Humans’ gastrointestinal tracts are home to 10,000 species of bacteria, which get energy from our half-digested lunches. (Another estimate puts the number of species as high as 36,000.) In exchange, they help us break down food and keep harmful bacteria out, and have also been shown to help regulate fat storage and provide vitamins.
But a recent review published in BioEssays suggests that these bacteria might be a little too big for their britches, bossing their hosts around and demanding certain kinds of foods. “Microbial genes outnumber human genes by 100 to 1 in the intestinal microbiome,” the article says, so the microbes are winning the numbers game at least. But it’s not like they’re all on the same team. The authors (who hail from the University of New Mexico and the University of California, San Francisco) note that many different species compete for space and nutrients in our intestines, and the more dominant ones may have more influence on their humans.
They may do this by inducing cravings: “Individuals who are “chocolate desiring” have different microbial metabolites in their urine than “chocolate indifferent” individuals, despite eating identical diets,” the study says. Or, they may influence people’s moods—crying in infants with colic has been linked to changes in the gut microbiome. And one thing parents do to stop their babies’ crying is feed them.
The article suggests some potential mechanisms by which the bacterias exert their influence: They may change the expression of taste receptors, making certain foods taste better; they may release hunger-inducing hormones; or they may manipulate the vagus nerve (which connects the stomach to the brain) to control their hosts’ eating behavior.
And different bacteria want people to eat different things—some crave sugar, some crave fat. Some microbes found in people in Japan are especially good at digesting seaweed.
Humans, of course, are not entirely powerless against the prodding influence of our gut flora. The relationship works both ways—the food someone chooses to eat influences their microbiome. And probiotics can change gut populations too. Certain probiotics have been shown to reduce fat mass or improve mood.
But microbes’ potential influence on cravings does offer a convenient excuse—the next time you’re trying to convince your friends to order a pizza, try shouting “My gut bacteria demand tribute!” and see where that gets you.