The microbes in your gut may have a say in what you want for dinner, according to new research.

The findings only apply to fruit flies at the moment. But they add to the evidence that microbes influence the behavior of the creatures they inhabit, from flies to people.

Fruit flies are a good place to begin to study how microbes affect complex behaviors like food choices, according to neuroscientist Carlos Ribeiro at the Champalimaud Foundation, because while the human gut contains hundreds of different kinds of microbes, flies have just five.

Craving killer

In the new study in the journal PLOS Biology, Ribeiro and his colleagues raised flies in a sterile environment and fed them a carefully controlled diet. When the flies were deprived of protein, they sought out yeast.

“Yeast is the steak of the flies,” Ribeiro said.

But when these sterile, protein-starved flies were inoculated with two of the five species of normal gut bacteria, they no longer sought out yeast.

“We’re not talking about a slight reduction,” Ribeiro added. “It’s really that the flies do not show an increase in protein appetite when they have these two bacteria.”

In effect, the microbes were telling the flies what to eat.

Also, while protein-hungry flies normally produce a lot fewer eggs, flies carrying these two bacteria did not see as big a drop in fertility.

Ribeiro doesn’t know why the bacteria would have these effects. But he noted that flies don’t live as long when they eat more protein-rich food.

“It might be advantageous not to overeat,” he said. “Maybe what the bacteria do to the fly is, it allows it to maximize reproduction while minimizing the shortening of its lifespan.”

Microbes were there first

“This is a pretty cool paper, I have to say,” said University College Cork neuroscientist John Cryan, who was not involved with the research. Cryan has studied how gut microbes affect anxiety and behavior in mice.

“What I’ve come to realize over the years is that there are very few elements of neurodevelopment and the brain that are not in some ways regulated by microbes,” he said. “We have to remember that the microbes were there first, before all species. We’ve all developed, including flies, in a microbial milieu, sending signals from the gut to the brain.”

While the fly study makes it tempting to blame our meat cravings on our microbiomes, “We have no idea whether any of this could be upscaled to a mammal,” Cryan said.

Ribeiro also has no idea how the bacteria are exerting their influence. His group plans to study metabolites the bacteria produce, and how the flies’ brain activity changes in the presence of the bacteria.

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