What causes hyponatremia?

Christopher C. Bell, MD
Sports Medicine

Hyponatremia is a condition of low sodium concentration in the blood. It is important to point out that it is not a condition of low sodium alone, but low sodium concentration. This means that the actual amount of sodium in your body is not the culprit, it's the amount of sodium in relation to your body fluids. Therefore, it is actually more a measure of your hydration status.

To explain further: if you are dehydrated, your body fluid level is low, and so your sodium concentration tends to be higher, on the hypernatremic side. If you are overhydrated, meaning you have a lot of fluid in your body relative to sodium, then you are on the hyponatremic side.

There are many causes of hyponatremia, but what I would like to focus on is what is called Exercise Associated Hyponatremia (EAH). This is a condition where athletes, typically endurance athletes such as marathoners, drink too much during exercise and basically dilute out their sodium. In most, if not all, cases those with EAH actually weigh more at the end of the race than at the beginning. In mild forms athletes may only have fatigue, nausea, and dizziness. However hyponatremia is a potentially lethal condition and there have been reported cases of athletes dying from it as well. 

EAH was first described in the 1980's and became more prevalent in the 90's and the early part of this century. The timing corresponded to some research that showed a detriment in performance with as little as 2% body weight loss during exercise (which has since been challenged with new research), attributed to dehydration. At this time there was also a surge of beginner and "weekend warrior" runners getting more involved in marathons and longer duration events. At races, especially marathons, runners were encouraged and even urged to drink as much as they could throughout the race to stave off the dreaded dehydration in the hopes of maximizing performance. The sad result was a lot of sick runners and a handful of deaths.

Oddly enough, there are still quite a few people out there who hold on to the idea of aggressive hydration during long events, especially marathons, including those within the sports medicine field and race organizers.

So, do not listen to anyone who recommends drinking a certain amount of fluid every "x" minutes! The official recommendation of leading experts is to drink to thirst only. In addition, drinking Gatorade or other sports drinks instead of water does not prevent EAH.

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