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 marathonCite error: Closing </ref> missing for <ref> tag

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