Staying Fat for Sarah Byrnes

Blurb

Staying Fat For Sarah Byrnes is a young-adult fiction novel by Chris Crutcher. It has been recognized by the American Library Association as one of the "Best of the Best Books for Young Adults". It is also one of fifty books on Young Adult Library Services Association's The Ultimate Teen Bookshelf.
From the back of the 2003 Harper Teen edition:
"Sarah Byrnes and Eric have been friends for years. When they were children, his fat and her terrible scars made them both outcasts. Later, although swimming slimmed Eric, she stayed his closest friend. Now Sarah Byrnes — the smartest, toughest person Eric has ever known — sits silent in a hospital. Eric must uncover the terrible secret she's hiding, before its dark currents pull them both under."