As the holiday season wraps up, people may reflect on the past few weeks of celebrations. The festive atmosphere, complete with holiday feasts ranging from gingerbread cookies and mulled wine to sausage rolls and pies, can increase cravings. Someone intending to have just one treat may find themselves reaching out for another, and another, until they have overindulged. This brings up a common question: What regulates food habits, both cravings and quantities?

Before feeling too guilty about giving into cravings, it is worth noting that several factors dictate these dietary desires. Several research groups have shown that among other things, the gut microbiome influences host feeding behavior.¹ 

“It really makes sense” that gut microbes regulate diet, said Amandine Everard, a physiologist who studies how gut microbiomes influence food reward behavior at the Catholic University of Louvain. “The gut microbes are feeding from the diet as well. So, it’s [of] interest for microbiota to manage the food intake and the type of food.” 

Researchers have used a variety of animal models to investigate how gut bacteria influence host feeding habits. For instance, when researchers investigated the role of bacteria in worm foraging, they found that bacteria naturally associated with Caenorhabditis elegans produced a neurotransmitter.² This acted on olfactory neurons, influencing the worms’ feeding habits by regulating their sense of smell. Similarly, when researchers looked into the role of gut bacteria in the food habits of Drosophila melanogaster, they found that bacterially-produced lactate altered host feeding decisions.³ 

Reports from mammals suggest a similar effect of gut microbiota in modulating host sensory and feeding behavior. When researchers treated mice with antibiotics resulting in gut microbiota depletion, the animals’ eating habits changed and they overconsumed several palatable foods.⁴ 

“We’ve learned a lot from antibiotic research,” said Kevin Kohl, an ecologist specializing in gut microbiome-host interactions at the University of Pittsburgh. “But we also know that antibiotics cause a lot of other changes to the animal,” he noted.

To circumvent this problem, Kohl and his colleague transplanted gut microbiota from rodents fed with different diets into germ-free mice.⁵ They observed that mice colonized with different microbiota showed different feeding behavior, specifically in nutrient uptake. Metabolomic and metagenomic analyses revealed that metabolism of certain microbes altered the levels of essential amino acids. This likely helped the mice meet their nutrient requirements, resulting in modified eating habits. 

There is evidence that gut microbes impact food preference in mice, said Everard. “[But it is] very tricky to show this causality in humans.” Indeed, a few reports where researchers have studied gut microbiome and food preference in humans highlight a correlation, but not a causality, between the two. 

When scientists compared the urine content of people who eat chocolates and those who avoid them, they observed different microbial metabolites, indicating distinct microbiome in people with different eating habits.⁶ Profiling the microbiome of people with eating disorders including anorexia nervosa and binge-eating disorder revealed altered bacterial composition compared to healthy individuals.⁷ 

Although researchers do not know the exact biological mechanisms underlying the connection between gut microbiome and food habits, they have obtained some clues. For instance, a research group found that a bacterial metabolite acts on neural pathways in humans and diminishes anticipatory reward responses, reducing eating driven by the rewarding aspects of consuming tasty food.⁸  

“We [must] understand the dialogue and the communication between the gut microbiota and the brain,” said Everard. This can reveal insights into the biological mechanisms of feeding behavior and eating disorders, which could inform strategies to develop therapy for such conditions.⁹ 

However, food choices in people are complex and the gut microbiome is just one of the factors that influences these, noted Everard. Kohl agreed. “There’s a lot of social and economic factors that dictate a lot of food choice,” he said. “And then on top of that, there’s still free will. Like, I don’t think the bacteria are controlling us.”