Medical emergency: If you or someone else is experiencing confusion, seizures, difficulty breathing, or loss of consciousness after consuming large amounts of water, call emergency services immediately. This article is educational — it is not a substitute for emergency care.
What is hyponatremia?
Hyponatremia — sometimes called water intoxication in its acute, exercise-related form — occurs when the sodium concentration in the blood falls below a safe level. Sodium is an electrolyte that plays a critical role in maintaining fluid balance inside and outside cells, supporting nerve function, and regulating blood pressure.
When large amounts of fluid (particularly plain water without sodium) are consumed rapidly, the kidneys may be unable to excrete the excess fluid fast enough. Blood sodium becomes diluted. Cells — including brain cells — begin to take on water and swell. This is what causes the neurological symptoms associated with severe hyponatremia.
Hyponatremia can occur in multiple clinical contexts — it is not exclusively caused by drinking too much water. Certain medical conditions and medications can also cause or contribute to it. This page focuses on the form most relevant to general hydration: dilutional or exercise-associated hyponatremia.
How common is it?
For healthy adults following general hydration guidance and not engaging in extreme endurance exercise, overhydration to the point of clinical hyponatremia is uncommon. The kidneys of a healthy adult can excrete approximately 800–1,000ml of water per hour under normal conditions, which means ordinary daily drinking — even at the higher end of general recommendations — does not typically overwhelm normal renal function.
The situations where overhydration becomes a genuine risk are more specific: prolonged endurance exercise with aggressive water-only intake, extreme wellness protocols recommending very high daily water volumes, or the presence of underlying medical conditions affecting fluid regulation.
Exercise-associated hyponatremia.
Exercise-associated hyponatremia (EAH) is the most well-documented form in otherwise healthy individuals. It has been reported primarily in endurance sports events — particularly marathon and ultra-marathon running, triathlons, and military training — and is more common in female athletes, slower runners (who spend more time on course), and those who aggressively follow "drink as much as possible" strategies.
The American College of Sports Medicine, in its exercise and fluid replacement guidelines, explicitly recommends against drinking to a set schedule during endurance events. Current guidance from ACSM and other sports medicine bodies recommends drinking to thirst during exercise — a strategy that research suggests naturally prevents both dehydration and overhydration in most athletes.
For exercise sessions lasting longer than 60–90 minutes, particularly in heat, including sodium (via electrolyte drinks or food) is widely recommended to help prevent dilutional hyponatremia.
Possible signs of overhydration.
These symptoms have many possible causes. The appearance of any severe symptom warrants emergency medical attention regardless of cause.
Who is at higher risk?
Endurance athletes
Marathon runners, triathletes, and ultra-endurance athletes are the most commonly affected group. Drinking only plain water (without sodium) during events lasting several hours can dilute blood sodium. The practice of drinking "as much as possible" — rather than to thirst — during endurance events has been associated with exercise-associated hyponatremia.
Individuals following extreme water intake advice
Some wellness and detox trends recommend drinking very large quantities of water per day — quantities well above established adequate intake benchmarks. Following such advice without medical supervision carries risk, particularly for people with conditions affecting kidney or heart function.
People with certain medical conditions
Conditions that impair the body's ability to excrete excess water — including kidney failure, heart failure, liver cirrhosis, and certain hormonal disorders — can make overhydration a genuine risk even at moderate fluid intakes. People with these conditions should have fluid intake guided by their healthcare provider.
Young children
Infants and young children have smaller body sizes and less developed kidney function. Giving infants large amounts of plain water — rather than breast milk or formula — can be dangerous. Paediatric hydration should always follow guidance from a healthcare provider.
Individuals using certain medications
Some medications (including certain antidepressants, NSAIDs, and diuretics) can affect fluid balance or sodium levels. People taking such medications should discuss appropriate fluid intake with their prescribing physician.
Evidence-informed safe hydration guidance.
For healthy adults in ordinary daily life, following general hydration guidance does not carry a meaningful overhydration risk. The following principles reflect current evidence from ACSM, the National Academies, and sports medicine research:
- Drink to thirst — For most healthy adults in non-exercise daily life, thirst is a reliable guide. Drinking to satisfy thirst without forcing excess intake is appropriate for most people.
- During exercise, drink to thirst — ACSM guidance recommends drinking to thirst during exercise, rather than to a rigid volume schedule, as a strategy to prevent both dehydration and overhydration.
- Include electrolytes for long exercise — For sessions lasting over 60–90 minutes, particularly in heat, include sodium to prevent dilutional effects from plain-water intake.
- Use urine colour as a rough guide — Pale straw-yellow urine is a practical indicator of adequate hydration. Persistently colourless urine may suggest drinking more than needed.
- Consult a professional if you have medical conditions — Anyone with conditions affecting kidney, heart, or liver function, or who is pregnant, should have fluid intake guided by a healthcare provider rather than general online calculators.
About our calculator: The water intake calculator on this site provides general estimates for healthy adults. It is not designed for people with medical conditions affecting fluid balance, and its outputs should be treated as a starting point rather than a prescription. Always discuss any concerns about fluid intake with your healthcare provider. Full medical disclaimer →
Sources & References
- • American College of Sports Medicine (ACSM) — Exercise and Fluid Replacement Position Stand.
- • Rosner MH & Kirven J. — Exercise-associated hyponatremia. Clin J Am Soc Nephrol. 2007.
- • Speedy DB et al. — Hyponatremia in ultradistance triathletes. Med Sci Sports Exerc. 1999.
- • National Academies of Sciences — Dietary Reference Intakes for Water (2004).
- • Noakes TD. — Overconsumption of fluids by athletes. BMJ. 2003.
- • NHS (UK) — Hyponatremia clinical guidance.