Exercise-Induced Hyponatremia

By Shannon Mitchel, MD – October 25, 2012

With a summer as hot and humid as Texas has had, it is difficult to imagine someone drinking himself to death with water. But that is exactly what happened at the Texas Water Safari Canoe race in June. The gentleman who passed away, Brad Ellis of Dripping Springs, was a healthy 30-year-old man who had been training intensely for the event. He even kept a blog to document his workouts. On May 7, 2012, Ellis wrote this after a “tune up race” to prepare for the big event in June:
“The only issue I am having and I’ve been this way my whole life, is the amount of water I consume. I was going through a gallon of water every two hours and most people would consume a gallon or less for the entire day. Everyone who saw the amount of water I had on the canoe for myself told me it was too much, but to me, I need it.” (Boldface is mine.)

Hyponatremia, which is the most common electrolyte disturbance, occurs when there is too much water (as compared to salt) in the human body. This water-to-salt balancing act is a highly regulated system as we use salt to drive our cells and, thus, all of our bodily functions. Lately, much has been written about exercised-induced hyponatremia. Particularly since the new book by Tim Noakes, MD,Waterlogged, came out, more and more runners are worrying about drinking fluids while exercising. Reading Noakes’ book, it is easy to believe that, unless you were going to be out in the heat for several days, it would not be necessary to drink or consume salt while exercising at all, and, if you must drink water, do so only to quench thirst (and no salt is ever needed). And yet, in August, an experienced ultra-runner died when he attempted to run a mere three miles in Death Valley without any salt or water.

All of this has left many of us confused. We still wish to exercise or run regardless of the heat and to do so in a safe and healthy manner, but who is right when it comes to the correct amount of water to drink? What do we need with regard to salt and water, and how much of it do we need?

I have found that, when there are conflicting opinions on a matter of science, it is always best to simply return to the basics. In this case, that would be the science of the metabolism of salt and water in our bodies.

To begin, the brain senses either a decrease in blood volume or a concentrated blood volume, meaning there are more solid particles (mostly salt, but it can also be potassium, sugar, chloride, magnesium, calcium and a few other particles that may be present in our blood) than water in the blood. When the brain senses one of these conditions, it signals the pituitary gland to release something called arginine-vasopressin (also known as the antidiuretic hormone or ADH). This hormone goes to work in the kidneys and tells them to decrease the flow of water into the urine. At the same time, the hormone signals another portion of the brain to tell you that you are thirsty. When all is well again, the concentration of water to particles in the blood goes back to normal and the hormone signals the brain to stop sending the ADH out into the body. Without the ADH, the kidney begins to work normally again, freely adding water to the urine. Of course, as you can imagine with all hormone systems in the body, it is not that simple.

First of all, there are other things that can signal the release of ADH. For example, pain, nausea, and low blood sugar are all triggers that may tell the brain to release the ADH. All of these triggers may be present during a long-distance endurance event. Nevertheless, as with all properly functioning bodies, the checks and balances in the system will usually keep things from going awry. So, even if you are experiencing a condition such as pain or stress—either of which would signal the unnecessary release of the ADH—the brain should sense that the concentration of water to salt was normal and the ADH should still be turned off before any harm can be done.

So, scientific talk aside, where is the harm in your kidneys retaining water and releasing salt? How can this be fatal? Well, as I mentioned before, sodium drives our cells. Sodium moves in and out of the cell as each cell does its respective job. However, if the solution the cell lives in becomes too dilute, (meaning there is too much water as compared to salt), the cell will start to move salt that is needed inside the cell to the outside of the cell in order to keep the salt-to-water ratio equal. Over time, this continual process leaves no salt to drive the cellular functions inside the cells and they become unstable. Eventually, the cells break down. If this deterioration happens in the lungs, fluid enters into the lungs. If it occurs in the brain, swelling usually follows and this can lead to death in the most extreme cases.

This begs the question, however: If the body is so regulated and this system has so many checks and balances, how can people actually drink so much water that they die?

As with any system, there are disorders. Syndrome of the Inappropriate Antidiuretic Hormone (SIADH) is said to be present to some degree in about seven percent of people in the normal community and about 22 percent of hospitalized patients. People with this condition have an inaccurate barometer for the concentration of liquid to particle in the blood and will be thirsty much more often than someone who does not have this disorder. So, their blood will stay dilute, (people with SIADH would always have less salt in their blood than the standard of normal, but the body would perceive this ratio as normal).

