We’ve known for decades that that the lush collection of bacteria that populate our guts plays a part in digestion. The famous after-effects of bean-eating—tactfully known in the 16th century as “windinesse”— are due to our resident microbes, chomping up an assortment of bean oligosaccharides (short chains of linked sugars) that our own enzymes can’t deal with, and generating in the process an unfortunate excess of bloating gas.

While gut bacteria play an essential (if not always socially tactful) role in human nutrition, a wealth of recent research now shows that they do far more. In fact, the key to whether we’re fat or thin, cheerful or depressed, healthy or chronically ill, may lie in the gut microbiome. Biologically, we’re