Hyponatraemia and Endurance Sports: When Plain Water Isn't Enough — And Drinking Too Much Becomes the Risk

"Drink more water" is good general advice — but for endurance athletes, drinking too much water relative to sodium intake has caused fatalities, and understanding why reveals the limits of simple hydration targets

Exercise-associated hyponatraemia (EAH) — dangerously low blood sodium levels caused by a combination of excessive fluid intake and sodium losses during prolonged exercise — has been documented in marathon and ultra-endurance events, and in rare severe cases has been fatal. This is the opposite problem from dehydration, but it's caused partly by an over-application of "drink plenty of water" advice in a context where the calculation needs to be more nuanced.

What hyponatraemia is and why it happens during exercise

Hyponatraemia means abnormally low concentration of sodium in the blood. During prolonged exercise (typically the cases most documented involve exercise durations of several hours, such as marathons or longer events), it can develop through a combination of:

Sodium loss through sweat: sweat contains sodium (the exact concentration varies between individuals — "salty sweaters" lose proportionally more sodium than others), and over several hours of sweating, meaningful sodium losses can accumulate.

Fluid intake exceeding fluid losses: if someone drinks more fluid (especially plain water, with no sodium) than they're losing through sweat and other routes, this dilutes the blood's sodium concentration — and combined with ongoing sodium losses through sweat, blood sodium can drop to dangerously low levels.

The key insight: EAH isn't primarily caused by "not enough sodium intake" in isolation — it's caused by the relationship between fluid intake and sodium balance. Someone who drinks exactly to match their fluid losses, even with zero sodium intake, is less likely to develop dilutional hyponatraemia than someone who drinks more than their fluid losses, even if that person has some sodium intake.

Why "drink as much as possible" advice contributed to this problem historically

In earlier decades, some hydration guidance for endurance events emphasised drinking as much as tolerable, partly motivated by concerns about dehydration and heat illness — concerns that are legitimate, but the "more is better" framing for fluid intake specifically has been revised in subsequent guidance.

Current guidance generally emphasises:

• Drinking according to thirst, rather than according to a fixed schedule or a "drink as much as possible" target, as a primary strategy for many recreational participants in endurance events
• Being aware that thirst is generally a reasonably reliable guide for most people in most conditions, contrary to some earlier messaging that suggested thirst couldn't be trusted

Why thirst-based drinking helps prevent EAH: thirst responds to the body's actual fluid balance — drinking only when thirsty tends to result in fluid intake that's closer to (though not perfectly matched to) actual fluid losses, compared to drinking on a fixed schedule regardless of thirst, which can lead to overdrinking in some individuals, particularly slower participants in long events who have more time to drink and may be losing fluid at a slower rate than faster participants.

Who is at higher risk

Research on documented EAH cases has identified some patterns, though individual risk assessment isn't precise:

Event duration: longer events (where there's more time to both lose fluid through sweat and to drink) are associated with higher reported EAH risk compared to shorter events — marathon and ultra-distance events feature more prominently in EAH case reports than shorter races.

Body size and sex: some research has found smaller body size may be associated with higher EAH risk — possibly because smaller individuals have a smaller total blood volume, meaning a given amount of excess fluid intake represents a larger dilutional effect. Some studies have found differences in EAH incidence between sexes, though the relationship is complex and likely involves multiple factors including body size, sweat rate, and drinking behaviour patterns rather than sex alone.

Slower finishing times: participants who take longer to complete an event have more time both to drink fluids and potentially to overdrink relative to their actual fluid losses (which may also be lower at slower paces, with lower sweat rates) — this combination has been identified as a risk factor in some research.

Non-steroidal anti-inflammatory drug (NSAID) use: some research has examined a possible association between NSAID use during endurance events and hyponatraemia risk, though this remains an area of ongoing investigation rather than a settled finding.

Sodium intake during exercise: what it does and doesn't do

Sodium-containing sports drinks and electrolyte supplements are often marketed with hyponatraemia prevention as part of their value proposition. The actual relationship is more nuanced:

Sodium intake alone doesn't prevent EAH if fluid intake significantly exceeds losses: as noted above, the core issue is the fluid balance — drinking large volumes of even a sodium-containing drink, in excess of fluid losses, can still contribute to dilutional effects, just to a lesser degree than drinking the same volume of plain water.

Sodium intake may help replace ongoing sodium losses: for events long enough that sodium losses through sweat become significant, replacing some of this sodium through diet/drinks during the event is a reasonable component of an overall strategy — but this is a complement to appropriate fluid intake, not a substitute for it.

Individual sweat sodium concentration varies substantially: some people lose much more sodium per litre of sweat than others ("salty sweaters," sometimes identifiable by visible salt residue on skin or clothing after exercise) — for these individuals, sodium replacement during longer events may be more relevant than for someone with lower sweat sodium concentration.

Practical guidance synthesis

For most people doing most exercise (shorter durations, moderate conditions), the risk of EAH is low, and general hydration guidance (drink according to thirst, have access to fluids during exercise) is appropriate without needing detailed sodium calculations.

For longer endurance events (several hours or more), particularly in hot conditions:

• Drinking according to thirst is generally a reasonable primary strategy for most participants
• Avoiding a fixed "drink X amount per hour regardless of thirst" approach, particularly avoiding overdrinking relative to thirst
• For those who sweat heavily or are prone to cramping, including some sodium intake (sports drinks, electrolyte tablets, salty snacks) as part of a fuelling strategy is reasonable, alongside (not instead of) thirst-appropriate fluid intake
• Awareness of EAH symptoms (which can include nausea, confusion, swelling, and in severe cases seizures — symptoms that can be confused with dehydration or heat illness, which is part of why EAH has historically sometimes been misdiagnosed and treated with more fluids, worsening the condition) is valuable for event organisers and medical staff at endurance events specifically

How to use the Water Intake Calculator on sadiqbd.com

1. For general daily hydration: the calculator provides a reasonable starting estimate based on body weight and activity level
2. For endurance exercise specifically: treat any calculated "during exercise" fluid target as a rough guide rather than a fixed prescription — thirst-based adjustment during long events is generally more appropriate than rigidly following a calculated number
3. Combine with sodium awareness for longer events: for events lasting several hours, particularly in heat, factor in some sodium intake as part of overall fuelling, alongside thirst-guided fluid intake

Frequently Asked Questions

How can I tell if I'm a "salty sweater"? Visible white residue on skin or dark clothing after exercise (dried sweat leaving salt deposits) is a commonly cited informal indicator. Some sports science labs offer sweat testing that can provide a more precise estimate of sweat sodium concentration, which can inform individualised electrolyte strategies for those doing significant amounts of endurance training.

Is hyponatraemia only a risk during exercise? No — hyponatraemia can occur in various medical contexts unrelated to exercise (certain medications, kidney conditions, hormonal conditions, and other causes), and is a topic relevant to general medicine, not just sports medicine. The exercise-associated form discussed here is a specific subset with its own particular causes related to the fluid and sodium balance during prolonged physical activity.

Is the Water Intake Calculator free? Yes — completely free, no sign-up required.

Try the Water Intake Calculator free at sadiqbd.com — calculate your personalised daily hydration target based on your weight, activity level, and climate.