Originally Posted by
goldfinch
People die in marathons from drinking too much water, which causes a dangerously low level of sodium. There is a real danger of misdiagnosis and people think they are dehydrated and drink more water. Arguably it is more dangerous than dehydration.
Here is a good article discussing the issue:
http://www.rrm.com/rrmsamplehyponatremia.htm
The people who die in marathons are not the elite runners, and not even experienced runners, most likely. They are the people who are spending 5 or 6 hours running and guzzling water without anything else. At least that has been the case in the cases I have read about.
Part of becoming a distance cyclist is learning to manage your food and water intake and what works for you. If you are going for a 6 hour ride you should know what to expect and how to prepare for it.