Exercise Associated Hyponatremia (EAH)

My last post was about how dehydration can negatively affect your performance. Today, I want to touch on a touchy subject (in part’s of the science industry), hyponatremia. I say touchy because a few years ago, a well-known researcher started advocating not drinking in marathons and that we didn’t need to replenish sodium/electrolytes during endurance events. This certainly scared a number of athletes that follow nutrition/hydration science, however with some basic guidelines, hyponatremia will not happen to you. And I will advocate to drink during exercise, in addition to taking an electrolyte supplement. Interestingly, hyponatremia was first studied at the Comrades marathon, and than more closely at the Boston marathon in 1985.

Hyponatremia, what is it?

  • Clinically, hyponatremia is hypo:low, natremia: the presence of sodium in the blood
  • When the plasma Na+ levels fall below 135mEq/L and the athlete is asymptomatic
  • When the plasma Na+ levels fall below 130mEq/L and the athlete shows symptoms
  • Becomes life threatening when levels fall below 120mEq/L
  • On a cellular level, what is happening is that the cell is swelling with water and diluting sodium.


  1. Headache
  2. Swollen hands and feet
  3. Confusion
  4. In coordination
  5. Muscle weakness
  6. Vomiting- this is a tell-tale sign and one to be especially on the lookout for
  7. Seizures
  8. Cardiac Arrest
  9. Death

Symptoms will start off gradual and get more pronounced. Once the CNS (central nervous system) becomes effected, things go from bad to worse.

There are 2 main causes for EAH:

1. Excessive consumption of fluid

2. Excessive loss of Na+ and/or inadequate Na+ intake

Both causes of EAH are hard to study due to the health risk consequences for athletes.

1.I’ll talk about both, but excessive consumption first. Excessive consumption means you are taking in more fluid than you are losing. And then you are actually gaining weight. Commonly, an increase in weight was seen as 1-9% of the person’s starting body weight. In fact, when I started IM back in 2005/6, we were weighed. Now a days I haven’t seen anyone being weighed, but I’m possibly missing it.

1-9% is huge. If an athlete weighs 150lbs, that is 1.5-13.5 pounds. That being said, you can also gain weight and not have clinical EAH. So, just because you gain a pound after an exercise session, that does not equal EAH. It could mean you are headed down that path though.

In a study done by Edelman et al, they saw a minimum of 2.2% gain in athletes, which equated to a surplus of 1.6L of fluid. That’s not just taking in 1.6L of fluid, that’s a surplus. Another study done was at the Houston marathon in 2000. Runners that drank 60 aid station cups of fluid (that’s 4+ cups at each aid station if they are spaced 2 miles apart) developed EAH. So you can see, it’s quite an excessive amount.

Excessive consumption can happen to any athlete, at any age, weight, gender, etc. One common trend that researchers see is that it happens to slower runners. This isn’t because of their pace, but because of their ability to consume more fluid than faster runners. Let me say that all slow runners will not develop EAH, so don’t get nervous. The other key is that it occurs in events over 4 hours in length.

Also, smaller athletes need less fluid excess to dilute their serum sodium levels.

*So, how do you prevent EAH from excessive consumption? Don’t drink in excess* The key is finding out your sweat rate, then drinking to match your own sweat rate. I’ll include a basic sweat rate test below.

Basic guidelines are drinking 14-27oz/hour of an electrolyte fluid. Some athletes are up above 48oz per hour though.

2. The second cause of EAH is excessive loss of sweat/Na+, or the inadequate intake of Na+.

-Noakes et al did a study in 2005 that showed that 20% of athletes that developed EAH did not over drink. So, they lost too much sweat, or didn’t take in enough.

Excessive loss of sweat is due to prolonged sweating, lack of  heat acclimatization, being under trained and drinking no to low Na+ drinks. Sweat rate  and make up is determined by several factors: body weight, temperature, pace, metabolic efficiency and genetics. Some of us are heavy sweaters, others are not. Sex and age are not factors in sweat rate. I am a heavy sweater, and when I wear black bike shorts, I have a nice caking of white salt on my shorts. That is a basic test to see how salty your sweat is. The other option is a lab test, which is highly impractical for athletes.

*Heavy sweaters need to be more vigilant because they don’t have to gain weight to develop EAH*

Sweat electrolyte losses include:

  • Sodium-10-70mEq/L
  • Potassium- 3-15mEq/L
  • Magnesium- .2-1.5mEq/L
  • Calcium- .3-2mEq/L
  • Chloride- 5-60mEq/L

I don’t want you to remember that, I just want to show you that Sodium is the most important electrolyte to replace. Heat acclimated athletes will show lower sweat sodium concentrations of up to 50% less. So, heat acclimate for at least 2 weeks prior to your race.

Basic guidelines: Consume 400-800mg of Na+ per hour when exercising 4+ hours. This is very individual and some only need 400mg, while other triathletes take in upwards of 1500mg/hr. This could potentially be a bad idea for most though. Hypernatremia is also an issue, just much less. 


  1. Hyponatremia can be deadly, but will only occur if you over consume water, or under consume Na+
  2. Find out your sweat rate and drink to replenish your own sweat rate
  3. Replace sodium in a drink or tablet form

Sweat Test:

  1. Take body weight before a one-hour moderate/high intensity bike or run.
  2. Record the amount of liquid consumed during the workout. Towel dry and weigh yourself again after the workout.
  3. Calculate the weight change and remember to add in the amount of liquid consumed during the workout.
  4. Every pound lost during your workout is equal to 16 oz of fluid.
  5. Drink 20oz of fluid per pound lost to replenish lost fluid-this does not need to be done all at once.





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