What makes human milk’s composition ideal for infants?

Human milk is the preeminent source of infant nutrition. Although its precise composition varies depending on the mother’s diet, health, lifestyle and geographic location, its unique properties remain essential to support the nourishment of full-term and premature newborns. The WHO recommends exclusive breastfeeding for the first 6 months after which breastfeeding can continue alongside suitable, complementary, solid foods¹.

Breastfeeding has numerous short and long-term beneficial effects for infants and mothers. Human milk has optimal nutritional value and beneficially influences absorption, metabolism, development of the gut microbiota and gut maturation. Breastmilk also plays a key role in reducing the risk of infections and allergies as well as supporting brain and eye development².

Exclusive breastfeeding for at least 3 months is associated with a lower incidence and severity of diarrhoea, respiratory infection and otitis media. Exclusive breastfeeding for at least 6 months is associated with a lower incidence of allergic disease in at-risk infants.

Breastfeeding is also associated with a lower incidence of obesity during childhood and adolescence and of hypertension and hypercholesterolemia in adulthood.

Human milk composition is influenced by gestational and postnatal age. A large review concluded that breastfeeding was associated with a reduced risk of acute otitis media, non-specific gastroenteritis, severe lower respiratory tract infections, atopic dermatitis, asthma (in young children), obesity, type 1 and 2 diabetes, childhood leukaemia, sudden infant death syndrome (SIDS) and necrotizing enterocolitis3.

Infants’ immune responses are modulated by environmental factors in the first month of life, which may explain why breastfeeding in the first month of life has a protective effect against subsequent infections and atopy4. In addition, breastfeeding is associated with an earlier return to pre-pregnancy weight, and decreased risk of breast and ovarian cancer pre-menopause and of hip fractures and osteoporosis post-menopause5.

Besides macronutrients (e.g. lactose, triglycerides and proteins), which provide energy and building blocks to the newborn infant, human milk also contains a large number of compounds that modulate functional aspects of metabolism.

The key to unravelling the health benefits of human milk is an understanding of the causes and consequences of the variation in its composition.

Many of the non-digestible factors in human milk contribute to epithelial cell growth, mucosal barrier and immunity. The development and maturation of the gastro-intestinal tract and its specific and digestive functions are of utmost importance. Maturation is a continuous process from foetal development until early childhood⁶. A crucial phase starts after birth with the infant’s first feeds.

Human milk contains low amounts of protein, and a ratio of caseins and whey proteins that is tailored to meet infants’ functional and nutritional needs⁷. It also contains functional proteins such as growth hormones and interleukins inducing cell growth and cell differentiation6.

Human milk contains large amounts of human milk oligosaccharides (HMOS) with complex molecular structures that have an important effect on the gut microbiota and the developing immune system including allergy and infection^8,9.

Individually, the complex pattern of these HMOS varies on the basis of different gene expression and longitudinally within a specific group¹⁰.

The main effect of human milk on the gut microbiota may be derived from HMOS, which undergo selective fermentation by the beneficial gut microbiota. Short-chain fatty acids (SCFA) are produced during this fermentation, which help the mucous layer lining the gut to develop and therefore inhibit the binding of potential pathogens.^9,11,12.

The milk of healthy mothers usually contains the optimal ratio of Omega-3 and Omega-6 long-chain polyunsaturated fatty acids (LCPs). These form complex structures important for metabolism and brain development, vary over the course of lactation^13,14, and are influenced by the diet of the mother15.

More recently, a clear association between genotypes of lipid metabolising enzyme and fatty acid levels in diverse human tissues shows that these gene cluster variations are, in addition to nutritional regulation of fatty acid synthesis, a very important regulator of LCPs synthesis^16-18.

Additionally, LCPs are known to stimulate differentiation, support gut maturation, reduce transcellular permeability¹⁹, influence mucin synthesis²⁰, and promote appropriate inflammatory and anti-inflammatory responses²¹.

Another functional lipid class is short-chain fatty acids (SCFAs), which can induce specific mucin expression and have been found to improve the gastro-intestinal extrinsic barrier by enhancing epithelial mucus expression²².

Human milk is a source of anti-oxidative compounds²³, and also contains living cells derived from the maternal system.

The relevance and physiological effect of low amounts of bacteria (e.g. bifidobacteria and lactobacilli) found in human milk samples is currently being explored²⁴.

Conclusion

There is an ongoing need to improve understanding of the contribution of specific human milk composition on digestion and absorption; the development of the gut and its microbiota; the immune system and the brain. There is an increased scientific interest to gain more insights into the complex interplay of macronutrients and trace compounds in human milk, which is key to understanding further health benefits.

Further exploration into the many benefits of human milk and breastfeeding behaviour on infant and maternal health must continue in order to help support breastfeeding but also to help in understanding the nutritional needs of mothers and their infants in their early phase of life.