The Karvonen formula is one of the bests that can be used to define your heart rate zones and anaerobic threshold. It is based on the heart rate reserve, so it takes into account both your resting heart rate and maximum heart rate. Therefore it gives a much better result than the formulas that use only the maximum heart rate.
During long-lasting activities you will lose water and some important electrolytes — salts and minerals — through sweat, and burn a lot of calories. Therefore it is important to refuel during and after such activities. This drink will help you to hydrate and to replenish electrolytes.