Imagine a 20-year-old woman who refuses to eat anything except carrots and toast because she is afraid of gaining weight, even though she is 5-foot-8 and weighs only 99 pounds. She exercises to the point of exhaustion five mornings a week because, though she is bone-thin, she thinks her thighs are too flabby. Her periods are irregular, but she has never gone more than three months without menstruating. Another woman, who is also 20 and also 5-foot-8, has an opposite eating pattern. She goes without eating all day, and starting at 6 p.m. she eats nonstop, whatever she can get...