Glossary

Hyponatremia

Updated April 9, 2026

Hyponatremia means blood sodium concentration has fallen too low, usually below 135 mmol/L. In nutrition practice it matters most in endurance sport, where athletes sometimes drink more fluid than they can clear while replacing little sodium. The Complete Guide to Hydration and How to Set Up a Race-Week Nutrition Plan both point to this problem because it can turn a well-meant hydration plan into a medical emergency.

What causes it

In exercise settings, hyponatremia usually comes from overdrinking relative to sweat loss and urine output. Water intake rises faster than sodium can be maintained, so plasma sodium becomes diluted. Hew-Butler and colleagues summarized the consensus position in 2015 and kept the same prevention logic in the 2019 Wilderness Medical Society update, with overconsumption of hypotonic fluids as the central driver in most cases.¹²

This is why the phrase "drink as much as possible" has done real harm in endurance sports. The safer rule is to let thirst, conditions, and known sweat-rate data shape intake.

What the evidence says

The most useful point is that sodium capsules do not rescue a bad drinking plan. Hoffman and Stuempfle reported in 2015 that sodium supplementation did not clearly prevent exercise-associated hyponatremia during prolonged exercise when athletes still drank according to their own behavior.³ McCubbin and da Costa found in 2024 that personalized sodium replacement raised plasma sodium concentration more than placebo during a 5-hour hot run, though overall body-water balance and thermophysiological strain did not change much.⁴

That result is helpful because it draws the line clearly. Sodium matters. It does not cancel out overdrinking.

The condition remains defined by the lab value, though the field signs matter first. Hyponatremia can present with headache, nausea, swelling, confusion, vomiting, and in severe cases seizures, respiratory compromise, coma, or death.¹²

How athletes get into trouble

Most athletes do not plan to overdrink. They slide into it through anxiety, fixed bottle schedules, or a belief that thirst is too late. Slower marathoners, triathletes, ultrarunners, and anyone taking fluid at every station without matching conditions are the group that shows up most often in the case series and consensus documents.¹²

Weight gain matters because it is one of the fastest field clues. A person who finishes heavier than they started during a long race has very likely retained excess fluid.

Prevention that holds up

The prevention rule is less dramatic than the internet makes it sound. Drink to thirst or to a measured plan built from repeated race-like sessions. Use sodium-containing fluids or foods when the event is long enough or hot enough to justify them. Do not force intake above what the conditions require.

This is where sodium-intake, hydration, and electrolyte-balance need to be read together. Sodium helps. A sound fluid plan helps more.

Where athletes misread the risk

Hyponatremia usually gets misread as the opposite of dehydration, as if one problem always cancels the other. In real races, the fear of dehydration often drives the oversized drinking plan that creates hyponatremia in the first place.

Sodium also gets asked to do more than it can do. It can support fluid retention and improve a long-event plan, though it does not make unlimited drinking safe. A large fluid plan stays large even when sodium capsules are added on top.

Risk always depends on exposure. A smaller athlete moving for five hours has a different margin for error than a larger athlete racing for ninety minutes in cool weather. Body size, pace, aid-station frequency, and total event time all belong in the same decision.

If this topic is relevant to your training, keep Sweat Rate, Sodium Intake, and Hydration in the same reading path. Good hydration strategy is usually about stopping the plan from getting too large, not pushing it to extremes.