If you walk down the aisles of any supermarket, you’ll see what dietary emulsifiers can accomplish. This common class of food additives binds water and oils together, preventing mixtures of the two from splitting. They stabilise ice-cream and other frozen desserts, mayonnaise, salad dressings, and virtually every kind of processed food. “Anything that sits in a package on a supermarket shelf, and can stay there for a while, probably has emulsifiers in it,” says Andrew Gewirtz from Georgia State University.

These additives may confer stability to food, but they can also bring discord to the gut—at least in mice. Gewirtz has found that two common emulsifiers—caboxymethylcellulose (CMC) and polysorbate-80 (P80)—can change the roll call of bacteria in a mouse’s