The aerobic system, also called the oxidative system (aerobic means a proces which functions with the aid of oxygen) is the most important source of ATP in rest and during lower intensity exercise. This system generates ATP from both carbohydrates and fats. When the carbohydrate stores are depleted protein can also be used. Depending on the intensity of exercise, a certain ratio of carbohydrates and fats is used. At rest this ratio is around 70% fat and 30% carbohydrates. When exercise starts, the use of fat as an energy substrate decreases and carbohydrate use increases with the increase in intensity. When intensity rises to 100% VO2max, almost all ATP will be generated from carbohydrates.