Food is essential in our lives. It is unthinkable to have a family celebration without a delicious feast in front of us; most of the time we meet friends around a table, and we try to taste local dishes and culinary specialties when traveling abroad.
Emotional aspects, as well as culture, memories, and also nutritional needs mediate our food choices and those food choices impact not only our general health, but also our ability to reproduce, our lifespan, and even our mental state and mood. But, how our brain controls and manages those cravings and preferences is still a mystery for neuroscience.
Now a group of researchers from the Champalimaud Center for the Unknown, based in Lisbon (Portugal) and from the Monash University, in Australia, led by cell biologist Carlos Ribeiro, have shed some light on this issue.
“Our study is the first to show how two specific bacterial species in fruit flies affect the decision of whether to eat protein-based food or not, ” says Ribeiro
In an experiment with fruit flies they have discovered that cravings may start very far away from the brain, in the gut. And in particular, they have shown that gut bacteria ‘talk’ to the brain and control the food flies eat. They have published their results in PLOS Biology.
Ribeiro and colleagues were studying the brain mechanisms behind food choices with Drosophila melanogaster when a coincidence made them take a look at the insects’ gut microbiota. They were offering a diet lacking essential amino acids to the flies and they saw the animals developed cravings for protein-rich food. Amino acids and proteins are key to maintain healthy stem cells and in cases where there is a lack of these, serious health problems can occur.
Nevertheless, some flies did not show this protein-craving behaviour, although they lacked the same amino acids in their diet. “We did not understand what was going on, why some flies behaved like that but not all of them”, explains Carlos Ribeiro, main author of the research, to GMFH editors.
They took a closer look at the flies and surprisingly they discovered that, even if the food and the tubes they used were germ-free some bacteria had indeed colonized the guts of some of those insects.
They decided to do an experiment to test the impact on food choices of the five main bacterial species that normally inhabit the guts of wild fruit flies. The microbiome of Drosophila is quite dynamic and is determined by food substrate and also by the provenance of the flies. However, “it is quite clear that Acetobacteraceae and Lactobacillaceae are almost always found to be present”, says Ribeiro.
They found that those two bacterial species could be responsible for the increased appetite for protein in flies that were following a diet lacking essential amino acids. “Our study is the first one in showing how two specific bacterial species in the fruit fly impact the decision of eating or not eating protein food”, states Ribeiro.
In the study they also identified other species responsible for sugar cravings.
According to the authors, their findings show that with the right microbiota, fruit flies can cope with unfavourable nutritional situations. Although they have not been able to discover the underlying mechanism by which bacteria can impact the brain and behaviour, they believe that changes in diet can cause bacteria to provoke metabolic alterations that influence the brain. Now, researchers explain, they are embarking on metabolomics studies to check this hypothesis.
But, what about humans? We are far more complex than flies. For instance, we have 100 trillion microbes in our body of hundreds of different species, whereas flies only have five main bacterial species. “For the moment, we cannot repeat these experiments in humans or mammals, and cannot identify which bacterial species may be affecting our food preferences”, confesses Ribeiro. But, he says, one of the species identified in flies, Lactobacillus, is also present in human intestines.
Leitão-Gonçalves R, Carvalho-Santos Z, Francisco AP et al. Commensal bacteria and essential amino acids control food choice behavior and reproduction. PLoS Biology 2017 doi.org/10.1371/journal.pbio.2000862