my guess would be dysbiosis, leaky gut syndrome

that means basically gut flora is out of balance, too many bad bacteria and they’re taking over, they cause inflammation in the intestines, that enlarges the pores, and then stuff leaks through into the bloodstream

doctors don’t ever acknowledge this because it’s a major cause of chronic diseases and it would hurt drug sales if they addressed and treated this

you can look up treatment for leaky gut, it’s mostly diet change (no inflammation causing foods like wheat and dairy) and tons of probiotics