The short-chain fatty acid propionate is used as a food preservative in cheeses and baked goods because it inhibits mold growth. It is also produced by microbes in the human gut. Now researchers report that it also has metabolic effects on animals and people. Amir Tirosh of Sheba Medical Center in Israel, Gökhan S. Hotamisligil of Harvard T.H. Chan School of Public Health, and their coworkers found that propionate induces increased glucose production and levels of the hormones norepinephrine, glucagon, and fatty acid–binding protein 4 (FABP4) in mice (Sci. Transl. Med. 2019, DOI: 10.1126/scitranslmed.aav0120). Mice lacking FABP4 or the liver glucagon receptor weren’t affected by the preservative. Some of the effects seen in animals were not observed in cell culture, suggesting they may be mediated by the sympathetic nervous system. Secretion of glucagon from pancreatic α cells and FABP4 from adipose cells didn’t change in response to propionate. Chronic exposure to the preservative led to weight gain in mice. In a small human study, 14 healthy participants ate meals supplemented with either propionate or a placebo. Afterward, people who ate propionate had elevated levels of norepinephrine and glucagon and decreased insulin sensitivity.