Taking Up Space, by Alyson Gerber

Sarah loves playing basketball, but lately she's noticed that she has trouble keeping up.  Her uniform is getting tighter too.  Maybe she's just getting fat?  Certainly, that's what her Mom would think.  Mom would never say such a thing, of course, but Sarah knows how concerned her mother is about food.  So Sarah starts to develop her own rules about eating: deciding what she can eat and how much.  But when her friends start to notice her behavior, she is forced to come clean or give up basketball.

This being seventh grade, there's also plenty of drama floating around including a cute boy who teaches Sarah how to cook (and also enjoy eating).  Together, they decide to try competing in a cooking contest.  When Sarah develops a crush on the boy this triggers a problem because one of her teammates already has a crush on him.  So when Sarah doesn't promptly come clean to her friend, it drives a rift between Sarah and the rest of the basketball team.

An important topic, but this take on puberty and eating disorders is a clunky recitation.  Gerber has a good sense of the dynamics of middle school, but she doesn't handle dialogue well.  The kids talk aloud like they are IM'ing each other which sounds awkward, but the adults are the worst talking largely in mini lectures (except for Sarah's parents whose sole purpose seems to be to apologize and agree with everything she says).  The whole thing is stiff and artificial -- more of a PSA than a story.