Microbes in the gut could affect our metabolism and circadian rhythms - regular daily fluctuations in mental and bodily functions - and may cause some people to gain weight, a new study has found.
The study including researchers from the US Department of Energy's Argonne National Laboratory and the University of Chicago found evidence that gut microbes affect circadian rhythms and metabolism in mice.
Previous studies on jet lag and night shifts have showed 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 their 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 hypothesise that high-fat diets change the compounds that microbes produce, thus disrupting the liver's circadian clock signalling.
"The earlier explanation for microbiome-related weight gain was that some bacteria make calories from food more available to your body, but this is a fundamental alternative explanation," said co-author Jack Gilbert, a microbial ecologist from US Department of Energy's Argonne National Laboratory.
The study was published in the journal Cell Host and Microbe.
