Your body tells you what it needs - when it is hungry and for what, when it needs to rest, when it needs to move. You just have to tune in.
At the same time, you need to tune out your mind. It will tell you it is hungry because you are stressed, it will tell you to eat that donut because it conjures up happy times, it will tell you to quit running or working out because it doesn't want you to expend energy needlessly.
Which one will you listen to?