The three main components of breast milk are fat, lactose and human milk oligosaccharides (HMOs). While the first two provide calories and nourish the baby’s developing brain and body, the HMOs are not digestible by humans but perform the task of feeding the baby’s developing community of gut microbes. In fact, human milk is the result of thousands of years of human evolution and this timespan has allowed its optimization for both the health of the child and their developing microbial community.
The mother’s womb is a sterile environment but, as soon as the baby leaves this bacterium free incubator, microbes start to colonize this free environment (the baby’s gut) and through complex interactions between different microbes the infant microbiota (made up of 25 million genes) is created. The infant gut microbes have the capacity to digest HMOs and extract energy from them. The main strain of microbes so fond of these HMOs are the beneficial Bifidobacterium (mostly found in the gut of healthy babies) and the Bacteroides which have an amazing capacity to thrive off plant material preparing the baby for life on solid foods and orchestrating the major transition in microbiota that occurs with its introduction.
Even though, 50 years of research have elapsed, formula companies have not yet been able to reproduce the complex chemical structure found in HMOs with the result that babies fed formula have a different microbiota compared to those who are breast fed.
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Marcobal A, et al. “Bacteroides in the infant gut consume milk oligosaccharides via mucus-utilization pathways.” Cell Host Microbe. 10.5 (2011): 507-14.
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