a growing infant and contributes to the development of the brain, nervous system and other tissues. Breast milk contains a range of fatty acids, including essential fatty acids that the body cannot produce in sufficient quantities on its own.

Fat concentration varies between individuals, between feeds and as milk is removed from the breast. It is influenced by factors including how full the breast is and the stage of milk removal. This natural variation is one reason that breast milk should not be thought of as having a single, fixed nutritional composition (Ballard and Morrow, 2013; Kent et al., 2006).

Proteins

Breast milk contains proteins that provide amino acids for growth and development, as well as proteins with other functions. These include whey and casein proteins, enzymes and immune-related proteins such as lactoferrin.

Lactoferrin binds iron and has antimicrobial properties, while other proteins help with digestion, nutrient transport and immune function. The relative proportions of proteins change over the course of lactation, with colostrum having a different profile from mature milk (Ballard and Morrow, 2013).

Vitamins and minerals

Breast milk contains vitamins and minerals required for growth and development. The concentrations of some nutrients are influenced by maternal nutritional status, while others are more tightly regulated by the body. Breast milk is not identical in composition for every lactating person, and some nutrients, including vitamin D, may require particular consideration under UK guidance.

Vitamin supplementation should be considered in line with current NHS advice and the individual circumstances of the infant and lactating person, rather than assuming that breast milk always provides every nutrient in the same quantity (NHS, n.d.).

Antibodies and other immune components

Breast milk contains antibodies, immune cells and other components that contribute to immune protection. Secretory immunoglobulin A (sIgA) is a particularly important antibody in human milk. It helps protect the surfaces of the infant's mouth, gut and respiratory tract by binding to microbes and supporting mucosal defence.

Breast milk also contains immune cells, cytokines, enzymes and other substances involved in the regulation of immune responses. These components are part of a wider biological system that supports the infant while their own immune system continues to develop (Ballard and Morrow, 2013; Verhasselt et al., 2024).

Enzymes, hormones and growth factors

Breast milk contains enzymes that assist with digestion and other biological processes, alongside hormones and growth factors involved in metabolism, intestinal development and physiological regulation. Their presence illustrates that breast milk is not simply a mixture of nutrients; it is a biologically active fluid that interacts with the developing infant in several ways (Ballard and Morrow, 2013; Cacho and Lawrence, 2017).

How breast milk is made

Breast milk is produced in specialised cells within structures called alveoli in the breast. These cells take up substances from the maternal circulation and use them to synthesise milk components. Breast milk is therefore made using materials supplied through the blood, but it is not simply blood that has been filtered into milk. The mammary gland actively produces and transports the different components of milk (Ballard and Morrow, 2013).

Two hormones play important roles in lactation. Prolactin supports milk synthesis, while oxytocin causes the small muscle-like cells surrounding the alveoli to contract, moving milk towards the ducts and nipple. This movement is known as the milk ejection reflex, often called let-down. Milk ejection can occur more than once during a feed, and its timing and strength vary between individuals and circumstances (Neville, 2001).

Milk production is also influenced by milk removal. As milk is regularly removed, the breast receives signals that support continued production. When milk remains in the breast for longer periods, production generally slows. This is one reason that effective milk removal and the frequency of feeding or expressing can influence milk supply (Kent et al., 2006).

The Association of Breastfeeding Mothers (2023) explains the process in its illustrated and animated video, How Breastmilk Is Made, illustrated and animated by JoJo Ford.

How a baby's saliva may relate to breast milk

The relationship between a baby's mouth and breast milk is an area of interest because lactation involves interaction between the infant and the lactating person's immune system. Breast milk contains antibodies and other immune components, and maternal immune responses can influence the antibodies present in milk. These responses are part of the body's wider immune activity, including responses to microbes encountered by the mother (Verhasselt et al., 2024).

It is sometimes explained that a baby's saliva is absorbed through the nipple or areola and directly signals the mother's body to produce specific antibodies for that baby. Although communication between the infant and the lactating person's immune system is biologically plausible and an area of ongoing research, this precise pathway should not be presented as an established mechanism. The extent to which infant saliva directly changes the specific antibody content of breast milk, and how this might occur, is not yet fully understood.

What is established is that breast milk contains immune components that support the infant's mucosal defences, and that its composition is dynamic. The potential for interactions between the infant and the lactating person's immune system is an important area of research, but it is helpful to distinguish what is known about breast milk's immune function from explanations of how particular changes might be triggered.

