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While dehydration can be dangerous, the danger from Hyponatremia is far greater. Hyponatremia is where the sodium (salt) levels in the blood becomes too dilute. For more details see [[The Science Of Hydration]] and [[Practical Hydration]]. ==Symptoms==Initial In athletes, mild Hyponatremia may have no clinical symptoms tend to be a gain in weight and a general swelling and 'puffiness', most noticeable in the hands. More or weakness, dizziness, headache, nausea/vomiting, but more severe Hyponatremia is likely to have symptoms are caused by a swelling of the brain (cerebral edema) , including nauseaaltered mental status, seizures, pulmonary edema, vomitingcoma, headache and malaise death<ref name="ref10RosnerKirven2006"/>. The link between Hyponatremia and [[Cramps]] is not clear, but there is evidence that increased sodium intake can alleviate the problemboth problems. ==Causes==The cause of Hyponatremia is poorly understood, but commonly believed there appear to be related to excessive several likely causes in runners. # Excessive water intake can cause 'dilutional Hyponatremia'<ref name="RosnerKirven2006"/>. Healthy kidneys can excrete about 2 pints (1 liter) of fluid per hour, but this may be reduced by exertion or illness <ref name="ref1ref3"/>. So drinking &gt;6 pints in 4 hours could easily exceed the kidneys capacity to cope. The recent rise in Hyponatremia may be due to earlier advice to athletes to "drink as much as possible" <ref name="ref13"/>, combined with a general concern about salt intake.# [[Sodium Deficiency]], combined with adequate or excessive hydration.# [[NSAIDs and Running|NSAID]] usage, which is been linked to hyponatremia.# Syndrome Of Inappropriate Antidiuretic Hormone Hypersecretion (SIADH) is where the body secretes too much of the hormone (vasopressin) that reduces the urine output of the kidney. It's not clear if this is an independent cause of hyponatremia, or if this is a mechanism underlying one of the other causes. However, I would argue some research<ref name="SiegelVerbalis2007"/> suggests that many cases of Hyponatremia is are due to SIADH caused excessive fluid intake in the absence by a combination of over hydration, NSAID usage and Rhabdomyolysis. One key aspect of sufficient electrolytesSIADH is that because it prevents urine production, people mistakenly assume that they are not urinating due to dehydration and drink even more. =Likelyhood=Hyponatremia can be common in endurance athletes:* In a 1997 Ironman triathlon, almost 4% of competitors received attention for Hyponatremia <ref name="ref4"/>. * In a study of the 2002 Boston Marathon, 13% of finishers had some level of Hyponatremia, and 0.6% had critical Hyponatremia <ref name="ref2Almond-2005"/>. * In the Houston marathon<ref name="Houston"/>, 21 runners who received medical help were suffering from Hyponatremia. Runners with Hyponatremia were generally slower and drank more.* However, a study<ref name="nzmarathon"/> of the 2002 Christchurch Marathon in New Zealand, where aggressive hydration was not recommended, none of the 134 runners measured had Hyponatremia.=Risk Factors==The study <ref name="ref12"/> revealed that the risk factors for Hyponatremia in marathons include * A slow finish time (&gt;4 hour)

* BAA suggests a 'slight build' is also a risk factor<ref name=Preventing Hyponatremia="ref12"/>.  Healthy kidneys can excrete about 2 pints (1 liter) of fluid per hour, but this may be reduced by exertion or illness <ref name="ref3"/>. So drinking &gt;6 pints in 4 hours could easily exceed the kidneys capacity The key to cope. The recent rise in preventing Hyponatremia may be due is to earlier advice to athletes ensure adequate electrolyte intake and to "drink as much as possible" <ref name="ref13"/>, combined with a general concern about salt intakewhen thirsty. See [[Practical Hydration]].== References ==

<ref name="ref1RosnerKirven2006">M. H. Rosner, J. Kirven, Exercise-Associated Hyponatremia[http://cjasn.asnjournals.org/cgi/content/abstract/, Clinical Journal of the American Society of Nephrology, volume 2/, issue 1/151 , 2006, pages 151–161, ISSN [http://cjasnwww.asnjournalsworldcat.org/cgiissn/content/abstract/2/1/1511555-9041 1555-9041]</ref> <ref name="ref10">Hyponatremia, doi [http://endx.wikipediadoi.org/wiki/Hyponatremia http:/10.2215/enCJN.wikipedia02730806 10.org2215/wiki/HyponatremiaCJN.02730806]</ref> <ref name="ref2Almond-2005">CS. Almond, AY. Shin, EB. Fortescue, RC. Mannix, D. Wypij, BA. Binstadt, CN. Duncan, DP. Olson, AE. Salerno, Hyponatremia among Runners runners in the Boston Marathon., N Engl J Med, volume 352, issue 15, pages 1550-6, Apr 2005, doi [http://contentdx.nejmdoi.org/cgi10.1056/contentNEJMoa043901 10.1056/abstract/352/15/1550 NEJMoa043901], PMID [http://contentwww.nejmncbi.org/cgi/contentnlm.nih.gov/abstractpubmed/352/15/155015829535 15829535]</ref> 

[http://en.wikipedia.org/wiki/Water_intoxication http://en.wikipedia.org/wiki/Water_intoxication]

[http://www.bostonmarathon.org/BostonMarathon/WelcomeBooklet.asp#hydration http://www.bostonmarathon.org/BostonMarathon/WelcomeBooklet.asp#hydration]