The idea comes from a 2018 study involving more than 10,000 people in the US, UK and Australia. Participants submitted stool samples and reported what they typically ate. Researchers analysed the microbes in those samples and found that people who consumed more than 30 different plant foods a week tended to have a more diverse gut microbiome than those who ate fewer than 10.

But that doesn’t mean 30 is a magic number. Whether you eat 25 plants a week or 30 is probably less important than some would have you believe.

“Thirty is pretty arbitrary,” says Prof Daniel M Davis, head of life sciences at Imperial College London and the author of Immune Health: A Myth-Busting Guide. “It’s not as though researchers compared 10 plants, 20 plants, 30 plants and 35 plants and found a clear cut-off point.”

Nor does the study prove that eating 30 plants a week directly improves health. While a more diverse microbiome has been linked to a lower risk of some diseases, scientists still don’t know exactly what a “healthy” microbiome looks like, he says.

“People eating more than 30 plants a week are probably doing a gazillion other things differently as well,” says Davis. Exercise, sleep and stress also affect our microbiome, and with our complex lives it’s hard to work out the importance of each of these alone.

Davis says we should be particularly wary of people using the “30 plants” message to make money. The idea has fuelled a market for expensive supplements that promise to help people hit the target, but there’s little independent evidence they work.

“It hasn’t been proven that reaching 30 plants a week will prevent illness or lead to specific health benefits,” Davis says. “What we do know is that eating a variety of fruits, vegetables, legumes, nuts and wholegrains is generally good for you.”