The single most effective way to avoid hyponatremia is to stop drinking more fluid than your body loses. Hyponatremia occurs when blood sodium drops below 135 mEq/L, almost always because there’s too much water in the body relative to sodium, not because you haven’t eaten enough salt. Your kidneys can process roughly 600 to 900 mL of water per hour under normal conditions, so exceeding that rate consistently, or drinking well beyond your sweat losses during exercise, sets the stage for dangerous dilution.
Why Overhydration Is the Main Problem
Sodium is the primary mineral that keeps fluid balanced between and within your cells. When you take in more water than your body can excrete, the extra fluid dilutes sodium in your bloodstream. Water then shifts into your cells to equalize the concentration, causing them to swell. When brain cells swell, symptoms appear: headache, nausea, confusion, and in severe cases (sodium below 120 mEq/L), seizures or coma.
This process can be driven by drinking too much plain water, losing too much sodium through prolonged sweating, or a combination of both. Some medical conditions and medications also impair the kidneys’ ability to clear excess water, making even moderate fluid intake risky.
Match Your Fluid Intake to Your Sweat Losses
The most reliable prevention strategy during exercise is simple: don’t drink more than you sweat out. Weigh yourself before and after a workout without replacing fluids. Every kilogram (2.2 pounds) lost equals roughly one liter of sweat. That number is your hourly sweat rate for that intensity and temperature, and it becomes your target intake for similar sessions.
If you don’t know your sweat rate, drinking when you’re thirsty is a safe fallback. The National Athletic Trainers’ Association identifies thirst-based drinking as likely sufficient to prevent overdrinking during activity. Competitive athletes who need to optimize performance may find that thirst lags behind actual losses, so a personalized hydration plan based on measured sweat rate works better for them.
A key rule of thumb: you should never gain weight during exercise. Weight gain during a long workout or race signals that you’ve consumed more fluid than you’ve lost, which is the primary setup for exercise-associated hyponatremia. Weigh yourself before and after events to confirm you’re staying on track.
Add Sodium During Long Exercise
For any activity lasting more than an hour, plain water alone isn’t ideal. The American College of Sports Medicine recommends including 0.5 to 0.7 grams of sodium per liter of fluid consumed during prolonged exercise. That’s roughly the concentration found in most commercial sports drinks, though some people with heavy, salty sweat may need more.
Sports drinks containing 4% to 8% carbohydrates, consumed at a rate of 600 to 1,200 mL per hour, can deliver both energy and electrolytes without slowing fluid absorption. But even a sodium-containing drink will cause problems if you drink it in volumes that far exceed your sweat output. The sodium helps, but volume control matters more.
For ultra-endurance events lasting four hours or longer, the risk climbs. Athletes who hydrate excessively with water or low-sodium beverages during these events are the classic candidates for symptomatic hyponatremia. Salty snacks, electrolyte capsules, or higher-sodium drink mixes become more important as duration increases.
Know the Symptoms
Hyponatremia and dehydration produce nearly identical early symptoms: nausea, headache, muscle cramps, and fatigue. This overlap is dangerous because a dehydrated person should drink more, while a hyponatremic person should not. Without a blood sodium test, the most useful clue is body weight. If you’ve been drinking steadily and your weight is the same or higher than when you started, low sodium is more likely than dehydration. If you’ve lost significant weight during activity, dehydration is the more probable cause.
As sodium drops further, symptoms progress to confusion, disorientation, and vomiting. Severe cases can involve seizures. These typically occur when sodium falls below 120 to 125 mEq/L, especially when the drop happens rapidly over less than 24 hours.
Medications That Raise Your Risk
Certain prescription drugs impair your kidneys’ ability to excrete excess water. The two most common categories are diuretics (particularly thiazide-type water pills) and antidepressants in the SSRI class, which includes medications like sertraline, citalopram, and fluoxetine. Studies report that up to 15% of SSRI users develop some degree of low sodium, though most cases are mild.
These medications work through different pathways but share a common result: your kidneys hold onto more water than they should. If you take either type, the risk compounds when combined with high fluid intake, hot weather, or endurance exercise. Being aware of this interaction is the first step. If you’re on one of these medications and planning a marathon or similar event, discussing a hydration strategy with a doctor beforehand is worth the effort.
Why Older Adults Are Especially Vulnerable
Aging changes your body’s water balance in several ways at once. By age 75 to 80, total body water content drops to around 50%, which means there’s less buffer against dilution. At the same time, the thirst mechanism weakens, the kidneys become slower at excreting a water load, and levels of antidiuretic hormone (the hormone that tells kidneys to retain water) tend to run higher than necessary for any given level of hydration.
The result is a narrow margin of safety. Roughly 7% of healthy older adults walk around with sodium levels at or below the low end of normal. In nursing homes, over 50% of residents experience at least one episode of hyponatremia during a 12-month period, often triggered by medications, illness, or intravenous fluids given during hospitalization.
For older adults, prevention means careful attention to both fluid intake and medication use. Drinking large volumes of plain water “to stay healthy” without accounting for reduced kidney function can push sodium down. When diuretics, SSRIs, or other implicated medications are in the mix, periodic blood sodium checks become important, especially after starting a new drug or during illness.
Practical Prevention Checklist
- Know your sweat rate. Weigh yourself before and after exercise to calculate hourly fluid losses, then match your intake to that number.
- Never gain weight during exercise. If you finish heavier than you started, you drank too much.
- Drink to thirst when in doubt. If you don’t have a personalized plan, your thirst response is a reliable guard against overdrinking.
- Include sodium after the first hour. Use a sports drink or electrolyte mix containing 0.5 to 0.7 grams of sodium per liter for prolonged activity.
- Be cautious with medications. Thiazide diuretics and SSRIs both impair water excretion. Factor this into your hydration approach.
- Watch fluid intake during illness. Vomiting, diarrhea, and fever can disrupt sodium balance. Oral rehydration solutions are better than plain water when you’re sick and losing fluids.
- Avoid aggressive “hydration challenges.” Drinking a gallon of water a day regardless of your size, activity, or climate ignores your body’s actual needs and can push sodium below safe levels.