During exercise, the body primarily utilizes fats for energy, especially during lower-intensity, longer-duration activities. As intensity increases, the body shifts towards burning more carbohydrates. This process begins once glycogen stores start to deplete. The availability of oxygen influences fat utilization; aerobic conditions favor fat oxidation, while anaerobic conditions lean towards carbohydrate use. Thus, activities like steady-state cardio are more fat-efficient, making them effective for endurance training and fat loss goals.