Apatient was instructed not to eat for twelve hours before coming to the hospital for a procedure. what is the most likely reason for these instructions?

Ihave a really high metabolism and only weigh 70 pounds (13 almost 14). people always ask me two questions: do you ever eat and how do you eat so much and stay so skinny.i am worried that a high metabolism wont me forever. what are some safe steps i can take to maintain being fit. (also, i already work out i am a dacer so i get cardio a lot)