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What makes human milk’s composition ideal for infants?

Dr Bernd Stahl, R&D Director of Human Milk Research at Nutricia Research, Utrecht, Netherlands, discusses the results of research into the anti-allergenic effect of beneficial bacteria found in the gastrointestinal tract of breastfed babies.

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 infection8,9.

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 lactation13,14, 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 synthesis16-18.

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.

  1. NHS Choices. Why breastfeed [Online]. 2012. Available at: http://www.nhs.uk/conditions/pregnancy-and-baby/pages/why-breastfeed.aspx [Accessed May 2013].
  2. Kramer MS, Kakuma R.The optimal duration of exclusive breastfeeding: a systematic review. Adv Exp Med Biol 2004;554:63-77.
  3. Ip S et al. Breastfeeding and maternal and infant health outcomes in developed countries. Evid Rep Technol Assess 2007;153:1-186.
  4. Belderbos ME et al. Breastfeeding modulates neonatal innate immune responses: a prospective birth cohort study. Pediatr Allergy Immunol 2012;23:65-74.
  5. Turck D. Breast feeding: health benefits for child and mother. Arch Pediatr 2005;3:S145-65.
  6. Wagner CL et al. Host factors in amniotic fluid and breast milk that contribute to gut maturation. Clin Rev Allergy Immunol 2008;34:191-204.
  7. Moro G et al. Postprandial plasma amino acids in preterm infants: influence of the protein source. Acta Paed1999;88:885-9.
  8. Stahl B et al. Oligosaccharides from human milk as revealed by matrix-assisted laser desorption/ionization mass spectrometry Anal. Biochem 1994;223:218-26.
  9. Boehm G, Stahl B. Functional Dairy products. 1st ed.Woodhead: Publ Cambridge,2003.
  10. Thurl S et al. Variation of human milk oligosaccharides in relation to milk groups and lactational periods. Br J Nutr 2010;104:1261-71.
  11. Newburg DS. Oligosaccharides and glycoconjugates in human milk: their role in host defense. J Mammary Gland Biol Neoplasia 1996;1:271-83.
  12. Coppa et al. Human milk oligosaccharides inhibit the adhesion to Caco-2 cells of diarrheal pathogens: Escherichia coli, Vibrio cholerae, and Salmonella fyris. Pediatr Res 2006;59(3):377-82.
  13. Harzer G et al. Changing patterns of human milk lipids in the course of the lactation and during the day. Am J Clin Nutr 1983;4:612-21.
  14. Szabo E et al. Fatty acid profile comparison in human milk sampled from the same mothers at the sixth week and the sixth month of lactation. JPGN 2010;50:316-20.
  15. Peng Y et al. Fatty acid composition of diet, cord blood and breast milk in Chinese mothers with different dietary habits. Prostaglandins Leukot Essent Fatty Acids 2009;81:325-30.
  16. Caspi et al. Moderation of breastfeeding effects on the IQ by genetic variation in fatty acid metabolism. Proc Natl Acad Sci U S A 2007;104:18860-5.
  17. Lattka E et al. Do FADS genotypes enhance our knowledge about fatty acid related phenotypes? Clin Nutr 2010;29(3):277-87.
  18. Lattka E et al. Heinrich Genetic variants of the FADS1 FADS2 gene cluster as related to essential fatty acid metabolism. J Curr Opin Lipidol 2010; 21:64-9. [Epub ahead of print 2009 Nov 30].
  19. Yamagata K et al. Polyunsaturated fatty acids induce tight junctions to form in brain capillary endothelial cells. Neuroscience 2003;116: 649–56.
  20. Tetaert D et al. Dietary n-3 fatty acids have suppressive effects on mucin upregulation in mice infected with Pseudomonas aeruginosa. Respir Res 2007;8:39.
  21. Tiesset H et al. Dietary (n-3) polyunsaturated fatty acids affect the kinetics of pro- and anti-inflammatory responses in mice with Pseudomonas aeruginosa lung infection. J Nutr 2009;139(1):82-9. [Epub ahead of print 2008 Dec 3].
  22. Willemsen LE et al. Short chain fatty acids stimulate epithelial mucin 2 expression through differential effects on prostaglandin E(1) and E(2) production by intestinal myofibroblasts. Gut 2003;52:1442–7.
  23. Szlagatys-Sidorkiewicz A et al. Longitudinal study of vitamins A, E and lipid oxidative damage in human milk throughout lactation. Early Hum 2012; 421-2. [Epub ahead of print 2011 Nov 13].
  24. Martín R et al. Isolation of bifidobacteria from breast milk and assessment of the bifidobacterial population by PCR-denaturing gradient gel electrophoresis and quantitative real-time PCR. Appl Environ Microbiol 2009;75:965-9.

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