Exercise-Associated Hyponatremia (EAH) is a potentially dangerous condition where the blood’s sodium concentration drops below 135 millimoles per liter during or within 24 hours of sustained physical activity. This imbalance is primarily caused by overhydration with hypotonic fluids, such as plain water or standard sports drinks, which dilutes the sodium content in the bloodstream. The resulting dilutional hyponatremia can lead to cerebral edema, which is the swelling of the brain. In severe cases, this can cause seizures, coma, or even death. Prevention strategies focus on balancing fluid volume and sodium intake to maintain a safe sodium concentration throughout prolonged exercise.
Controlling Fluid Volume: The Thirst-Based Approach
The most effective method for preventing dilutional hyponatremia is to follow the body’s natural guidance system and simply drink to thirst. This approach allows the body’s osmoreceptors to regulate plasma osmolality and circulating volume, preventing both dehydration and overhydration. Drinking on a fixed, scheduled basis, such as consuming a set amount of fluid every 15 minutes, is now considered a dangerous practice that often leads to excessive fluid intake.
Overconsumption of hypotonic fluids is the main mechanism causing EAH. When a person drinks more water than their body loses through sweat, urine, and insensible losses, they create a positive fluid balance. The excess water retained in the body dilutes the total sodium content, causing the serum sodium concentration to drop.
The stress of prolonged exercise can also inappropriately stimulate the release of arginine vasopressin (AVP), or antidiuretic hormone. This hormone signals the kidneys to retain water, compounding fluid retention even if the athlete is not significantly overhydrating. Limiting fluid intake to what the thirst mechanism dictates helps athletes avoid overwhelming the body’s ability to excrete excess water, preventing sodium dilution.
Strategic Sodium Intake and Electrolyte Use
While overhydration is the main cause of EAH, sodium loss through sweat can be a contributing factor, particularly during ultra-endurance events. Sweat is hypotonic, meaning it contains less sodium than blood plasma, but cumulative losses over many hours can be substantial. The American College of Sports Medicine suggests that athletes in prolonged events should aim for a sodium intake between 300 and 600 milligrams per hour to maintain fluid balance.
The concentration of sodium in sweat varies significantly between individuals, sometimes referred to as being a “salty sweater.” Some athletes may lose as little as 200 milligrams of sodium per liter of sweat, while others can lose over 1,500 milligrams per liter. Standard sports drinks, which typically contain about 460 milligrams of sodium per liter, may be sufficient for the average person, but they are not designed to correct a dilutional issue.
Athletes participating in events lasting longer than four hours, or those who are known to be heavy sweaters, may need to supplement their intake with higher sodium concentrations. This strategic replacement can involve using sodium-enhanced sports drinks, salty snacks, or sodium capsules. Sodium supplements alone cannot prevent EAH if an athlete continues to over-consume fluids. The goal is to balance both fluid volume and sodium concentration to avoid dilution and electrolyte deficit.
Identifying Risk Factors and Pre-Event Planning
Prevention of EAH requires athletes to be aware of their personal risk factors and incorporate careful planning before and during an event. A primary risk factor is longer exercise duration, particularly events lasting over four hours, which increases the time available for fluid imbalance to develop. Other factors include a slow running pace, inexperience with endurance events, and extremes in body mass index (BMI).
The use of non-steroidal anti-inflammatory drugs (NSAIDs) like ibuprofen before or during an event is also a risk factor, as these drugs can impair kidney function and promote water retention. Athletes should monitor their body weight before and after long training sessions to estimate their individual fluid loss rate. A weight gain during exercise is a clear indication of overhydration and necessitates an immediate reduction in fluid intake.
Athletes and support crews should be familiar with the early warning signs of EAH, which can include persistent nausea, vomiting, dizziness, and malaise. If these mild symptoms are coupled with a noticeable weight gain, fluid restriction and the consumption of oral sodium (like salted pretzels or a hypertonic saline solution) may be necessary. Recognizing these signs and adjusting hydration strategy in real-time is an important part of prevention.