When plant nutrients run low, some gut bacteria may turn on their host and begin consuming the intestines’ protective lining. The health effects of a plant-based diet may depend not only on what gut bacteria eat but also on whether they begin feeding on the gut itself. They also challenge the widespread assumption that many biologically important compounds found in the body come exclusively from gut bacteria. When gut bacteria process the amino acid phenylalanine, they can produce phenylpropionate and hippuric acid. Together, the studies provide a clearer map of how food, gut bacteria, and mammalian cells contribute to the body’s chemical environment.