Why diversity is treated as a marker of gut health
A more diverse gut microbiome, meaning a greater number and evenness of bacterial species, is broadly associated in research with better metabolic, immune, and even mental-health measures. The reasoning is that a varied community can perform more functions, resist colonization by pathogens, and recover better from disturbances. Diversity is a useful summary marker, but it is a correlate, not a guaranteed cause of health, and a high count alone does not define a 'good' microbiome.
How modern diets erode the community
Low-fiber, high-sugar, ultra-processed Western diets tend to reduce diversity over time because many beneficial microbes depend on fermentable fiber for fuel; antibiotics and very restrictive diets can also lower it. Increasing the range of plant foods eaten is one of the more consistent ways to support diversity, though responses are individual.
This is general educational information, not medical advice. Specific gut symptoms or microbiome testing results should be interpreted with a qualified professional.