Questions about hyponatremia and running

I thought the comments on the hyponatremia issue were interesting enough to warrant a rant.

With the Boston Marathon four days away, I hope publicity from this article doesn’t lead to an excess incidence of dehydration and hyperthermia. The timing of publication in the NEJM makes for an interesting experiment. As a marathoner myself, my advise to my brother who is running on Monday: don’t miss any water stops.

I disagree. The data supports the concept of drinking when thirsty. The data do not support drinking at all water stops. Drinking at all water stops puts one at risk for hyponatremia.

Did the study address any difference between runners who drank water and runners who drank sports drinks or other liquids with ion concentrations closer to serum?

If not, do you have any thoughts or knowledge about any difference for risk in these two groups?

As a commentor made clear, sports drinks do not help here. The concept seems counterintuitive, but I will try to explain. Think of a sports drink as having the same solute concentration as blood. When ADH is stimulated (see yesterday’s rant if that does not make sense), the urine concentration becomes at least twice the serum concentration (sometimes 3 times as great). If we assume urine concentration twice serum concentration, then it is also twice the concentration of the sports drink. If we drink in a liter of sports drink, we only urinated 500 ccs of urine to excrete the solutes. This leaves 500 cc of new free water – which dilutes the serum and lowers the sodium. The other problem with sports drinks is the glucose – which increases water absorption, making the sports drink more effective at increasing free water (and thus lowering serum sodium).

Speaking as a somewhat avid runner (and marathoner) who has been a distance athlete for years, I’m really curious about the athletic and training backgrounds of the people who do contract hyponatremia.

This phenomenon occurs over time, thus the less fit have more time to drink – and usually drink much greater amounts. High performers finish before they have time to drink enough. I would wonder what happens to distance athletes (as opposed to less fit participants) in iron man competitions or 50 mile runs. That would make for a very interesting study.