Gut microbes affect circadian rhythms

We know from studies on jet lag and night shifts that metabolism—how bodies use energy from food—is linked to the body’s circadian rhythms. These rhythms, regular daily fluctuations in mental and bodily functions, are communicated and carried out via signals sent from the brain and liver. Light and dark signals guide circadian rhythms, but it appears that microbes have a role to play as well.

All humans have a set of bacteria, viruses and fungi living in our guts, called the gut microbiome, which helps us digest food—and also interacts with the body in a number of other ways: there is evidence they affect allergies, mental health, weight and other metabolic conditions.

Researchers found that mice with a normal set of gut microbes showed evidence of a regular daily microbial cycle, with different species flourishing in different parts of the day and producing different compounds as a result. These compounds appear to act on the liver—they affected how circadian clock genes were expressed in the liver.

A high-fat diet reduced the variation in the microbial cycle; the circadian clock genes were disrupted, and the mice gained weight.
Meanwhile, “germ-free” mice raised without a normal gut microbiome showed evidence of a disrupted circadian clock cycle, but did not gain weight even on a high-fat diet.

The researchers hypothesize that high-fat diets change the compounds that microbes produce, thus disrupting the liver’s circadian clock signaling.