Is the gut or the brain running the show?

We’ve known for a while that the combination of nutrients and the microbiome impact health — but now new research has taken this concept further, suggesting that microbes might also be able to control behaviour.

In a study led by Portugal’s Champalimaud Centre for the Unknown, conducted in collaboration with Monash University, scientists conducted experiments on the fruit fly Drosophila melanogaster in order to dissect how the complex interaction of diet and microbes affects food preference. The results were published in the journal PLOS Biology.

The scientists initially showed that flies deprived of amino acids showed decreased fertility and increased preference for protein-rich food. Indeed, the team found that the removal of any single essential amino acid was sufficient to increase the flies’ appetite for protein-rich food.

The scientists next tested the impact on food choices of five different species of bacteria that are naturally present in the guts of fruit flies in the wild. The results exceeded the scientists’ expectations: two specific bacterial species could abolish the increased appetite for protein in flies that were fed food lacking essential amino acids.

“With the right microbiome, fruit flies are able to face these unfavourable nutritional situations,” said Zita Carvalho-Santos from the Champalimaud Centre for the Unknown.

So exactly how could these bacteria act on the brain to alter appetite?

“Our first hypothesis was that these bacteria might be providing the flies with the missing essential amino acids,” noted Santos; however, the experiments did not support this hypothesis. Instead, Santos said, the gut bacteria “seem to induce some metabolic change that acts directly on the brain and the body, which mimics a state of protein satiety”.

So not only do gut bacteria appear to have an influence on the brain, but this influence occurs through a new and previously unknown mechanism. And with human beings playing host to hundreds of bacterial species, as opposed to the fruit fly’s five, it can be assumed that such influence is taking place on an even greater scale.