While abnormal, this ratio might not be enough to cause symptoms until something else went wrong. Often, this condition is not found until you are quite sick in the hospital. You may also be diagnosed if a routine blood test shows slightly less sodium than normal, but, even then, further tests are needed to sort out what may or may not be done, particularly if you are otherwise healthy. SIADH is often associated with other chronic medical conditions (such as cirrhosis of the liver, congestive heart failure, and certain cancers) but it can occur for no particular reason. It can be treated with either water restriction or medication when caught at a mild level but some patients, such as those who fall into unconsciousness as a result, will often need treatment with salt or extra salty water via IV.

Which brings us back to our canoe race fatality. Based on Ellis’ own accounts, he was used to drinking much more than everyone else and had done so his entire life. This seems to point to the possibility that he did suffer from SIADH. He knew he was consuming more water than others but was still thirsty. As this thirst is present in those with SIADH, you can see why telling someone to simply drink until his/her thirst is quenched can lead to problems. If you have a problem with system regulation, you will continue to be thirsty long after your actual need has ceased.

The increased incidence of hyponatremia at endurance events has been blamed on the sports drink industry as well as on training mistakes. Yet I believe it is simply a numbers game. If hyponatremia is the most common electrolyte problem and we have at least seven percent of the population walking around with SIADH and no knowledge of the condition, why would we not expect an increase in exercise-induced hyponatremia? I have seen at least one large review study propose that most who require hospitalization or die from hyponatremia at races started out with SIADH. The added stress of the event combined with the perception of increased thirst tipped these athletes over the scale. Given how well regulated this balancing mechanism is in an otherwise healthy person competing in these events, this seems to be the only rational explanation.

So, then, what is the right thing to do? Can a person simply get a test for SIADH? Unfortunately, the answer is “probably not.” It is a complicated diagnosis to make and may only show up when you are under stress or in pain. That being said, if you find that you are significantly thirstier than most other people and you drink more than your training partners, it would certainly be worth going to get this checked out with your physician.

This all leads us back to the hot Texas summers and safely managing exertion in the heat. I have always been a proponent of salt intake when you are sweating. If your kidneys are working properly and you take in a little more salt than you need, your body will get rid of the excess. On the other hand, if you skimp on the salt and only drink water (and just happen to have SIADH), you could become one of those people who have trouble. Since one of the treatments of SIADH is taking salt, this should be a safer bet as well.

How will I know during a race? Unfortunately, this is the tough part. You can be either dehydrated, of perfect fluid volume, or you may have too much fluid and have some edema and, in any of these cases, you can still have an imbalance of salt and water. Confusion is a symptom, but it is also a symptom of low blood sugar or sleep deprivation, so, again, it is very difficult to determine. If you pass out, the medical staff at the scene may think you are dehydrated and start an IV. The problem with this course of action is that if you have hyponatremia, this might not be the best treatment. Depending on the severity of the salt deficit, a normal saline IV, which seems magical when you are dehydrated, could easily make things much worse when the problem is hyponatremia.

The only way to be certain of what the problem actually is would be to do a blood test. So, in my opinion, it is best not to administer IVs in the field unless the medical staff is 100 percent familiar with the athlete and his/her history. Fainting at the finish line is often neither dehydration nor hyponatremia but rather brief cardiovascular collapse. While this sounds far worse than anything yet discussed, it basically means your blood vessels were shunting blood to your muscles during your exercise. When you crossed the finish line, you stopped pushing so hard and your blood pressure simply dropped, which kept blood from getting up to your brain. Generally, folks feel better after just lying down for about 15 minutes without anything further than reassurance and monitoring.

I realize this is confusing. It is hard to put all of this into layperson terms without either accidentally scaring someone or boring them to death. But it is a fact that hormones and their regulation is complicated. At present, there are many conflicting opinions about hyponatremia, so even getting someone to a basic understanding takes more than a cursory article. It is hard to combat three-second headline grabbers with a long discussion of the hows and whys. Which is why, as always, if you have any questions about your own particular case, you should have a thoughtful discussion with your doctor. Regardless of where you think you fall on the sponge-to-camel spectrum, just don’t try to go out to run Death Valley or the Lady Bird Lake trail in Austin’s heat without any hydration at all.


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