Just an update on this thread. There was a program on a few days in the UK called 'Panorama: The truth about sports supplements'. Unfortunately, I don't think you can view this outside the UK, or at least I can't find a link for it. Anyway, a generally interesting program looking at evidence for sports shoes, BCAAs, rehydration drinks etc.
One of the things they looked at was the obsession with hydration. These were the conclusions from the top-echelon researchers:
1. No one knows where this 2% dehydration results in x% performance loss idea comes from.
2. Over years of monitoring marathon events, there have been zero deaths that could reasonably be attributed to dehydration. There have, however, been 16 (from memory) attributed to over-hydration
caused by drinking too much water. There have been 1600 incidents of severe illness (again, from memory) caused by over-hydration.
3. One of the factors contributing to this has been the belief that we need to take action before thirst strikes. This is a message heavily promoted by sports drinks manufacturers, but again there's no evidence that this is the case.
Most interestingly, they showed the South African army, who are carrying 60kg (132lb) packs in searing heat for hours at a time. The army carried out its own research into hydration...as a result of this, now the soldiers are not told they have to drink x amount. Their instruction is just 'drink when you feel thirsty'.
The sports shoe stuff was fascinating too.