The World Health Organization advices mothers to exclusively breastfeed babies for the first 6 months of life, as this has shown to lead to multiple health benefits to newborn infants. These health benefits are related to the presence and concentration of specific immunologically-active human milk components like serum proteins and human milk oligosaccharides (HMOs). This PhD thesis provides information on the variability in serum proteins, serum protein N-glycans and human milk oligosaccharides in milk of individual mothers and over lactation, and offers insights in the degradation of proteins during infant (0–3 months) in vitro digestion. This new knowledge on the variability of immune-active human milk components helps in better understanding the benefits of breastfeeding and may assist in improving infant formula.
Variation in type and levels of serum proteins and HMOs in milk exists among mothers, between countries, and over lactation. The 15 most abundant HMOs cover >95% of the total HMO content. The mother’s secretor (Se) and Lewis (Le) histo-blood groups are related to specific fucosylated HMOs. Based on the total neutral fucosylated HMO concentrations in both Chinese and Dutch human milk, for the first time, Se+Le+ subgroups were identified. Variation among mothers was found in the composition of serum proteins in both colostrum and mature milk, although the group of immune-active proteins, enzymes, and transport proteins were the most abundant for all mothers. These three protein groups encompass many of the 15 most abundant proteins, covering >95% of the total protein concentrations, in both the Chinese and Dutch milk serum proteome. The serum protein N-glycan composition in human milk from 2 different lactational periods was investigated. Analysis showed that neutral fucosylated and nonfucosylated N-glycans dominated the human milk serum glycoproteome. Based on the levels of the individual N-glycans, for the first time, a clear difference can be observed between the milk of secretor and nonsecretor mothers. It also indicated that specific fucosylated N-glycans can be synthesized in the gland of nonsecretor mothers and even in much higher concentrations compared to secretor mothers. In an in vitro infant digestion model, the total milk protein content decreased from the start to the end of infant in vitro digestion with large variation between mothers, especially in the gastric phase (remaining between 25–80%). After intestinal digestion, still some undigested proteins could be found, ranging from 0.5% to 4.2% of the initial protein content, although no differences could be observed between colostrum and mature milk. More than 40 serum proteins could be detected after intestinal digestion. Overall, caseins are digested more than most serum proteins during digestion. Especially immune-active serum proteins, which are digested to a lower extent, might protect infants from pathogens. The levels of protease inhibitors and total protein content, both higher in colostrum and by other papers hypothesized to be related to the degree of protein degradation, both did not appear to correlate with the degradation of human milk proteins during intestinal digestion.