Study after study fail to disprove the hypothesis that gut microbiome diversity is a strong correlate with longevity.
Here's a meta study for you:
Here we introduce the MicrobiomeHD database, which includes 28 published case–control gut microbiome studies spanning ten diseases. We perform a cross-disease meta-analysis of these studies using standardized methods. We find consistent patterns characterizing disease-associated microbiome changes.
What more do people want? Why is this something people fight?