Wintergirls.....

This question is for middle school students. I think it would be interesting to see how they perceive the issues in this book. Here is my question:
Many adolescents in the United States have some sort of eating disorder like the two main characters, Lia and Cassie, in the book Wintergirls. The two girls took an oath to be the skinniest girls in school. What do you believe Lia's motives were? Do you believe all the problems Lia was going through were because of her issues with weight and food? Or something deeper? Explain your answer and support it with evidence from the book.