This is because microbes are looking out for their own interests, says Athena Aktipis, assistant professor at Arizona State University’s department of psychology. And they’re good at doing this.

“The gut microbes that are best at surviving inside us end up being more frequent in the next generation. They have the evolutionary advantage of being better at affecting us in ways that get us to preferentially feed them,” she says. (Find out more about how microbes affect our bodies in our recent BBC Future series Microbes and Me).

Different microbes in our guts prefer different environments, such as more or less acidic, and what we eat affects the ecosystem in our guts and what’s available for the bacteria to survive on. They can manipulate us into eating what they need in a few different ways.

They can send signals from the gut to the brain via our vagus nerve and make us feel under the weather if we’re not eating enough of a certain nutrient, or make us feel good when we eat what they want, by releasing neurotransmitters such as dopamine and serotonin. They can also alter our taste receptors so we consume more of something to get the same taste of sweetness, for example.

No one has observed this happening yet, Aktipis says, but it’s based on scientists’ understanding of how microbes behave.

But, she adds, these microbes aren’t always necessarily signalling for us to eat things that are good for us. After all, some bacteria cause disease and death.

“There’s a notion that the microbiome is part of us, but if you have an infectious disease making you feel sick, you would say that microbe is invading your body, not that it’s part of your body,” she says. “You could be getting hijacked by an impaired microbiome.”