These aren't independent variables. The point is that a half an hour run only burns around 300-400 calories, a deficit which could more easily be accomplished b

These aren't independent variables. The point is that a half an hour run only burns around 300-400 calories, a deficit which could more easily be accomplished by diet. Also, it's possible that the run would increase your appetite. The point isn't that exercise _can't_ work, it's that aiming for dietary changes has a higher average ROI than exercise.