Sacrifice is really this: giving up something to achieve something much greater/better.

If you want to have a better healthier body, you have to stop eating hamburgers all the time and sitting on the couch watching tv all day. You have to give up the burgers, spend less time watching tv, and go exercise until you get in shape. You have got to change the habit. You will need the discipline to form the "better" habit. So the sacrifice here, is getting rid of the old habit, to get into practicing the better habit, to get you to achieve a better healthier body.