The Holy Trinity by checkthemargins
I haven't read it yet, so i cant say whether its any good or not.

Summary: In response to de_nugis's prompt for the "ohsam" fic challenge over on LJ: s7, Sam/Dean or gen; Sam is compensating for having Lucifer in his head by going into hyperfunctional mode. I want to see touches of Mystery Spot OCD Sam: hospital corners, perfect efficiency, everything under control, everything in place. It works. He still hallucinates, but he can keep going through it, he can wall himself off from being affected by fear of his breaks with reality and by the trauma of the things he remembers and hallucinates. He just has to keep going further and further into his coping mechanism, because if it breaks it will all come crashing down and that will not be good.

Wow really late, but I found this post and had just the fic in mind!

Brittle (http://sammehsayum.livejournal.com/5878.html) by sammehsayum (aka thecapn)

Summary: Sam Winchester has an eating disorder.
Warning: Main Character suffering from an eating disorder and graphic descriptions thereof, mentions of past suicide and self harm in secondary character.

I couldn't agree more. This fic is perfect. *-*

YAY! Personally, thecapn is my favorite author, so I never turn down an opportunity to rec her fics.