Prevent cardiac arrest when dieting?

im a 16 year old guy and i started dieting a few days ago. I heard of some teens going into cardiac arrest and actually dying from there sudden change of diet. Every day i will eat sultana bran, 3 pieces of fruit, roast beef and salad sandwhich on whole meal bread, and pumpkin soup. So am i at risk of going into cardiac arrest and if so how do i prevent it?