Another powerful effect on energy comes from the impact that cardio fitness has on circulation and blood pressure in your body. In short, using your heart a lot will make you better at pumping blood around your body, which provides you with oxygen, nutrients etc. The best form of exercise for this benefit is steady state cardio. Running long distances gives your heart’s left ventricle time to enlarge and that means you can pump more blood around in your body more easily. Oxygen is a major player when it comes to cell metabolism, which is a fancy way of saying reactions that help facilitate biological functions. In addition to running, any form of exercise will help increase your VO2 Max, your resting heart rate – which effectively has a positive correlation to your energy levels.