Experts worldwide have long recognised that breastfeeding is the preferred method of infant nutrition1-5. The unique composition of breast milk changes according to the nutritional needs of the newborn and, through its non-nutritional components, helps babies adapt to life outside the uterus.
Breast milk contains significant amounts of protein, fat and carbohydrate as well as vitamins and minerals, to support healthy growth and development of infants. There are also other important components in breast milk which protect the infant from infection and help support the developing immune system.
Protein
Protein is required in an infant’s diet for growth, maintenance and repair of body tissues, as well as making enzymes that control many body functions. There are two distinct protein components in breast milk; whey and casein. Mature breast milk consists of about 40% casein, which forms a curd and takes longer to digest7 and about 60% whey and is more easily digested7.
The main whey protein in mature breast milk is alpha-lactalbumin. Breast milk protein contains about 27% alpha-lactalbumin, and approximately 33% other whey proteins8,9. The alpha-lactalbumin is rich in essential amino acids, in particular tryptophan and cysteine9.
Fats
The fat in breast milk provides a rich source of energy (calories) for the infant and accounts for about 50% of the energy content of breast milk. Infants have much greater energy requirements than adults relative to their body weight, due to the fast rate of growth in early life.
Fat is also the vehicle for the transfer of the fat-soluble vitamins A, D, E and K and other substances such as prostaglandins.
Fat molecules exist as ‘fork-shaped’ triglycerides, which consist of different fatty acids bound to the sn-1, sn-2, or sn-3 position of a glycerol backbone.
Human breast milk contains various fatty acids, with one of the most abundant being palmitic acid. Palmitic acid makes up 20–25 % of the total fatty acids in breast milk11.
In human breast milk, 60-70% of palmitic acid exists on the sn-2 position whereas the sn-1 and sn-3 positions are commonly occupied by unsaturated fatty acids such as oleic and linoleic12,13.
Breast milk also supplies other essential fats: linoleic acid and alpha-linolenic acid,
docosahexaenoic acid and arachidonic acid, which are vital constituents of brain and neural tissues14.
Carbohydrates
The carbohydrate in breast milk provides about 40% of the total energy in breast milk15. It is the second biggest source of energy in an infant’s diet after fat 16, helping to spare protein, which can then be used for new tissue growth. The predominant carbohydrate in breast milk is lactose, which splits into glucose and galactose on digestion. Lactose aids the absorption of some minerals such as calcium17. The absorption of calcium is very important during infancy as it helps to ensure optimum bone growth 18.
Breast milk contains a significant quantity of carbohydrates, mainly monosaccharides and oligosaccharides. Oligosaccharides in breast milk provide substrates for bacteria in the infant colon and thereby promote the growth of intestinal flora, and supporting gut health19.
Overall, breast milk contains the correct balance of nutrients required for optimal growth and development.1-5 These nutrients are supplied in an optimal form that is readily absorbed by the infant’s immature digestive system for use by the body6.