Hyponatremia
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.
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 the problem.
2 Causes
The cause of Hyponatremia is poorly understood, but there appear to be two likely causes in runners.
- 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.
- For ultrarunners, I would argue that Hyponatremia is more likely to be caused by a insufficient electrolytes combined with appropriate drinking.
3 Likelyhood
Hyponatremia can be common in endurance athletes:
- In a 1997 Ironman triathlon, almost 4% of competitors received attention for Hyponatremia [5].
- In a study of the 2002 Boston Marathon, 13% of finishers had some level of Hyponatremia, and 0.6% had critical Hyponatremia [6].
- In the Houston marathon[7], 21 runners who received medical help were suffering from Hyponatremia. Runners with Hyponatremia were generally slower and drank more.
- However, a study[8] 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[9] 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
- ↑ Hyponatremia http://en.wikipedia.org/wiki/Hyponatremia
- ↑ Exercise-Associated Hyponatremia http://cjasn.asnjournals.org/cgi/content/abstract/2/1/151
- ↑ Water Intoxication http://en.wikipedia.org/wiki/Water_intoxication
- ↑ USATF Announces Major Changes in Hydration Guidelines for Long Distance Runners http://www.rrm.com/archive03/042803n2.htm
- ↑ Micronutrient Information Center - Sodium http://lpi.oregonstate.edu/infocenter/minerals/sodium/
- ↑ Hyponatremia among Runners in the Boston Marathon http://content.nejm.org/cgi/content/abstract/352/15/1550
- ↑ 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
- ↑ Study of hematological and biochemical para... [Clin J Sport Med. 2004] - PubMed - NCBI http://www.ncbi.nlm.nih.gov/pubmed/15523206
- ↑ BAA - HYDRATION, DEHYDRATION AND HYPONATREMIA http://www.bostonmarathon.org/BostonMarathon/WelcomeBooklet.asp#hydration
