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.
Interesting, I automatically assumed that it was some scientifically approved fact given the amount of publications it's found in, which is pretty much every book relating to health.
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.
Over-hydration can also cause people to stop peeing which then appears to the unwary that the individual is not hydrated enough and so they take on even more water; there is a name for it but I cannot remember what it is but once a person relaxes then the peeing mechanism switches on and the fluid is passed out of the system, whereas taking in even more water just means the mechanism stays switched off and that urine cannot be passed, whilst in the state of over-hydration.
A simple test of adequate hydration is to pinch the skin on the back of the wrist, where the arm meets the hand, if it springs back immediately then there is adequate hydration, if it takes time to return to its normal positioning without a ridge etc, then more fluid is required...that's just another test that can be used in addition to the colour of the urine checking.
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.
And these days, people are being led to believe that they require isotonic style drinks for rehydration as well instead of basic water which always worked for each of us in the past...obviously a commercial angle which will become factual, eventually.
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.