Can water be bad for you?

I am not talking here about water borne illness--anyone reading my publications or blog posts knows my feelings about that!  I am, instead, focusing on problems related to the over-consumption of water from any source. Believe it or not, this is actually a growing problem.  It was nicely reviewed recently in an article in the New York Times:

 http://well.blogs.nytimes.com/2015/08/26/for-athletes-the-risk-of-too-much-water/?_r=0

The physiology here is actually quite simple.  When water is added in excess to the body, it dilutes the amount of sodium in the circulation, a condition called "hyponatremia."  Hyponatremia can cause a host of complications, including devastating and occasional fatal brain injury.

 Fortunately, the body has a couple very effective mechanisms to prevent over-consumption of water.  The first is our thirst mechanism.  Once one has drunk enough water to result in even very minimal hyponatremia, our thirst shuts off and there is simply no desire to drink any more.  Secondly, the kidneys are very good at excreting excessive water; this is why your urine looks so clear after drinking a lot of fluid.

Dangerous hyponatremia can result if we consistently over-ride our thirst mechanism and/or if the ability of the kidney to excrete excess water is impaired.  Vigorous athletic activity can result in both of these.  Hormones produced during exercise may impair kidney water excretion, and admonitions to drink heavily during sport may push us even when our thirst says "no more!"

Most of the reports of this condition have been during endurance sports such as marathon running.  I have not heard of well-documented cases occurring during conventional backcountry travel or hiking. 

One of the lessons of this subject is the importance of listening to one's body.  Fear of dehydration has often led to the admonition for athletes to drink even if they are not thirsty.  This is probably a mistake.  Mild dehydration may impair athletic performance a bit (although even that is debatable), but is rarely fatal.  Probably the best advice is simply pay attention to one's feelings.  If you are thirsty, drink.  If you aren't, don't.