Sous vide meats with fat vs no fat..does it make a difference?

[torolover]

I sometimes sous vide short ribs or duck legs with fat such as grapeseed oil or butter for long cooking times Sometime I don't use any fat when I sous vide meats.

Does it really make a difference in terms of keeping the meat more moist and juicy? Anyone with scientific explanations or just experience?

[nickrey]

See this post for Nathan Myhrvold's discussion of this.