Early observational studies suggested a correlation between eating more than 30 plant types per week and higher gut microbial diversity, though this may be influenced by healthy user bias.

“the idea that dietary diversity um is correlated with microbial diversity really came out of the american gut project so they basically looked at the eating habits and gut profiles of all the people who participated and so it was kind of like a massive cross-sectional study and they basically found that people who were eating more than 30 types of plants per week had more diverse guts than people who were eating less than 10 types of plants per week”