We tend to worry about dehydration during physical activity, but athletes also need to be aware of the risks of over-hydration. The result is a condition is known as hyponatremia and, like dehydration, can be very dangerous.
On the milder side, drinking too much fluid can cause bloating and discomfort. But when over-hydration is severe, it can be very serious, and even fatal. Hyponatremia occurs when the sodium concentration in your blood gets too diluted by excess fluid; this can happen when you drink more fluid than you lose through sweating. As an electrolyte, sodium helps regulate the amount of water in the fluid around your cells; this important balance is disturbed when your sodium levels are too low.
We tend to hear more about sodium as a bad thing for our health, as too much of it can raise blood pressure and increase risk for cardiovascular and other health issues. But we also need sodium—it helps maintain blood pressure and supports nerve, muscle, and tissue function.
Most sodium in the body is found in fluids outside the cells. If your sodium levels become too low (hyponatremia), fluid moves into the cells, causing the cells to swell with water, particularly brain cells.
Athletes Are at Risk
The risk of hyponatremia among endurance athletes may be more common than you realize. Though long-distance runners and cyclists tend to be more concerned about dehydration, research has shown the over-hydration is also a significant risk. In a 2014 study of endurance cyclists, most participants reported that they were concerned with dehydration during a long-distance event. After an almost 250-mile race, however, only two of 18 cyclists appeared to be moderately dehydrated, whereas almost 40% of participants had sodium levels in the low-normal or below-normal range.
Symptoms of hyponatremia include:
- Loss of appetite
- Muscle spasms or cramps
- Muscle weakness
Hyponatremia can be very dangerous and even life threatening. If you have symptoms, call your healthcare provider or dial 9-1-1 immediately. Treatment for hyponatremia includes IV fluids with electrolytes, medication, and restriction of how much water you drink.
You can reduce your risk of hyponatremia by monitoring your urine—if you’re urinating regularly and it’s clear in color, you’re likely getting enough fluids and don’t need to increase intake. You may also want to consider a sports drink with electrolytes during activity, so that you’re replacing sodium as well as fluids.
 Black KE, Skidmore P, Brown RC. Fluid Balance of Cyclists during a 387-km Race. European Journal of Sport Science. 2014;14 Suppl 1:S421-8. doi: 10.1080/17461391.2012.711860.