The makeup of human breastmilk

The makeup of human breastmilk

The composition of breastmilk is really very elaborate, with literally hundreds of things that contribute to its nutrition and health properties

The composition of breastmilk is elaborate, with hundreds of compounds contributing to its nutrition and health properties. One of the unique factors of breastmilk is that it not only changes within one feed but also during the course of lactation, to reflect a baby’s needs. The first ‘breastmilk’ is called colostrum, which is a protein and essential fatty acid rich, yellowish and thick fluid produced during late pregnancy and during the early days after birth. Although it is only produced in small amounts, its protein content is very high and it is rich in antibodies, which are important for infection protection. Colostrum is also a rich source of fat-soluble vitamins such as vitamins A and E and minerals. Due to these benefits, it is recommended that if possible, a baby receives colostrum rather than infant formula at this time.

Around day three or so after a baby’s birth, a mother’s milk ‘comes in’ and changes to a thinner whiter appearance.

Once a mother’s milk has ‘come in’ and a mother’s breasts become firmer and fuller, at the start of the feed a baby receives milk that is lower in fat and calories and is thirst quenching. As the feed progresses and the breast drains, the baby receives milk that is high in fat and calories.

Breastmilk is nutritionally complete. It contains all the nutrients a baby needs during the first six months of life, including fat, carbohydrate, protein, vitamins, minerals and water to help a baby grow, develop and reach their full potential. Many of the nutrients in breastmilk are absorbed and used easily an infant. All of this makes it nature’s perfect first food for a new baby’s developing body.


About half of the energy from breastmilk comes from fat (3.5 grams per 100ml of breastmilk). Most of the fat in breastmilk comes from the essential omega 3 and 6 fats, including the omega 3 fat called docosahexaenoic acid (or DHA) and the omega 6 fat called arachidonic acid (or ARA). These play an important role in brain and eye development. Fat in breastmilk is released in small drops, with the amount of fat increasing as the breast empties and the feed continues. So, there is more fat in breastmilk produced from softer, emptier breasts as feeding progresses, compared with breastmilk produced at the beginning of a feed from firmer breasts. So a mother should let her baby continue feeding on the milk from softer breasts so that her baby can get all the important nutrients breastmilk provides during the course of one feed.


The milk sugar called lactose is the main carbohydrate in breastmilk, with around 7 grams per 100ml of breastmilk. This is more than in most other mammalian milk types. The lactose in breastmilk is an important source of energy for a baby and his or her developing brain. Lactose also enhances calcium absorption and promotes growth of lactobacilli in the gut (good bacteria). Notably, it is extremely rare for babies to have the genetic disorder that means they cannot digest lactose. In fact, babies of all  ethnicities are well adapted to digest lactose and will only experience temporary or secondary ‘lactose intolerance’ if there is some sort of gut damage, which will need medical attention and which will subside with treatment. Otherwise, a baby consuming large amounts of breastmilk may experience lactose overload but this can be managed with the help of an Australian Breastfeeding Association Counsellor. Breastmilk also contains other carbohydrates called oligosaccharides (sugar chains), which give protection against infection and help to develop healthy gut function.


There are around 1.3 grams of protein per 100ml breastmilk, which is around one third of the total protein of other mammalian milk. Most of the protein in human breastmilk comes as either whey protein or casein protein. The ratio of whey and casein is also different in human breastmilk compared to other mammalian milk: there is more whey compared to casein protein in breastmilk. For this reason, infant formula is designed to contain a similar ratio of whey and casein protein as that found in human breastmilk.

As a baby’s digestive system matures, more milk casein protein appears in human breastmilk. Casein protein is classified into different groups, including alpha-casein, beta-casein and kappa-casein. Beta-casein is classified further as A1 or A2 beta-casein types. Human breastmilk contains only the A2 type and not the A1 type and unlike cows’ milk, human breastmilk contains very little of the other casein protein groups. So, most of the casein in human breastmilk is A2 type beta-casein (however, some cows’ milk proteins may pass into a mother’s breastmilk from her own diet).

Vitamins and minerals

Breastmilk contains all of the essential vitamins and minerals your baby needs during his or her first six months of life, unless a mother’s own stores are compromised (a GP or healthcare professional may advise supplementation). Having said that, Vitamin D needs a special mention: if a mother has good Vitamin D levels during pregnancy, her breastfed baby will continue to receive enough Vitamin D provided he or she receives sufficient casual exposure to sunlight. This can be achieved by taking him or her out for walks, but sensible sun protection strategies need to be followed and always avoid high ultraviolet (UV) light times. Sun protection recommendations do vary with latitude and UV index but as a general rule, if the UV index is less than three, sun protection is not needed unless in alpine regions or outside for extended periods or one is near a highly reflective surface, such as snow or water.

Anti-infective factors

Breastmilk also has many unique factors which play an important role in the development of the gastrointestinal system and protection against infection. These include antibacterial factors like lactoferrin and lysozyme, immunoglobulins (or antibodies) which are directed against a number of microorganisms that can threaten a baby, white blood cells which can destroy germs and oligosaccharides and nucleotides, which help to promote healthy gut and immune function.

Other bioactive factors

Breastmilk contains other bioactive factors that can help digestion and absorption of nutrients. For example, there are enzymes in breastmilk itself that help a baby to digest breastmilk fully and hormones that act as messengers by carrying signals between body cells. There are also proteins in breastmilk that stimulate the development of an infant’s intestinal lining (epidermal growth factors) and improve vitamin and mineral availability (lactoferrin), so that he or she is better able to digest and absorb nutrients and are less easily infected by foreign proteins.


Breastmilk has a high water content and supplies enough water to keep a baby hydrated. So breastfed infants do not need extra fluids even in hot weather, provided they have access to the breast.

Find further information

Infant Feeding Guidelines. Canberra: National Health and Medical Research Council (2012). National Health and Medical Research Council. External Link
Australian Breastfeeding Association (2012). Breastfeeding Management in Australia. 4th Ed. Brodribb W, editor. Australian Breastfeeding Association, East Malvern, Australia.
The World Health Organization’s Global Strategy on Infant and Young Child Feeding (WHA55 A55/15). WHO. External Link

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