In exchange, the microbes provide us with essential vitamins, metabolize digestive compounds, called bile acids, that are produced by the liver, and detoxify foreign chemicals that our bodies have never experienced—so-called xenobiotics. Most important, they digest dietary fiber and complex sugar molecules that our digestive system can’t break down or absorb on its own, and thus provide us with a substantial number of additional calories that we would otherwise lose in our stool.
