Depends. People mistake the purpose of exercise. The primary goal of exercise isn't to burn calories. The primary goal is to change the way your metabolism works, so that you can process food far more efficiently. If you exercise vigorously, it helps to change your sugar metabolism, making it easier to store more calories without converting them to fat, while reducing your insulin sensitivity, two very important aspects of health. Sugar (our most important source of calories) is stored in lean muscle, and thus, the more of it you have, the better your ability to handle its intake. If you eat sugar and can't store it due to lack of space, it gets converted to fat, which we basically don't have a storage cap for. So while diet is important, exercise is at least as important. I would skew that 90% figure pretty much to 50%.