[coldtea]

>Is this "gut problems are behind everything" theory is just simply quackery?

I'd say it's more a blind shot into a wide domain we know little about or its mechanisms, often told with high confidence ("just boost your gut microbiome with X food/probiotic"), as dietary advice usually is.

I think a lot of it is also mistaking causes and effects or correlation for causation.

If anything of the sort trully and reliably worked for fixing X (not just having this or that paper with some mediocre results under dubious conditions, or some "testimonial" like stories of "it worked for me", from people we know nothing about their specific issues and comorbidities and other factors), then it would be packaged and marketed in an instant, not as dietary supplements and bro advice, but as official treatments.