Chapter 24, according to the lessons at school how did anorexia play a part in the inventing the Pretties?

From the text:

Back in the days before the operation, Tally remembered, a lot of people, especially young girls, became so ashamed at being fat that they stopped eating. They’d lose weight too quickly, and some would get stuck and would keep losing weight until they wound up like this “model.” Some even died, they said at school. That was one of the reasons they’d come up with the operation. No one got the disease anymore, since everyone knew at sixteen they’d turn beautiful. In fact, most people pigged out just before they turned, knowing it would all be sucked away.