Milk Designed for Bacteria

Breast milk contains over 200 oligosaccharides — complex sugars that human infants cannot digest directly. One major job of these sugars is to feed Bifidobacterium, a keystone gut bacterium that helps crowd out harmful species and shape the infant microbiome. Formula-fed babies develop measurably different gut flora that can persist for months.

By age three, your microbiome stabilizes at roughly 1,000 species. For years these microbes were said to weigh 1 to 2 kilograms, but that figure was never well supported — current estimates put the total far lower, likely under half a kilogram, with the gut bacteria that dominate the count adding up to only a couple hundred grams. Every pet you touched, every fistful of dirt, every shared toy contributed new residents to this internal ecosystem.