What is the theme in Heartburn by Nora Ephron?

One major theme in the novel is infidelity, specifically, the way infidelity affects the foundation of one's beliefs. Mark's infidelity causes Rachel to question her own identity and value as a person.... it makes her feel as if she is unworthy and needs to change. Over the course of the novel, Rachel comes to understand that she was NOT the one lacking in substance.... Mark was lacking.