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Beneficial gut microbes and the body work together to fine-tune fat metabolism and cholesterol levels, according to a new preclinical study by investigators from Weill Cornell Medicine and the Boyce Thompson Institute at Cornell University’s Ithaca campus.

The human body has co-evolved with the beneficial microbes that live in the gut (termed the microbiota), resulting in mutually favorable relationships that aid in the digestion of food and absorption of essential nutrients required for survival of the host and the gut microbes. A central aspect of these relationships is the production of bioactive molecules that promote the breakdown of food, enabling nutrient absorption by the host. One of the most important groups of such molecules are termed bile acids (also known as ‘bile’) which are produced from cholesterol in the liver and then delivered to the intestine where they promote fat digestion.

Scientists have known for some time that gut bacteria modify bile acids into a form that stimulates a receptor called FXR, which reduces bile production. The new study, published Jan. 8 in Nature, reveals that an enzyme produced by intestinal cells converts bile acids into a different form that has the opposite effect. This altered form, called bile acid-methylcysteamine (BA–MCY), inhibits FXR to promote bile production and help boost fat metabolism.