The last of the big three you need to know about is the oxidative system. This is the slow energy system and provides a sustained release of energy for low-to-moderate activities that last a bit longer. Long runs, swimming, hiking, hot yoga, yard work—these are all activities that are more intense than your resting state but can be sustained for a long time. This system uses a mixture of carbohydrates and fats for fuel. While carbohydrates are still important and will benefit your body when it’s using the oxidative system, they aren’t quite as important as they are to the medium energy system.