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.