What You Need To Know About Hyponatremia

By: Dr. Jeff, NYX Endurance Athlete & Physician

As endurance athletes, we are very aware of the importance of proper nutrition and hydration. We often focus on fluid intake to prevent dehydration, and carbohydrate intake to prevent bonking (hypoglycemia, due to glycogen depletion). We are seemingly constantly barraged with ads and information about various hydration, carbohydrate, and electrolyte products in sports magazines, social media posts, and sports-related e-mails. But do you really understand the electrolyte component of these products and why they are so important?

Electrolytes, as far as we athletes should be concerned, refer mainly to sodium (Na+) and potassium (K+). Na+ and K+ are very tightly regulated in the body. If an imbalance occurs, bad things happen. The most common and most dangerous electrolyte abnormality in endurance athletes is hyponatremia. In this article, I will discuss what hyponatremia is, what causes it, why should we care about it, and how can we avoid it.

Sodium is important for proper function of almost every cell in our bodies. We loose a lot of Na+ (as salt) when we sweat and it needs to be replaced. Hyponatremia, simply put, is low blood sodium. It is a silent menace since we often have no clue it is occurring until it becomes a serious problem.

There are several causes of hyponatremia including medications (e.g. diuretics, anti-depressants), liver failure, congestive heart failure, and other medical conditions but by far, the main cause of hyponatremia in endurance athletes is inadequate sodium intake to replace what has been lost and rehydrating with water which dilutes the sodium concentration of the blood.

The symptoms of hyponatremia are mainly neurologic and are related to the rapidity of the change in Na+ concentration in addition to the severity (how low the Na+ level is) of hyponatremia. When hyponatremia occurs in a race situation, it is typically rapid onset. The body tries to equalize the Na+ concentration in cells with that of the blood so when blood Na+ levels fall, water is transported into cells to even out the concentration. Brain cells fill up with water and swell, which is why many of the symptoms related to hyponatremia are neurologic in basis. Early symptoms of hyponatremia include nausea and fatigue but as the blood Na+ level continues to drop, symptoms can progress to headache, severe lethargy, obtundation (an altered level of consciousness), seizures, coma, and even respiratory arrest leading to death. Note that I did not state muscle cramps as an early symptom. In fact, muscle cramps, which are often attributed to electrolyte imbalances, are not typical of hyponatremia so we cannot rely on this symptom as an indicator that our sodium levels are getting low.

Once a person becomes hyponatremic, the reversal of this severe and, at-times, life-threatening condition often requires hospitalization and very close management by a physician knowledgeable and skilled in its treatment. Simply consuming a lot of salt will likely be inadequate and can lead to other potentially detrimental effects.

The good news is that hyponatremia is entirely preventable. Knowing your salt loss during exercise (by doing a sweat test for example) can give you the information needed to devise a strategy for salt replacement. Salt loss will vary depending on the ambient temperature so performing a sweat test in various climate conditions can yield important information that you can use throughout the race season. Whether you hydrate with fluids containing carbohydrates or not, depending on the duration of your exercise, it is always recommended to hydrate with fluids containing electrolytes rather than plain water. This is going to be especially important if you train long hours in the heat or race long course triathlons (including 70.3 and 140.6 distances) where salt losses are large. In my opinion, it would be money well spent to hire a good sports nutritionist to help you plan your hydration and electrolyte needs while training and racing. It could just save your life!

Dr Jeff

Dr. Jeff Krebs is a Board Certified Internal Medicine physician specializing in Hospital Medicine. He has practiced in San Diego for over 34 years. As a long-time competitive athlete, he has a special interest in sports medicine.