Eggs or yogurt, vegetables or chips? We make decisions about what we eat every day, but those choices may not be entirely ours. New research from the University of Pittsburgh on mice shows for the first time that microbes in animals’ guts influence what they choose to eat, creating substances that cause cravings for different types of food.

“We all have these cravings — like if you ever just want to eat a salad or really need to eat meat,” said Kevin Kohl, assistant professor in the biology department at the Kenneth P. Dietrich School. . of Arts and Sciences. “Our work shows that animals with different compositions of gut microbes choose different types of diets.”

Despite decades of speculation by scientists about whether microbes could influence our preferred diets, the idea has never been directly tested in animals larger than a fruit fly. To explore the question, Kohl and his postdoc Brian Trevelline (A&S ’08), now at Cornell University, gave 30 mice lacking gut microbes a cocktail of microorganisms from three wild rodent species with natural diets. very different.

The duo found that the mice in each group chose foods rich in different nutrients, showing that their microbiome altered their preferred diet. The researchers published their work today in the Proceedings of the National Academy of Sciences.

While the idea of ​​the microbiome affecting your behavior may seem far-fetched, it’s no surprise to scientists. Your gut and your brain are in constant conversation, with certain types of molecules acting as intermediaries. These by-products of digestion signal that you have eaten enough food or perhaps that you need certain types of nutrients. But microbes in the gut can produce some of these same molecules, potentially hijacking that line of communication and altering the meaning of the message for their own benefit.

One of these messengers will be familiar to anyone who has had to take a nap after a turkey dinner: tryptophan.

“Tryptophan is an essential amino acid that is common in turkeys but is also produced by gut microbes. As it makes its way to the brain, it is transformed into serotonin, which is an important signal to feel full after a meal. meals,” Trevelline said. “Eventually it turns into melatonin, and then you feel sleepy.”

In their study, Trevelline and Kohl also showed that mice with different microbiomes had different levels of tryptophan in their blood, even before they were given the chance to choose different diets – and those with more of the molecule in their blood had also more bacteria. who can produce it in their gut.

It’s a compelling gun, but tryptophan is just one thread in a complex web of chemical communication, according to Trevelline. “There are probably dozens of signals that influence eating behavior on a daily basis. Tryptophan produced by microbes could be just one aspect of that,” he said. However, it does establish a plausible way that microscopic organisms could alter what we want to eat – it’s one of the few rigorous experiments to show such a gut-brain connection despite years of theory by scientists.

There’s still more science to do before you start worrying about your food cravings, though. Besides not having a way to test the idea in humans, the team didn’t weigh the importance of microbes in determining diet against anything else.

“It may be that what you ate the day before is more important than the germs you have,” Kohl said. “Humans have a lot more to do than we know of in our experience. But it’s an interesting idea to think about.”

And this is just one behavior that microbes could modify without our knowledge. It’s a young field, Kohl points out, and there’s still a lot to learn.

“I am constantly amazed at all of the roles we are discovering that microbes play in human and animal biology,” Kohl said.

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Material provided by University of Pittsburgh. Original written by Patrick Monahan. Note: Content may be edited for style and length.