Hyponatremia is a serious condition of low blood sodium. It’s a leap past sodium deficiency, but remarkably the earliest signs and symptoms can oftentimes be quite subtle.
Exercise-associated hyponatremia is seen most often in endurance athletes. They lose both sodium and fluids through sweat, but they focus only on aggressively replacing the fluids. Electrolytes are, at best, an afterthought. That’s a recipe for hyponatremia. Drinking too much plain water causes blood sodium levels to plummet.
Since sodium is crucial for brain function, the hallmarks of hyponatremia are neurological. Today I’ll be diving into those signs and symptoms to help you identify them in yourself and others. That way, we can take corrective measures before anyone needs medical attention.
What Is Hyponatremia?
Hyponatremia is a condition of low serum sodium. A person is considered hyponatremic when blood sodium levels fall below 135 mEq per liter.
Ultimately, hyponatremia is a problem of fluid balance. When someone has low sodium levels, it means they have excess body water and insufficient solutes (aka, sodium) in their blood.
Normally, the human body excels at preventing fluid imbalances. It’s always monitoring the osmolality (concentration of solutes) in your blood — and it’s always ready to make adjustments if things get wonky. Here are few examples:
- If blood osmolality is too high, your body increases sodium excretion through urine. Rising osmolality also triggers thirst. Then you drink water and fluid balance normalizes.
- If blood osmolality is too low, your body secretes aldosterone to reabsorb more sodium through the kidneys.
- If blood volume is too high — from, say, drinking too much water — your brain suppresses antidiuretic hormone (ADH) so you can pee out the excess.
There are many more examples, but you get the idea. When the fluid balancing system runs properly, hyponatremia is rare. But this system can be derailed in a number of ways.
What Causes Hyponatremia?
When someone has hyponatremia, it means that either something is wrong with their fluid balancing system, or their fluid balancing system has become overwhelmed.
To the first point, various medical conditions can derail our fluid and electrolyte balance. The most common are kidney failure, heart failure, liver disease, cancer, and any illness that causes vomiting or diarrhea. Less common medical conditions include cystic fibrosis, Gitelman syndrome, Bartter syndrome, hypovolemic POTS, orthostatic hypotension, and Addison’s disease. Many drugs can also result in hyponatremia. This list includes diuretics, oxytocin, SSRIs (antidepressants), nicotine, antipsychotics, NSAIDs (anti-inflammatories), and more.
All of those medical causes are beyond the scope of this article. The less-talked-about issue which I want to discuss today is drinking too much plain water. When you overhydrate with plain water, you overwhelm the body’s fluid balancing system, temporarily diluting blood sodium levels. Many elite endurance athletes have experienced this danger firsthand, on account of well-meaning but dangerous advice that more water is always better.
Exercise-Associated Hyponatremia
Elite endurance sports create the perfect storm of hyponatremia risk. Athletes are often encouraged to drink beyond thirst, and most marathon courses have watering stations every couple of miles. This water, unfortunately, does not contain sodium.
Exercise also chews through glycogen (your body’s stored glucose), for energy. Unfortunately, this simultaneously releases water that may contribute to fluid imbalance. Lastly, exercise can prevent you from peeing.
When you combine all of these factors with overwatering, it’s not surprising that a large percentage of endurance athletes wind up with asymptomatic cases of hyponatremia, and up to 1% of endurance athletes have symptomatic hyponatremia.
The symptomatic cases are no joke. Athletes stagger across the finish line like confused zombies. They need to be rushed to the medical tent to prevent serious damage being done.
Signs and Symptoms of Mild Hyponatremia
A person with marginally low sodium levels will often present with headaches, brain fog, muscle cramps, low energy, and fatigue. They feel “off,” in other words. That said, these symptoms are obviously not a guarantee that a person is hyponatremic — they overlap with many other conditions like heat exhaustion, hypovolemia, hypoglycemia, and ironically, dehydration.
Proper diagnosis should be handled by a medical professional. However, since blood sodium tests aren’t always available on command, it’s important to pay attention to the context. Was Johnny glugging plain water like a bathtub before his soccer game? That points away from dehydration, and toward sodium imbalance.
If hyponatremia is indeed to blame, the most common course of action is to restrict fluid intake and halt exercise until you pee. Urination restores blood sodium levels.
Signs and Symptoms of Moderate to Severe Hyponatremia
Moderate to severe hyponatremia is serious stuff. It’s a life-threatening condition.
In his book Waterlogged: The Serious Problem of Overhydration in Endurance Sports, Dr. Tim Noakes covers many of the points I just covered and more. His accounts of treating hyponatremic runners are super interesting.
The usual presentation, according to Noakes, is confusion, loss of consciousness, and epileptic seizures. “The other presentation that I frequently see is that people who finish the race become very withdrawn,” Noakes is quoted as saying. “They lie down in the fetal position. They don’t want to speak to anyone. They don’t want to look at the lights.”
For my list-lovers, here’s a list of moderate to severe hyponatremia symptoms:
- Confusion
- Seizures
- Lethargy
- Light sensitivity
- Loss of consciousness
- Brain damage
- Brain swelling
- Death
In these cases, you really need a doctor to call the shots. Along with restricting water intake and halting exercise, a doctor may decide to give the patient super salty fluids, either orally or through an IV. Fortunately, this can restore the patient fairly rapidly.
How To Prevent Exercise-Associated Hyponatremia
Fortunately, preventing hyponatremia is much less complicated than treating it. There are just two rules:
- Drink according to thirst (not beyond it)
- Stay Salty
Thirst is nature’s way of simultaneously preventing dehydration and overhydration. It’s a brilliant mechanism for maintaining fluid balance. Athletes who drink to thirst are unlikely to develop exercise-associated hyponatremia, whereas athletes who drink on a set schedule (or as much as they can) are playing with fire.
The second rule is to stay salty. Healthy people are likely to benefit from a baseline of 4–6 grams of sodium (2–3 teaspoons of salt) per day. Super sweaty athletes may need more than that to replace sodium lost via sweat. If you’d like to replace your fluid and electrolyte losses precisely, follow these guides to calculate your sweat rate and sweat sodium concentration. To stay salty, become good buddies with the salt shaker and try an electrolyte drink mix like LMNT.
In a sentence: Drink electrolyte water to thirst, and you’ll avoid hyponatremia without even thinking about it. I hope this article taught you something new!
