You do burn both fat and carbohydrates when you exercise, and the proportion varies with the intensity of exercise. A very low-intensity activity like walking taps mainly into fat stores, whereas high-intensity sprints pull much more heavily from carbohydrate stores. At about 60 percent of maximum exertion, your body gets about half of its energy from carbohydrate stores and half from fat stores (which is why many “experts” claim that you should work in the range of 60 to 70 percent of maximum exertion).
