If carbohydrates are not available, the energy will come from ketones, which are broken down from fats. Again, this is why many are doing the low carbohydrate and high protein diets. The brain rarely uses any fuel other than carbohydrates. However, many get discouraged while exercising and dieting because the scales do not seem to show any benefits. There is a reason for this.
Muscle weighs about twice as much as fat.
That is by volume, if you took a cup filled with fat and a cup filled with muscle and weighed them, the muscle would weigh about twice as much. Therefore, pat yourself on the back if you maintain a steady routine and the scales are not moving. You are converting fat to muscle. The athletes' rule of thumb for working out and eating has been not to eat two hours before a workout and not to eat one hour after. Whenever, the stomach is full, the majority of blood is circulating to the stomach, and it is needed in the muscles.