In the earliest years of life, the foods a child eats do more than nourish — they help determine which microbial communities take root in the gut and how those communities behave. A new study suggests that different dietary fibers, far from being interchangeable, each carry a distinct biological signature: some fortify the gut's protective lining, others invite greater microbial diversity, and still others quiet the early stirrings of inflammation. The findings are preliminary, drawn from laboratory experiments and observational data, but they point toward a future in which the timing and type