A doctor here. So, the question of what's more important actually depends on the individual, but yes, diet matters very much. You have to eat a balanced diet as a normal person. But if you have any disease, let's say a metabolic disorder, diabetes, or a thyroid disorder, the diet changes to low carb. Exercise is as much important as diet. The standard American diet is mostly carbs and processed junk food, which will lead to many diseases that even exercise can't avoid. So it really depends on what works for you.