Your digestive tract is a jungle, with more species than man will ever fully know about. When too many of certain types go extinct, the ecosystem breaks down, and bad shit starts to happen (come to think of it, literally).
It may take decades to figure out all of the specific species that matter most, and in what proportions. We aren't even certain which bacteria are common to all people, and which are specific to certain cultures / races / geographies / genders / who-knows-what-else. I bet people who eat a lot of KFC have a very different personal biome than I do.
In the meantime, here's 'one weird trick' that seems to just work.