A recent study led by the University of Pittsburgh has found that the microbes in animals’ guts influence what they choose to eat, by creating substances that prompt cravings for different kinds of foods. Although the hypothesis that the gut microbiome may influence animal dietary preferences has been speculated for years, this is the first study to explicitly test it in a laboratory setting in animals larger than a fruit fly.
The scientists gave 30 mice that lacked gut microbes a cocktail of microorganisms collected from three species of wild rodents with highly different natural diets. They found that mice in each group chose food containing different nutrients, thus proving that the composition of their gut microbiome influenced their preferred diet.
“We all have those urges – like if you ever just felt like you need to eat a salad or you really need to eat meat,” said study co-author Kevin Kohl, an assistant professor of Biology at Pittsburgh. “Our work shows that animals with different compositions of gut microbes choose different kinds of diets.”
Since in mammals the gut and the brain are in constant conversation, certain kinds of molecules act as intermediaries. These by-products of digestion signal to the brain that the animal has eaten enough, or that they might need different types of nutrients. However, this new study shows that microbes in the gut are often producing the same molecules, potentially hijacking the line of communication between gut and brain, and changing the meaning of the circulating chemical messages in order to benefit themselves.
One messenger molecule discovered to play a fundamental role in this process is tryptophan. “Tryptophan is an essential amino acid that’s common in turkey but is also produced by gut microbes. When it makes its way to the brain, it’s transformed into serotonin, which is a signal that’s important for feeling satiated after a meal,” explained study lead author Brian Trevelline, a postdoctoral fellow at Cornell University. “Eventually that gets converted into melatonin, and then you feel sleepy.”
The experiments performed on mice revealed that those with different microbiomes had different levels of tryptophan in their blood, even before given the option to choose a diet. Moreover, those with higher levels of tryptophan in their blood also had more bacteria that could produce it in their gut.
While tryptophan may play a fundamental role in gut-brain communication, it is just one thread in a complicated web of chemicals. “There are likely dozens of signals that are influencing feeding behavior on a day-to-day basis. Tryptophan produced by microbes could just be one aspect of that,” said Dr. Trevelline.
Further research is needed to clarify and analyze the complex role that microbes play in human and animal biology, and to assess their importance in determining dietary preferences.
The study is published in the journal Proceedings of the National Academy of Sciences.