It all depends on what time of day you are doing your cardio and how many grams of overall carbs you are taking in. I am dieting right now and I do my cardio in the morning and have only a whey isolate shake before I go. That way there are no carbs in my system and my body burns fat. Then I train in the evening, Ifind I get better workouts that way and maintain more muscle mass as my muscles are full of glycogen by the time evening rolls around.