Does maternal diet affect breast milk?

A varied, balanced diet supports the nutritional health of the lactating person. Some aspects of breast milk composition, including the profile of certain fatty acids, can be influenced by maternal dietary intake. However, the relationship between diet and the overall nutritional composition of breast milk is complex, and changes in what someone eats do not reliably produce a predictable increase in the total fat content of their milk or their baby's weight gain (Bravi et al., 2016; Dror and Allen, 2018).

There is no special diet that is required to produce good-quality breast milk. In general, a balanced diet that includes a variety of foods is appropriate, and drinking to thirst is a reasonable way to maintain hydration. Drinking excessive amounts of fluid does not, by itself, increase milk production.

Where there are concerns about a baby's weight gain, it is important to consider the whole feeding picture. This includes how effectively the baby feeds, milk transfer, feeding frequency, milk production and the infant's clinical circumstances. Changing the lactating person's diet alone is not a reliable way to address poor weight gain.

How breast milk changes during a feed

The fat concentration of breast milk changes as milk is removed from the breast. Rather than being divided into two distinct types, milk has a changing composition throughout a feed. Fat concentration generally increases as the breast becomes less full, although the pattern varies according to breast fullness, the interval since the previous feed and the circumstances of milk removal (Daly et al., 1993; Kent et al., 2006).

The terms foremilk and hindmilk have traditionally been used to describe milk collected towards the beginning and end of a feed. However, these terms can give the misleading impression that one type of milk stops and another begins at a specific point. In reality, there is no abrupt transition. Milk with a lower fat concentration and milk with a higher fat concentration are part of the same continuously changing fluid, and their relative proportions vary throughout milk removal.

The fat concentration at any particular moment depends on the circumstances of feeding. Milk at the beginning of one feed may have a different fat concentration from milk at the beginning of another. There is no universal point at which milk becomes higher in fat, and no requirement for a baby to reach a particular stage of a feed before receiving nutritionally valuable milk (Daly et al., 1993; Kent et al., 2006).

Milk ejection and changes in milk fat

Changes in fat concentration should not be confused with the milk ejection reflex, commonly called let-down. Milk ejection occurs when oxytocin causes specialised cells surrounding the milk-producing alveoli to contract, moving milk into the ducts and towards the nipple. This response can occur more than once during a feed (Neville, 2001).

Milk ejection describes the movement of milk, whereas changes in fat concentration describe its composition. Although milk ejection and milk removal are related, a let-down is not a switch between two types of milk. Milk composition continues to vary as feeding progresses, including around successive milk ejections.

Understanding concerns about milk fat

The expression foremilk–hindmilk imbalance is sometimes used when families are concerned about symptoms such as green stools, wind, fussiness or frequent feeding. However, these observations alone do not establish that an infant is receiving too much milk with a lower fat concentration or too little milk with a higher fat concentration.

Milk fat varies naturally, and research examining milk macronutrients has not established a consistent relationship that would allow a particular symptom or stool colour to be used to identify an imbalance (Brockway et al., 2024). It is therefore important not to draw conclusions about milk composition from these observations alone.

There is no universal requirement to feed for a specified number of minutes or to follow a rigid schedule to ensure that a baby receives milk with a higher fat concentration. Feeding patterns need to be considered in the context of the individual infant, including feeding effectiveness, growth and any clinical concerns.

Understanding a baby's feeding cues can help families respond to their individual needs without trying to manage feeding according to perceived changes in milk fat. For more information, see Recognising Feeding Cues in Young Babies.

Where persistent symptoms, feeding difficulties or concerns about weight gain are present, assessment of the infant and the overall feeding pattern is more informative than attempting to correct an assumed imbalance.

How breast milk changes as a child grows

Breast milk changes throughout lactation. Colostrum, produced in the first days after birth, has a different composition from mature milk, including differences in its concentration of proteins and immune components. As milk production becomes established, its composition changes to meet the needs of the growing infant, although individual variation remains considerable (Ballard and Morrow, 2013).

Breast milk continues to provide energy, nutrients and biologically active components beyond the first year of life. Research has found that milk composition can change during prolonged lactation, including changes in some macronutrients and immune components. These changes do not mean that breast milk loses its value as a child grows (Lackey et al., 2021).

