Gingersoul is right when she said that even people come from perfectly healthy families develop eating disorders.  My thoughts on that, is it is more about negative body image, that what I suffer from. 

When I look in the mirror, it resembles that of a carnival fun house mirror.  I never know what I will see.  Sometimes I see my ribs jutting out, and my shoulder blades, which are so sharp, they look as though they may pierce through my skin at any moment.  Then other times, I see a woman who is, well, fat. 

There is SSSOOOOO much in the world today, that serve only to make perfectly healthy young girls feel fat.  Commercials, models, movies, Victoria Secret.  If you're not a size 0 or 2, then you're fat.   And the peer pressure, my GOD, it is unbelievable!!!! It isn't enough to be "athletic" and "a healthy weight" by standard, if you're ribs aren't showing, you're not thin enough. 

You're right to worry if this is your daughter.  Especially if her friends have eating disorders.  I guess my suggestion (and, truely, I am not the right person to give advice here) is to ENCOURAGE, ENCOURAGE, ENCOURAGE.  Work to build her self esteem.