Back in 2006 I was diagnosed with ankylosing spondylitis which led me to trying a zero starch "low klebsiella bacteria" diet developed by a UK Dr. (
http://www.kickas.org)
I had moderate success with it, and it got me buying into the whole leaky gut/ molecular mimicry theory of autoimmune disease. So I went all in on healing my gut, and have managed to put the AS into remission for the past 10 years.
Since then in all my reading, I've see this a common thread for most studies on glucosamine, chondroitin, gelatin/collagen, various peptides etc. supplementation is their benefit comes from healing the gut, thereby reducing endotoxins, the resulting inflammatory diseases and systemic stress. Maybe just my own confirmation bias, but I see this as the important vector vs a direct action on healing the joints.