Every mammal mother produces complex sugars called oligosaccharides, but human mothers, for some reason, churn out an exceptional variety: so far, scientists have identified more than two hundred human milk oligosaccharides, or H.M.O.s. They are the third-most plentiful ingredient in human milk, after lactose and fats, and their structure ought to make them a rich source of energy for growing babies—but babies cannot digest them. Why would a mother expend so much energy manufacturing these complicated chemicals if they were apparently useless to her child? Why hasn’t natural selection put its foot down on such a wasteful practice? Here’s a clue: H.M.O.s pass through the stomach and the small intestine unharmed, landing in the large intestine, where most of our bacteria live. What if they aren’t food for babies at all? What if they are food for microbes?
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