By SARAH FITZMAURICE Healthy appetite: Kim enjoyed a lunch date with a friend and tucked into a burger complete with french fries and a muffin washed down with a colaAs a glamour model and page three pin up she is the envy of many girls who wish they could have her killer curves. But fans of Kim Kardashian may be surprised and pleased to learn just how she keeps her voluptuous curves. The 30-