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The infant’s nutritional requirements in early life are primarily satisfied by a single and highly specific food source: breast milk. Human milk is recommended as the optimal nutrient source for infants and is associated with several short- and long-term benefits for child health. Health benefits of breast milk are multiple, but a reduced risk of infectious diarrhea and acute otitis media are the best documented [3], and evidence from developing countries demonstrates that under conditions of poor hygiene breastfeeding can be a matter of life or death [4]. It has also been suggested that breastfeeding is associated with a reduced risk of cardiovascular events, overweight, obesity, type 2 diabetes and disorders of the immune system, as well as with better cognitive development [5, 6].

There is no singular standard for breast milk composition: human milk composition is dynamic. Differences in the nutrient content of breast milk are observed within feeds, across the period of lactation (foremilk differs from hind milk, and colostrum is very different from transitional and mature milk) and between women [7, 8]. At least some of these changes in the composition of breast milk during lactation reflect changes in the requirements of infants, which are linked to changes in growth velocity and maturation of immunological and physiological functions. The complexity is also given by the diverse composition of breast milk. Nutrients (such as proteins, lipids, carbohydrates, minerals, vitamins and trace elements) are of high importance to meet the nutritional needs of young infants and ensure healthy normal growth and development, but human milk also contains numerous bioactive proteins and peptides including antimicrobial and immune-modulating factors, enzymes, hormones and growth factors [9, 10]. Several of these compounds affect the infant’s immune status, conferring a passive protection against infection, and facilitate immune development and maturation.

Although it is variable within mothers and across lactation, the macronutrient composition of human milk is conserved across populations. The mean macronutrient composition of mature, term milk is estimated to be approximately 0.9-1.2 g/dl for proteins, 3.2-3.6 g/dl for fat and 6.7-7.8 g/dl for lactose [11].

The proteins of human milk are divided into the whey and casein fractions comprising a large number of different proteins and peptides [12, 13]. The most abundant proteins are caseins, α-lactalbumin, lactoferrin, secretory IgA, lysozyme and serum albumin [9]. Nonprotein nitrogen-containing compounds, including urea, ureic acid, creatinine, creatine, amino acids and nucleotides, cover approximately 25% of human milk nitrogen.

Both total protein content and concentrations of individual proteins in human milk change throughout the first year of lactation to match the needs of the infant. Protein levels decrease in human milk over the first 4-6 weeks regardless of the timing of delivery (from 12-18 g/l in early milk to 8-9 g/l in mature milk) [14].