I watched a really interesting documentary the other day about sports products marketed at improving performance. They look at protein powder, sports drinks and trainers. It's very interesting and dispels many myths.
Most interesting of all, they explain the risks of overhydration, i.e water intoxication, and how much more dangerous it is than dehydration. Apparently lots of people die from water intoxication every year because of all the scaremongering (sports companies wanting to sell their expensive isotonic drinks) telling us to drink before we are actually thirsty.