Drinking too much water during a long workout might sound harmless, but if sodium levels in the blood drop too low, a condition called hyponatremia occurs. It is one of the most serious yet most preventable risks in endurance training.
What is hyponatremia?
Hyponatremia means that the concentration of sodium in the blood falls below normal levels, usually defined as below 135 mmol/L. During exercise, it is called exercise-associated hyponatremia (EAH) and most often occurs when you drink more fluid than the body loses through sweat and urine, while sodium losses are not compensated for.
What causes hyponatremia during exercise?
Research points to several recurring risk factors: excessive fluid intake relative to sweat loss, prolonged exercise (especially over 4 hours), lower body weight, and an inappropriate secretion of the hormone vasopressin which causes the body to retain fluid. Studies have also shown that weight gain during a race – a sign of drinking more than you have lost – has a clear link to lower sodium levels in the blood afterward.
Symptoms
With mild hyponatremia, symptoms are often vague or non-existent – nausea, headache, confusion, or swollen hands and feet. With more pronounced hyponatremia (below approximately 120 mmol/L), brain swelling can occur, which can cause seizures, unconsciousness, and in severe cases, respiratory failure. The symptoms can easily be mistaken for dehydration, making the condition extra risky because the wrong action – drinking even more water – worsens it.
Who is most at risk?
Endurance athletes and ultra-runners are the most studied, especially in races lasting longer than four hours in warm climates. Even less experienced recreational athletes who drink according to a fixed schedule rather than according to thirst, instead of listening to the body’s signals, are at increased risk.
How to reduce the risk
A few simple principles significantly reduce the risk of hyponatremia:
- Drink according to thirst rather than a strict schedule during long sessions.
- Weigh yourself before and after longer workouts – weight gain is a warning sign of overhydration, not a sign that you managed your fluid balance well.
- Supplement with sodium during sessions longer than about two hours, especially in the heat, instead of just drinking water.
- Be attentive to symptoms like headache, nausea, or confusion during and after exercise – if unsure, seek medical care.
Hyponatremia and sodium supplements
Since one of the root causes of hyponatremia is that fluid intake is not matched by sufficient sodium intake, it is one of the reasons why electrolyte supplements with sodium – rather than plain water – are recommended for longer and intense sessions. Each ELECT sachet contains 1,000 mg of sodium, making it easier to replenish sodium while you hydrate. If you want to estimate your own needs based on training type, intensity, weight, and weather, you can use our calculator. You can also read more about sodium needs for running and endurance training.
Please note: this article is for general information and does not replace medical advice. In case of symptoms of hyponatremia or severe dehydration, seek medical care.
Sources
- Hew-Butler T, et al. Exercise-Associated Hyponatremia: 2017 Update. PMC.
- Exercise-Associated Hyponatremia in Marathon Runners. Journal of Clinical Medicine, MDPI.
- Exercise-Associated Hyponatremia in Endurance and Ultra-Endurance Performance: A Narrative Review. PMC.
