Human milk oligosaccharides and the infant gut microbiome from an eco-evolutionary perspective

Human milk oligosaccharides (HMOs) are a family of glycans found in breastmilk with over 200 identified structures. Despite being the third-largest component in breastmilk, HMOs are indigestible by infants, which raises an intriguing question: we would expect evolutionary dynamics to have shaped breastmilk to efficiently fulfill the baby's nutritional needs; what, then, could be the role of HMOs? Tracking their fate offers an answer: they are metabolized by certain gut bacteria, suggesting that breastmilk has been structured to shape the developing infant microbiome. We suggest that ecological paradigms, in particular, the notion of priority effects, can help contextualize the importance of HMOs as agents shaping the gut microbiome. The fitness consequences of this process provide insight regarding the evolutionary forces that have shaped the composition of breastmilk. In this review, we offer an eco-evolutionary perspective and present empirical data associating the compositions of mothers' milk and their infants' gut microbiomes.