American Gut: An Open Platform for Citizen Science Microbiome Research [pdf]
msystems.asm.org
msystems.asm.org
> The self-reported dietary data suggested, unexpectedly, that the number of unique plant species that a subject consumes is associated with microbial diversity, rather than self-reported categories such as "vegan" or "omnivore"
Additionally, since bacteria strains tend to be specific to their environment, i.e. the plant that they're living on (which the plant itself thrives better in particular environments), it makes sense that a diverse diet would lead to a more diverse gut biom.
Its a naturally occurring feedback loop. And if you get sick of split peas every night, you can seek varied sources of polyphenolic rich foods in surprisingly varied places: pomegranate seeds, cocoa powder, etc. Stay healthy :)