Let's talk about Running and Hyponatremia

Author: Summit Coaching & Training | | Categories: Aerobic Training , Fitness Trainer , Fitness Training , Group Fitness Training , Group Trainer , Personal Trainer , Personal Training , Resistance Training , Running Coach , Ultra Marathon Coaching , Ultra Marathon Training

Let's talk about Running and Hyponatremia

Well, well, Sprummerall has arrived in Edmonton! That means us polar bears need to shed our winter coats and adapt to the higher temperatures that are coming. Every year around this time I am asked about hydration and electrolyte intake when running and racing. Makes sense, since the sweating season has arrived. To start Sprummerall 2020 off on the right foot, here is some information on hydration and electrolyte intake to help you with your virtual races this year.

Exercise-Associated Hyponatremia and Hydration Status in 161-km Ultramarathoners

Hoffman, M.D., Hew-Butler, T., and Stuempfel, K.J. 2013. Exercise-associated hyponatremia and hydration status in 161-km ultramarathoners. Medicine and Science in Sports & Exercise, (454) 784-791.


Purpose: This work combines and reanalyzes 5 yr of exercise-associated hyponatremia (EAH) research at 161-km ultramarathons in northern California with primary purposes to define the relationship between postrace blood sodium concentration ([Na+]) and change in body weight; to examine the interactions among EAH incidence, ambient temperature, and hydration state; and to explore the effect of hydration status on performance. Methods: Prerace and postrace body weight and finish time data were obtained on 887 finishers, and postrace [Na+] was also obtained on a subset of 669 finishers. Results: EAH incidence was 15.1% overall (range, 4.6%–51.0% by year) and had a significant positive relationship with ambient temperature. Of the runners with EAH, 23.8% were classified as overhydrated (weight change, ≥0), 40.6% were euhydrated (weight change, <0% to –3%), and 35.6% were dehydrated (weight change, ≤3%) at the finish. There was a weak significant relationship (r = 0.17, P < 0.0001) between postrace [Na+] and change in body weight such that a lower [Na+] was more common with increased weight loss. Considering all finishers examined, 18.5% were dehydrated and 34.9% were overhydrated at the finish. There was a weak significant relationship (r = 0.092, P = 0.006) between change in body weight and performance in that faster runners tended to lose more weight. Top finishers varied in body weight change from ~1% gain to ~6% loss. Conclusions: EAH incidence can be high in 161-km ultramarathons in northern California. In this environment, EAH is more common with dehydration than overhydration and is more common in hotter ambient temperature conditions. Because weight loss 93% does not seem to have an adverse effect on performance, excessive sodium supplementation and aggressive fluid ingestion beyond the dictates of thirst are ill advised.

What Does This SH*T Mean for You??

This is a long one. Settle in!

Hyponatremia is low blood sodium concentration. For reference, normal blood sodium concentration (normonatremia) is 135-145 mmol•L-1, high blood sodium concentration (hypernatremia) is >145 mmol•L-1, and hyponatremia is <135 mmol•L-1. Endurance athletes commonly experience exercise-associated hyponatremia (EAH), although some may not know it because they may not have any symptoms (dizziness, nausea, fatigue and muscle weakness or cramps, headache; and in very serious cases seizures, coma, and the ultimate in poor health: death).

Various studies have found that up to 18% of Ironman triathletes, 28% of marathoners, 50% of endurance cyclers, and 50% of 100-mile ultramarathon runners can experience EAH (but EAH has also been reported to be as low as 0%–0.6% in other endurance events). To me, those are some pretty big percentages that deserve some investigation. Thankfully, our super cool friend Science likes to investigate things like this. Questions that the authors of this study asked were: 1) are ambient temperature and hydration status at the race finish related; 2) is there a level of body weight loss that impairs performance; 3) is there a relationship between post-race body weight and blood sodium concentration; 4) are there interactions among EAH occurrence, ambient temperature, and hydration status; and 5) is the effect of hydration status on EAH occurrence statistically different for this study than from a previous study. All the questions! All the learning!

Ok, what did they find? So many things! And it’s all COOL SH*T! It’s important to note that this study included runners at five years of Western States Endurance Run (WSER). The data that the authors compared against for question 5 were from athletes in eight endurance events (triathlons, marathons, cycle tour) from around the world, and that the maximum ambient temperatures at these races were generally lower than the lowest ambient temperatures in the years that WSER data were analyzed for this study.

General findings from this study: 18.5% of finishers were dehydrated, 46.6% were euhydrated (normal), and 34.9% were overhydrated. 15.1% of finishers were hyponatremic, 83.4% were normonatremic, and 1.5% were hypernatremic. Of the hyponatremic finishers, 35.5% were dehydrated, 40.6% were euhydrated, and 23.8% were overhydrated (so, basically, hyponatremia can occur at any hydration status). Looking at these numbers, most finishers were fine (aka normal hydration and normal blood sodium concentration). But what about the folks who weren’t?

To answer question 1: Ambient temperature and hydration status were related. Specifically, percentage of dehydrated runners was correlated with maximum ambient temperature. The hotter it gets, the more you sweat and the more likely you are to become dehydrated. Makes sense.

To answer question 2: Other studies done in a laboratory setting have shown that a 3% loss of body weight will impair performance. However, the authors of this study found that the faster runners tended to lose roughly 3% of their body weight. Science doesn’t have an answer for this yet.

To answer question 3: Increasing body weight loss is associated with lower blood sodium concentration. Body weight loss was attributed to water loss (not fat loss) for a legit reason: most runners start the race a little bit heavier than their average training weight because they’ve carb loaded and hydrated and stored a bunch of calories and water they will need to use in the race. But why does water loss lead to lower blood sodium concentration? The authors attribute this to the higher ambient temperatures, which lead to more sweating (hence the water loss), which leads to more sodium loss. Again, back to the temperature.

To answer question 4: Runners who were dehydrated tended to have a high occurrence of hyponatremia compared to those who were overhydrated. This can seem counterintuitive because you might think that low blood sodium usually happens when dilution takes place (overhydration). Why did this happen? See the explanation for question #3 - temperature. In addition, the WSER lasts up to 30 hours and the endurance events in the previous study lasted up to 18 hours, which means there’s more time to get your sweat on and more time to lose sodium.

To answer question 5: Results from the previous study show that athletes with EAH were more likely to be overhydrated, whereas in the current study runners with EAH were more likely to be dehydrated. Also, the current study found twice as many athletes with EAH as in the previous study. Why? See the explanation for question #3 – temperature, again. In addition, there is a proposed link between rhabdomyolysis and EAH (Ahhhh, good old rhabdo…). In a medium sized nutshell, it has been theorized that EAH could promote rhabdomyolysis through changes in cellular potassium or calcium concentrations that reduce cell membrane stability, or through osmosis of hypotonic extracellular fluid into muscle cells that causes cell swelling and decreases cell membrane stability (well that’s sciencey and complicated). And then there’s also a potential sciencey effect by creatine phosphokinase, which increases in concentration when you run really really far.

So, what it boils down to is this: Hyponatremia can occur at any hydration status. What seems to have the biggest effect on blood sodium concentration is the ambient temperature – the hotter it is, the more likely you are to experience hyponatremia. Also, given that body weight loss of 3% does not appear to impair performance when running outside of a laboratory (who runs in a lab?), the authors advise runners in 100-mile ultramarathons against excessive sodium supplementation and against aggressive fluid replacement beyond what your thirst tells you to drink.