Why doesn't she just switch to heroin?

From The Metro:

A 15-year-old girl who weighs 33 stone [said] "It's like a drug. Some people choose heroin but I've chosen food and it's killing me."

Anyway, if a 33-stone person lives for 20 years, isn't that the same total amount of life lived as a 10-stone person who lives for 66 years?