It's been known for a while that some gut disorders, including Crohn's, can manifest as skin lesions of various kinds, and bowel bypass patients can also develop skin symptoms. It's also known for a while that autoimmune patients have less bacterial diversity in their gut, and an increased abundance of certain types of bacteria, especially in the small intestine. Research has found that in such patients, gut bacteria can migrate from the gut to the bloodstream and then to the skin. Psoriasis patients, for example, have a much higher gut permeability than non-psoriatics (not to be confused with the pseudoscientific Internet hysteria around "leaky gut"!) and have deficient bile acids.

Treatment would involve resetting the gut somewhat by eradicating the known pathogens. Astonishingly, a Hungarian study in 2003 [3], which used an experimental treatment with bile acid supplementation, was able to completely clear psoriasis in 78% of its subjects (even more in a second, more acute group), which is unheard of, and about 58% were still clear 2 years later. I don't know why that study hasn't revolutionized things; it's only been cited 11 times according to Scopus.com.

[1] https://www.ncbi.nlm.nih.gov/pubmed/29908580

[2] https://www.nature.com/articles/nrmicro2974

[3] https://www.sciencedirect.com/science/article/pii/S092846800...