Benefits of human milk
Human milk provides all the essential components for the growth and development of the baby. This includes macronutrients (fats, carbohydrates and proteins), micronutrients (vitamins and minerals) and developmental factors (long-chain polyunsaturated fatty acids, known as LCPUFA, growth factors and cytokines). Human milk also provides vital protection, decreasing infection via immunoglobulins and anti-infective proteins. It is therefore recommended as the sole source of nutrition for all babies during the first six months of life, with continued breastfeeding in addition to solid foods for at least the first years.
Breast milk fat provides 50-60 per cent of the term baby’s caloric intake. Fat also performs a critical function in delivering free fatty acids and fat-soluble vitamins to the baby. Triacylglycerols, composed of saturated and unsaturated fatty acids, are the most abundant class of fat in human milk, representing more than 98 per cent of the total fat. LCPUFA including docosahexaenoic acid (DHA) and arachidonic acid (AA) are of particular importance because they accumulate in the membrane lipids of the brain and retina, where they provide important visual and neural functions. Indeed, babies fed higher levels of breast milk show higher plasma concentration levels of DHA and AA in the brain cortex, cerebral grey and white matter, as well as a higher IQ up to 15 years of age compared to babies fed formula not containing LCPUFA .
Lactose is the principal carbohydrate in milk, providing 30-40 per cent of energy to the baby. Lactose is a major source of energy to the baby once it has been broken down to glucose and galactose. Glucose is mostly passed into the peripheral circulation and used as a substrate for energy production, whereas galactose is absorbed via the liver and converted into glucose-1-phosphate, which is eventually either converted into glucose or used to replenish liver glycogen stores. Both galactose and glucose can also be used by the brain for energy, and galactose in particular is critical for the production of galactolipids (cerebroside), which are essential for the baby’s developing central nervous system.
Human milk oligosaccharideo (HMO) are complex carbohydrates ranging in length from three to ten monosaccharides. HMOs are the third-largest component in human milk after lactose and triacylglycerols. HMOs do not provide a major source of energy to the baby, since they are not digested in the small intestine. Instead, HMOs have an important immunological function, acting as prebiotics and promoting the intestinal growth of commensal bacteria, in particular Bifidobacterium longum subsp infantis and B.bifidum. They also act as decoys or receptor analogues to inhibit the binding of pathogens – including rotaviruses – to intestinal surfaces. Specific HMOs have also been associated with enhanced gastrointestinal protection against necrotising enterocolitis, which is likely to be especially important for preterm infants (gestational age <36 weeks) because of their increased vulnerability to NEC.
Proteins provide approximately 8 per cent of the energy delivered to the baby. Over 415 have been identified in human milk, many of which are active and have functional roles in protecting the baby. Although protein levels vary widely between mums, the protein level is higher in colostrum (30-70g/l) and then falls to a stable level in mature milk (7-14g/l). Proteins in human milk can be divided into three groups: caseins, whey proteins and proteins associated with the membrane of the milk fat globule. The whey proteins comprise the majority of the protein content in colostrum and decrease to around 60 per cent in mature milk.
Proteins such as β-casein have important antiseptic and anti-infective functions via protease inhibition of bacteria and viruses. In addition, peptides generated from the digestion of α-lactalbumin show potent anti-bacterial activity against gram-positive and gram-negative bacteria. Although multi-functional, other proteins in human milk, including secretory IgA, lactoferrin and lysozyme – and macrophages and free fatty acids – act as anti-infective agents, which are essential for the preterm infant. These agents work together to inactivate, destroy or bind to specific microbes, preventing their attachment to mucosal surfaces.
At the same time, human milk contains protective commensal bacteria that become part of the gut microflora and influence inflammatory and immunomodulary processes. Not only do commensal bacteria prevent overgrowth of pathogenic bacteria, they also acidify the gut, ferment lactose, break down lipids and proteins, and produce vitamin K and biotin.
Human milk provides micronutrients including fat-soluble vitamins, water-soluble vitamins, minerals and trace minerals to the baby, all of which are dependent on the mum's diet. Calcium and phosphate, although independent of maternal dietary intake, are an essential component of casein micelles and are required for bone mineralisation. Trace elements in breast milk include copper, zinc, barium, cadmium, caesium, cobalt, cerium, lanthanum, manganese, molybdenum, nickel, lead, rubidium, tin and strontium, and have a high bioavailability in breast milk only.
Human milk contains live maternal cells, which include blood-derived leukocytes, cells of the mammary epithelium and cell fragments. Leukocytes both protect the mum and have an immunoprotective function for the baby. Stem cells have also been identified in human milk, and have the potential to differentiate into mammary epithelial lineages under mammary differentiation conditions in vitro, as well as other cell types in corresponding microenvironments, including bone cells, brain cells, liver cells, pancreatic beta cells and heart cells. The function of stem cells in the baby is still unclear and warrants further research to understand their potential.
The components of human milk, especially live cells from the baby's mum, cannot be replaced from artificial sources. An exclusive breast milk diet can meet the nutritional needs of term babies for the first six months, with continued breast milk feeding in addition to solid foods for the first two years of life.
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