The topic of sports drinks can be daunting. There are so many choices, but are they all created equal? My personal experience is no they are not. In the past I have become dehydrated either by choosing the wrong type of sports drink or from being reluctant to pump my body full of the sugars found in most sports drinks. I have over the years learned that my body needs some type of replenishment and the science behind these drinks does offer some real help in this way. "Variables such as the length of your run, the temperature, and your pace affect what you should drink. For workouts that are about 30 or 45 minutes long, a few sips of water will usually do the trick. But for runs closer to an hour, it's important to choose a drink that has the right combination of water, carbohydrates, and electrolytes compounds such as sodium and potassium that help your body retain fluid."---Runners World
I know that after a long or hard run I will notice my skin has a white or salty layer of film. This is known as "electrolyte-sweat". It is critical to your body to replace these electrolytes and minerals.
A good sports drink should provide:(in an 8oz serving)
35-50 mg Potassium
Most of these can be found in your local grocery store, such as Gatorade, PowerAde, Gleukos, Cytomax. I have not mentioned the different gels out there because my experience with them has been minimal. I did have a GU pack on hand and looked at the carb, sodium, potassium ratio and found it was low in the sodium, only 50mg based on the above equation.
Here's a link to a video at Runners World that covers this topic: