I would guess that these are associative conditions.
Several years ago, I joined the Arthritic Association
http://www.arthriticassociation.org.uk/index.php?section=about_us
I had mild arthritis, and followed their diet plan, and it eventually disappeared.
Their diet is no so similar to that which is recommended for diabetes, ie a basically healthy diet with a few no nos to start with. Happily I was able to reintroduce most of these in due course.
It may be that both conditions are triggered, and relieved, by similar lifestyle habits.