I don't know about you, but where I'm from (Hamilton, Ontario, Canada), this has been a hot, steamy summer. I headed out for a long run a couple of weeks ago early on Saturday morning, to try to beat the heat. But I made a big mistake (yes, even trainers do that from time to time!). I didn't bring any water. Unlike a race, nobody sets up hydration stations for my training runs. I'm going to have to work on that. Because I needed water that morning. By the end of my run, I was looking longingly at roadside puddles, thinking "gosh, that one doesn't look like it has too much car exhaust by-product in it!". I managed to make it home without dropping to my knees to lap at the filthy water, and I made sure I drank plenty of water then and throughout the day. But I still felt the effects. And it wasn't pleasant, let me tell you. If you've ever experienced dehydration, you know that it's NOT a party.
Water is just about the MOST essential ingredient for a healthy life. It works hard, doing things like aiding digestion, transporting nutrients and helping to eliminate waste, lubricating joints and helping regulate your body temperature through sweating. Water is especially important during exercise. Dehydration can cause muscle cramps, dizziness, fatigue, and in extreme cases, heat stroke and heat exhaustion. The amount of water you need to drink before, during and after exercise is based on a lot of factors, but as a general rule, you should be drinking a cup or two of water about half an hour before exercise, half a cup or so about every twenty minutes during your workout, and 1-2 cups after your workout for every pound you sweat off.
What about those sports drinks? Well, unless you're exercising for longer than 60 minutes at a fairly high intensity, it's not necessary to down a big bottle of Gatorade. Water is sufficient. If you're running marathons or competing in triathalons, you will need to add a complex sports drink with electrolytes.
And I really don't recommend drinking from those roadside puddles!