Human milk contains a wide variety of bioactive molecules, including Ig and nucleotides. Recently, human milk oligosaccharides (HMO)3
are being recognized as a new class of potent bioactive molecules. HMO consist of a lactose core extensively elongated by β1–3 or β1–6 linkages to lactosamine units and further decorated with fucose or sialic acid residues in terminal positions connected with α1–2, –3, and –4 and α2–3 and –6 linkages, respectively (1
). The diversity of monosaccharide combinations and linkages results in a structurally complex array of linear and branched oligosaccharide structures. At present, the only source of oligosaccharide structures with the structural complexity of HMO is human milk, a fact that limits the applicability of these protective oligosaccharides in population groups other than breast-fed infants. Presumably, many of the health benefits that milk oligosaccharides provide for infants could also be available to humans of all ages if the same structures and functions could be provided in the diet.
The advantages of HMO are likely related to the structural and functional diversity of multiple components that act in synergy and confer protection to infants. The neutral HMO (containing the monomers N
-acetylglucosamine and fucose) are considered to be the most relevant factors for the development of the intestinal microbiota typical for breast-fed infants (2
) as well as for direct effects on the immune system (5
). In fact, the structural specificity for the consumption preferences of different intestinal bacteria has been described (6
). Recently, Bifidobacterium longum
), a bifidobacterium enriched in the gastrointestinal tracts of healthy breast-fed infants, was found to have a unique gene cassette that allows it to transport and metabolize the specific oligosaccharide structures found in human milk (9
), arguing for a specific coevolutionary relationship between this unique bacterium and the infant (10
). The acidic oligosaccharides (decorated with the monomer sialic acid), on the other hand, play an important role in the prevention of adhesion of pathogenic bacteria to the epithelial surface (11
) and have recently also been found to be metabolized by B. infantis
Various strategies have been used to mimic the structural complexity of HMO; much simpler structures, including fructo-oligosaccharides (FOS) and galacto-oligosaccharides (GOS), so far have been used in dietary products. These simple structures possess a stimulating prebiotic effect that increases the bacterial counts of bifidobacteria and lactobacilli; however, this effect has been inconsistent, with 16 studies showing a bifidogenic effect and 4 studies showing no effect in infants [reviewed in (12
)]. FOS and GOS also mimic some but not all of the other functions of HMO, including SCFA production, pathogen blocking, and immune modulation [reviewed in (12
)]. Additionally, these simpler oligosaccharides do not contain the structural complexity and diversity of HMO; thus, better sources of complex oligosaccharides that more closely mimic the structures and functions of HMO are needed to improve upon the existing supplementation strategy.
Recent research has demonstrated that bovine milk contains oligosaccharides that are analogous to HMO, suggesting a similar protective role (13
). The oligosaccharides found in bovine milk (BMO) are structurally similar to those in human milk, but their concentration in milk is low, particularly in mature milk compared with early milk (i.e. colostrum) (15
). Both human and bovine milk contain large amounts of the acidic oligosaccharides known as sialyloligosaccharides, especially at the early lactation stage (15
). Because mature bovine milk contains only trace amounts of these valuable components, up to now it has not been considered a viable source of oligosaccharides for human supplementation. In addition, the evaluation of BMO as a substitute for HMO has been hindered by the lack of precise analytical methods to accurately characterize and quantify these oligosaccharides. Recently, a high-throughput strategy to annotate the human and bovine milk glycomes by using high accuracy MS has been developed (17
). Using these novel techniques, dairy streams including whey permeate from cheese production have been identified as novel sources of oligosaccharides that mimic HMO (18
This review will describe the current status of knowledge of the effects of HMO on human health, give a brief overview of the structures and available sources of oligosaccharides, and then focus on bovine milk and dairy streams as a source for functional oligosaccharides that mimic the beneficial effects of HMO.
Analysis of oligosaccharides
As a result of the heterogeneity of oligosaccharides, technologies for their characterization have lagged far behind technologies for nucleic acids and proteins. In recent years, MS has become an essential tool for the analysis of oligosaccharides because of the breadth of information obtained with high resolution and sensitive instrumentation. The use of Fourier transform ion cyclotron resonance (FT-ICR) in the detection of oligosaccharides recently has been reviewed (19
). Several features of FT-ICR make it ideal for oligosaccharide analysis: the high resolution and mass accuracy readily yields the composition in terms of numbers of fucose, glucose or galactose, N
-acetylglucosamine, and sialic acid without the need for exhaustive and expensive derivatization methods. The 2 most useful ionization techniques for analyzing oligosaccharides, electrospray ionization and matrix-assisted laser desorption/ionization, can now be performed in FT-ICR, allowing for maximum detection of both neutral and acidic oligosaccharides. The availability of reliable tandem MS techniques such as collision-induced dissociation and infrared multiphoton dissociation combined with any ionization method in FT-ICR make it a vital toolset in acquiring detailed information about glycan structures (19
The systematic examination of oligosaccharides in bovine milk and dairy streams can also now be accomplished using newly introduced methodologies such as microchip liquid chromatography separation and high performance MS techniques, including time-of-flight and quadrupole time-of-flight analyzers (3
). Currently only a few commercial standards are available for bovine oligosaccharides; therefore, to date, the highest number of oligosaccharides identified in both human milk (over 70 fully annotated HMO) (3
) and bovine milk (40 BMO) (14
) has been obtained using a combination of FT-ICR MS, enzymatic digestion, and HPLC-chip/time-of-flight MS techniques.