BMI between 18.5 to 24.9 is required to have optimal cardiorespiratory fitness because weight directly affects the cardiorespiratory fitness.
If you starve yourself in a week, like by not eating anything, you will die in 3 days. Your body cannot survive without any food or water in a week! If you are thinking of doing it, don't!! If you want to loose weight try doing a healthy diet and exercise!
I hope this helped :)
