This has been a topic of an extended discussion on the FHL in recent
weeks and here is a post from FHL co-moderator, Pam Sessoms which has
info from Dr. Tom Willard whose doctorate is in the topic of veterinary
nutrition, and info from others who know their stuff well.  In case
anyone has these questions this will interest you:
 
http://www.smartgroups.com/message/readmessage.cfm?gid=1423922&messageid=4071
 
I found it very informative so I figured that it might help others, too.
Enjoy!  I did.  She does good work (but those here who aren't new already
knew that).
[Posted in FML issue 4121]