Our expert says:
It's most likely a blood pressure problem, and I certainly wouldn't worry about dehydration - remember that humans are the only animals that have to be told how much to drink, so that makes one wonder about how true it is that everyone has to drink a certain amount per day. As far as I'm concerned, if you drink to thirst, you will be fine.
As for the lightheadedness, if your blood pressure is chronically low, then there are forms of medication that you can use to treat this, and I would suggest that you seek medical advice in this regard.
The other thing is that this sort of thing usually passes with time - the body adapts pretty quickly to the strenuous exercise and this will then not happen after a while - so just as with training, as you get fitter, you find this is less and less of a problem, so if you can stick it out, it will subside. However, if you are struggling, then I would see a doctor about it.
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