Caffine obviously does have a dehydrating effect. However - to give a little cred to the positives - caffine has been shown to increase carb absorbtion in the body. So, for your 5k run type of events this is completely useless. But if you're doing some type of long-distance/endurance training or racing there could potentially be an arguement for it's use. This is why you'll see some of the supplements have it these days (i.e. the gel carb packets).
