While our previous work explored the effect of maternal diet on the macronutrient and micronutrient composition of human milk 23, very few studies have explored the impact of maternal diet on its bioactive components. Although challenging to distinguish between the effect of the maternal diet during gestation versus lactation, these and other studies 16 suggest that the maternal lactation diet alters components of milk that play a role in structuring the offspring gut microbiome, thereby potentially influencing lifelong metabolic health. Data from our laboratory and others suggest that a high fat maternal diet during gestation and lactation causes near and long-term dysbiosis in the offspring gut microbiome, even after weaning onto a control diet 9, 10, 11, 12, 13, 14, 15. Multiple studies have demonstrated that the infant stool microbiome of breastfed newborns, significantly varies from that formula fed, driven by the diverse microbial nutrients in human milk 8, 9, 10. Human milk contains a low abundance, low biomass, and yet surprisingly diverse community of bacteria that has been suggested to augment early and initial seeding of the infantвЂ™s gut via breastfeeding (e.g., Staphylococcus, Streptococcus, Lactobacillus) 22. Like breast feeding, alterations to the early infant microbiome have demonstrated benefits, like decreased necrotizing enterocolitis, and later in life chronic diseases, and immune function 19, 20, 21. One such candidate mechanistic link is the milk microbiome, given its presumptive role in contributing (at least in part) to the colonization and functional shaping of the developing infant microbiome 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18. However, the mechanisms by which such common maternal dietary modifications might alter the metabolism of her offspring via the breastmilk have yet to be elucidated. This suggests that (at least in mice) the maternal lactational diet in and of itself is capable of modulating the beneficial composition of milk. However, cross-fostering studies in mice have shown that exposure to a high fat maternal diet during lactation is sufficient to mitigate these beneficial effects and alternately render metabolic and gastrointestinal dysfunction in the nursing offspring 5, 6, 7. Human milk is recommended as the optimal nutrition source for neonates and infants as it confers protection against both immediately life-threatening conditions such as necrotizing enterocolitis, as well as later in life chronic diseases such as obesity, diabetes and inflammatory bowel disease 1, 2, 3, 4. These studies reveal a successive mechanism by which the maternal diet during lactation alters milk HMO composition, which in turn shapes the functional milk microbiome prior to infant ingestion. We find significant associations between the concentration of HMO-bound fucose and the abundance of fucosidase (a bacterial gene that digests fucose moieties) harbored by milk bacteria. Employing a human cross-over study design, we demonstrate that distinct maternal dietary carbohydrate and energy sources preferentially alter milk concentrations of HMO, including fucosylated species. Hypothesizing that the maternal diet itself might influence HMO composition, we sought to directly determine the effect maternal diet on HMO and the milk bacteria. Human milk oligosaccharides (HMO) are favorable macromolecules which are, interestingly, indigestible by the infant but serve as substrates for bacteria. DS (font: Times New Roman ,sans-serif 12.Human milk is the optimal nutrition source for infants, and oligosaccharides represent the third most abundant component in milk after lactose and fat. DS (font: Times New Roman ,sans-serif 12.00pt color:#FF0000)

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