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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.

1 Symptoms

Initial symptoms tend to be a gain in weight and a general swelling and 'puffiness', most noticeable in the hands. More severe symptoms are caused by a swelling of the brain (cerebral edema) including nausea, vomiting, headache and malaise [1]. The link between Hyponatremia and Cramps is not clear, but there is evidence that increased sodium intake can alleviate both problems.

2 Causes

The cause of Hyponatremia is poorly understood, but there appear to be several likely causes in runners.

  1. Excessive water intake can cause 'dilutional Hyponatremia'[2]. Healthy kidneys can excrete about 2 pints (1 liter) of fluid per hour, but this may be reduced by exertion or illness [3]. So drinking >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" [4], combined with a general concern about salt intake.
  2. Sodium Deficiency, combined with adequate or excessive hydration.
  3. NSAID usage, which is been linked to hyponatremia.
  4. Syndrome Of Inappropriate Antidiuretic Hormone Hypersecretion, or 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, some research[5] suggests that many cases of Hyponatremia are due to SIADH caused by a combination of over hydration, NSAID usage and Rhabdomyolysis. One key aspect of SIADH is that because it prevents urine production, people mistakenly assume that they are not urinating due to dehydration and drink even more.

3 Likelyhood

Hyponatremia can be common in endurance athletes:

  • In a 1997 Ironman triathlon, almost 4% of competitors received attention for Hyponatremia [6].
  • In a study of the 2002 Boston Marathon, 13% of finishers had some level of Hyponatremia, and 0.6% had critical Hyponatremia [7].
  • In the Houston marathon[8], 21 runners who received medical help were suffering from Hyponatremia. Runners with Hyponatremia were generally slower and drank more.
  • However, a study[9] of the 2002 Christchurch Marathon in New Zealand, where aggressive hydration was not recommended, none of the 134 runners measured had Hyponatremia.

4 Risk Factors

The study[10] revealed that the risk factors for Hyponatremia in marathons include

  • A slow finish time (>4 hour)
  • Consumption of >6 pints (3 liters) of water during the race
  • BAA suggests a 'slight build' is also a risk factor

5 Preventing Hyponatremia

The key to preventing Hyponatremia is to ensure adequate electrolyte intake and to drink when thirsty. See Practical Hydration.

6 References

  1. Hyponatremia http://en.wikipedia.org/wiki/Hyponatremia
  2. Exercise-Associated Hyponatremia http://cjasn.asnjournals.org/cgi/content/abstract/2/1/151
  3. Water Intoxication http://en.wikipedia.org/wiki/Water_intoxication
  4. USATF Announces Major Changes in Hydration Guidelines for Long Distance Runners http://www.rrm.com/archive03/042803n2.htm
  5. http://www.sciencedirect.com/science/article/pii/S0002934307001672
  6. Micronutrient Information Center - Sodium http://lpi.oregonstate.edu/infocenter/minerals/sodium/
  7. Hyponatremia among Runners in the Boston Marathon http://content.nejm.org/cgi/content/abstract/352/15/1550
  8. Hew TD, Chorley JN, Cianca JC, et al. The incidence, risk factors and clinical manifestations of hyponatremia in marathon runners. Clin J Sports Med. 2003;13:41–47 http://www.ncbi.nlm.nih.gov/pubmed/12544163
  9. Study of hematological and biochemical para... [Clin J Sport Med. 2004] - PubMed - NCBI http://www.ncbi.nlm.nih.gov/pubmed/15523206
  10. BAA - HYDRATION, DEHYDRATION AND HYPONATREMIA http://www.bostonmarathon.org/BostonMarathon/WelcomeBooklet.asp#hydration