Breast milk contains special indigestible sugars solely for feeding co-evolved infant gut microbiota.
“This is an awesome area of research, and, as you might've expected, not something that has yet been replicated in infant formula. Learn more about HMOs (at 37:14) and much, much more about the human gut microbiota with this video interview featuring Justin and Erica Sonnenburg, authors of The Good Gut.”