I recently mentioned the concept of hyponatremia in my mailbag response to my series on hydration (Read “Hydration Reloaded” here). It is clinically defined as a blood electrolyte (mainly good old sodium) concentration <135 mmol/L (normal values are about 136-142). This usually happens when an athlete exercises for an extremely long period of time in the heat while drinking mainly water or relatively dilute electrolyte drinks. The combination of high salt loss in the sweat, combined with lots and lots of water with inadequate salt intake, results in the blood becoming dilute. Figure 1 (taken from the Gatorade Sport Science Institute website article on hyponatremia), outlines both the risk factors leading to hyponatremia and also its primary symptoms.
Why is hyponatremia a bad thing? Your body is always going to try to equalize the concentration of the fluid in your blood with the fluid in your body tissues. So having all that dilute fluid in your bloodstream means that your blood plasma (the liquid portion of your blood) is going to get filtered into your tissues. Remember in high school biology class sticking a live sponge or starfish into a bucket of fresh water when the teacher’s back was turned? The poor creature didn’t like it very much and swelled like a blimp. The sponge didn’t like it much either! Now imagine the same thing happening to your brain when you’re hyponatremic. Now don’t you feel guilty?
Is Hyponatremia Really a Problem?
Just how big a problem is hyponatremia in healthy athletes? It’s difficult to get a true handle on this question. Most of the studies rely on gathering data on actual medical cases found during ultra-endurance events or in the military. Both populations tend to attract highly-determined and focused individuals who under-report medical conditions (Yo, I mean you Tyler!), so the actual prevalence may either be much higher, much lower, or dead-on accurate. Most of the rest of the studies are done on hospitalized people with specific renal diseases, so you can’t really generalize these findings to healthy athletes either.
However, as can also be seen in Figure 1, a big danger from hyponatremia is that the end result is a set of symptoms very similar to the more familiar problem of dehydration. Namely, victims end up with bad headaches, nausea, and confusion just like if they’re dehydrated. This makes it a potential matter of life and death for medical personnel at events to recognize the differences between hyponatremia and dehydration, because the last thing you want to give a hyponatremic victim is more water! In this case, the victim doesn’t need more water but rather salt tablets.
Based on both the existing evidence on both the prevalence and the potential risks of these two conditions, I would conclude that, compared to the more common problem of dehydration from NOT drinking enough while exercising in the heat, hyponatremia is generally not as much of a threat to the general athletic population. It is generally difficult to drink as much water as required to really overwhelm your body’s regulatory systems, unless you’re also on some particular drugs that may affect your fluid or salt retention. Also, most of us are drilled into drinking some sort of carbohydrate-electrolyte drink for long events. Therefore, I would rather err on the side of drinking too much rather than risk not drinking enough, especially as our thirst sensors take a long time before they kick in.
Now excuse me while I make another trip to the bathroom and then head back to the water cooler!
Read Hydration Part I here.
Read Hydration Part II here.
Read Hydration Part III here.
Stephen Cheung is an Associate Professor of Kinesiology at Dalhousie University in Halifax, Nova Scotia, Canada, with a research specialty in the effects of thermal stress on human physiology and performance. He has been an avid roadie since beginning university in the mid-eighties, and still has non-indexed downtube shifters on his winter bike and wool jerseys hanging in his closet. He can be reached for advice or comments at firstname.lastname@example.org