The World Health Organization recommends exclusive breastfeeding for the first six months, followed by the introduction of complementary foods alongside continued breastfeeding to two years and beyond (WHO, 2023). As children grow, complementary foods become necessary to meet their increasing nutritional requirements. Breast milk continues to contribute nutrition and biological support alongside those foods, including during natural-term breastfeeding, which may continue for several years until the child chooses to wean.

A final perspective on breast milk

Breast milk is a complex, biologically active food that provides energy, nutrients and immune components. Its composition changes naturally from colostrum in the early days after birth to mature milk and the milk produced during prolonged breastfeeding. These changes are part of normal lactation and do not mean that breast milk loses its value as a child grows.

Understanding the biology of lactation can help explain why milk composition varies, how milk production works and why the idea of two separate types of milk is misleading. However, lactating parents do not need to understand these mechanisms to breastfeed. They do not need to monitor milk fat, identify different stages of milk composition or learn the science of lactation to feed their baby.

The science behind breast milk is fascinating, but it should reassure families about the remarkable biology of lactation rather than create another expectation they feel they must meet. Where feeding is difficult or there are concerns about growth, appropriate support can help families understand what is happening and identify whether anything needs attention.

References

Association of Breastfeeding Mothers (2023) How Breastmilk Is Made [YouTube video]. Illustrated and animated by JoJo Ford. Available at: https://www.youtube.com/watch?v=5-i4tm7mGnc

Ballard, O. and Morrow, A.L. (2013) ‘Human milk composition: nutrients and bioactive factors’, Pediatric Clinics of North America, 60(1), pp. 49–74. doi: 10.1016/j.pcl.2012.10.002.

Bode, L. (2012) ‘Human milk oligosaccharides: every baby needs a sugar mama’, Glycobiology, 22(9), pp. 1147–1162. doi: 10.1093/glycob/cws074.

Bravi, F. et al. (2016) ‘Impact of maternal nutrition on breast-milk composition: a systematic review’, The American Journal of Clinical Nutrition, 104(3), pp. 646–662. doi: 10.3945/ajcn.115.120881.

Brockway, M.B. et al. (2024) ‘Human milk macronutrients and child growth and body composition in the first two years: a systematic review’, Advances in Nutrition, 15(1), article 100149. doi: 10.1016/j.advnut.2023.100149.

Cacho, N.T. and Lawrence, R.M. (2017) ‘Innate immunity and breast milk’, Frontiers in Immunology, 8, article 584. doi: 10.3389/fimmu.2017.00584.

Daly, S.E.J., Di Rosso, A., Owens, R.A. and Hartmann, P.E. (1993) ‘Degree of breast emptying explains changes in the fat content, but not fatty acid composition, of human milk’, Experimental Physiology, 78(6), pp. 741–755. doi: 10.1113/expphysiol.1993.sp003722.

Dror, D.K. and Allen, L.H. (2018) ‘Overview of nutrients in human milk’, Advances in Nutrition, 9(Suppl. 1), pp. 278S–294S. doi: 10.1093/advances/nmy022.

Kent, J.C. et al. (2006) ‘Volume and frequency of breastfeedings and fat content of breast milk throughout the day’, Pediatrics, 117(3), pp. e387–e395. doi: 10.1542/peds.2005-1417.

Lackey, K.A. et al. (2021) ‘Breastfeeding beyond 12 months: is there evidence for health impacts?’, Annual Review of Nutrition, 41, pp. 283–308. doi: 10.1146/annurev-nutr-043020-011242.

National Health Service (NHS) (n.d.) Breastfeeding vitamins. Available at: https://www.nhs.uk/best-start-in-life/baby/feeding-your-baby/breastfeeding/healthy-diet-when-breastfeeding/breastfeeding-vitamins/

Neville, M.C. (2001) ‘Anatomy and physiology of lactation’, Pediatric Clinics of North America, 48(1), pp. 13–34. doi: 10.1016/S0031-3955(05)70283-2.

Verhasselt, V. et al. (2024) ‘Antibodies in breast milk: pro-bodies designed for healthy newborn development’, Immunological Reviews, 328(1), pp. 192–204. doi: 10.1111/imr.13411.

World Health Organization (WHO) (2023) WHO guideline for complementary feeding of infants and young children 6–23 months of age. Geneva: World Health Organization.