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.
The exact amounts lost depend on the type of the activity, its duration and intensity, and of course the weather plays a major role, too. For example, on average I lose one liter of water and burn 500 kcal during one hour of running — can’t measure the amount of salts and minerals lost.
Drinking sports drinks is probably the easiest way of refueling. You can find a ton of different brands offering their drinks. These are usually very similar to each other, made of water, fructose, glucose and other sugars, and electrolytes. These drinks are usually expensive and may contain some unhealthy stuff.
It is very easy to make your own sports drink and all you need are a few basic ingredients.