Breastmilk composition – the research

Breastfeeding is the biologically normal way to feed babies. Hence, the composition of breastmilk is the biological norm for human babies.

Breastmilk contains many factors which help protect against infection and inflammation and contribute to the development of a child’s immune system and a healthy gut microbiome. 

Breastmilk is a dynamic fluid which changes in composition over the course of lactation and varies within and between feeds and between mothers. Its composition also varies between term and preterm infants.¹ 

The first fluid made by breastfeeding mothers is called colostrum. It is made in small amounts and is a rich source of immune protective factors (e.g. secretory IgA, lactoferrin, white blood cells) and developmental factors (e.g. epidermal growth factor). Around day 3 after birth, a mother’s breastmilk ‘comes in’ and this represents a period of increased milk production to support the needs of the rapidly growing baby. 

The nutritional components of breastmilk are either made in the milk making cells of the breast, are sourced from the mother’s diet or from maternal stores. 

The macronutrient (ie overall fat, protein and carbohydrate) composition of breastmilk is robust even across different populations of women despite variations in maternal nutritional status.² The average macronutrient composition of breastmilk is approximately 1.2 g/dL for protein, 3.6 g/dL for fat, and 7.4 g/dL for lactose (the main carbohydrate in breastmilk).³ The approximate energy content of breastmilk is 70 kcal/dL and is significantly associated with the fat content of breastmilk.³

The most abundant proteins in breastmilk are casein, α-lactalbumin, lactoferrin, secretory immunoglobulin IgA, lysozyme and serum albumin.⁴ 

Fat is the most highly variable macronutrient of breastmilk. The more full of milk the lactating breast is, the lower the overall fat concentration in the breastmilk. Whereas the more drained a lactating breast is, the higher the overall fat concentration.⁵ That’s because the first milk that flows at each feed is lower fat milk, so the concentration of fat is higher in the milk that’s left. The fatty acids of breastmilk vary in relation to maternal diet. If a mother consumes more long chain polyunsaturated fatty acids (eg omega-3 fatty acids in her diet), her breastmilk will contain higher proportions of those fats.⁶ 

The main carbohydrate in breastmilk is lactose. The other significant carbohydrates in breastmilk are called oligosaccharides, which comprise approximately 1 g/dL in breastmilk.⁷ 

The average Australian diet usually provides breastfeeding mothers with sufficient quantities of vitamins and minerals. There are only a few vitamins or minerals for which some breastfeeding mothers may need supplementation to ensure their breastmilk has adequate concentrations. 

• Vitamin B12 Breastfeeding mothers who are vegan (and perhaps vegetarian) may be deficient in vitamin B12 and therefore require supplementation to ensure their breastmilk concentration is adequate.⁸  

• Vitamin D Exposure to sunlight reduces the risk of vitamin D deficiency. However, the safe level of exposure to sunlight for children is unknown. Breastfed babies who are most at risk of vitamin D deficiency include those who are dark-skinned, whose mother is vitamin D-deficient and who receive too little sunlight (eg by living at higher latitudes).  

• Iodine Australia’s National Health and Medical Research Council (NHMRC) recommends pregnant and breastfeeding women take iodine supplementation. ⁹

Breastmilk contains various bioactive factors (eg living cells, antibodies, cytokines, growth factors, oligosaccharides, hormones). Bioactive factors are elements which have an effect on biological processes and thus impact bodily functions or conditions and ultimately our health. While science is yet to unravel the functions of all the bioactive components of breastmilk, what can be said with certainty is that breastmilk is far more than just nutrition. Breastmilk contains a variety of factors which play a significant role in infant health and development. Many of these factors in breastmilk work together meaning that supplementation with individual factors is unlikely to have the same effect. 

Breastmilk contains various growth factors which have significant effects on the developing baby’s gut, blood vessels, nervous system and endocrine system. ²

Breastmilk helps protect against infection and inflammation. The specific factors in breastmilk which help protect babies from infection and inflammation are vast and science is only beginning to unravel some of their complex functions. 

Breastmilk contains many different living cells (eg white blood cells, stem cells). In early lactation, a breastfed baby can consume up to 10 trillion maternal white blood cells each day! Secretory IgA is the most abundant class of antibody in breastmilk. Secretory IgA antibodies specifically protect mucosal surfaces. There are also many other anti-infective proteins in breastmilk, such as lysozyme and lactoferrin. 

The oligosaccharides in breastmilk differ in composition from those of any other mammal. They are prebiotics which selectively encourage the growth of beneficial bacteria (probiotics). In addition, oligosaccharides act as ‘decoys’ for pathogens. This is because pathogens can bind to the oligosaccharides in the baby’s gut and be passed out with the faeces. Therefore, the pathogens aren’t able to cross the baby’s gut wall and cause illness.¹⁰ 

Breastmilk is not sterile but rather contains its own microbial community. This likely helps to shape the development of a breastfed baby’s gut microbiome. Oligosaccharides may also influence the bacterial community make-up of breastmilk.^11,12

Breastmilk is a dynamic, multi-faceted fluid containing nutrients and bioactive factors needed for infant health and development. While many studies of breastmilk composition have been done, there are still many components of breastmilk still being identified.

© Australian Breastfeeding Association May 2022