Nagging is counter productive. Helping your partner make better choices might be a wiser move. Instead of a bowl of ice cream in the evening, maybe some sorbet or Frozen yougert and fresh berries. Popcorn instead of chips or nachos. Invite them on a walk in a field of wild daisies, or to the park to feed the ducks. Find the things your SO enjoys and change them up a bit. Be creative, and those better choices may come slowly, but they will happen. If you are there participating with them, not nagging, it is a joy, not a job.