Other sugars are also present, including some 150 oligosaccharides (there may be even more, scientists are really just beginning to understand them), complex chains of sugars unique to human milk. (I repeat: unique to human milk.) These oligosaccharides can’t be digested by infants; they exist to feed the microbes that populate a baby’s digestive system.
And speaking of microbes, there’s a ton of them in breast milk. Human milk isn’t sterile—it’s very much alive, filled with good bacteria, much like yogurt and naturally fermented pickles and kefir, that keep our digestive systems functioning properly. So mother’s milk contains not only the bacteria necessary to help a baby break down food, but the food for the bacteria themselves to thrive. A breast-feeding mother isn’t keeping one organism alive—but actually hundreds of thousands of them.