Too little fluid can add to heat strain; excessive intake can contribute to exercise-associated hyponatremia (EAH), a potentially fatal reduction in blood sodium. A fixed intake or weight target cannot guarantee safe hydration. This guide is educational, not medical advice: if you have kidney or heart disease, a history of hyponatremia, or medication that affects fluid balance, get an individual plan from a sports medicine clinician.
Correction, October 23, 2026: We have incorporated the published correction to Hoffman's body-mass model and removed the universal 80% sweat-replacement ceiling previously inferred from a sodium model. Neither model establishes a personal drinking or weight-loss target.
How Much to Drink per Hour
Drink to thirst rather than forcing a quota. The Wilderness Medical Society guideline recommends thirst-guided drinking to reduce EAH risk. The ISSN ultramarathon position stand gives a broad racing range of 450–750 mL per hour. This is context for planning fluid access, not a minimum you must force down or a maximum that fits every runner and condition.
In Costa's study of a 24-hour race, average fluid intake was about 378 mL per hour. That describes one group in one event; neither its average intake nor its average plasma-volume change establishes a safe amount for you. Weather, effort, individual losses and the ability to excrete water all matter.
| Information | What it tells you | How to use it |
|---|---|---|
| Published racing ranges | A broad reference from guidance | Plan access to fluid, then adjust to thirst and conditions |
| A rehearsal in similar conditions | Your intake, thirst and tolerance at that effort | Check bottle capacity, refill access and whether your plan needs individual advice |
| Heat, humidity or a change in effort | Conditions differ from the rehearsal | Reassess effort, cooling and fluid access rather than following the same number automatically |
| Symptoms or unexpected weight gain | A reason to reassess, not a diagnosis | Do not force fluid; seek race medical assessment for concerning symptoms |
Fluid and fuel together. Drinking 500 mL per hour of a 6% sports drink supplies 30 g of carbohydrate. This is label arithmetic, not a recommendation to drink that volume. Do not force extra fluid to meet a fuel target; consider tolerated food or gels separately. The 24-hour nutrition plan and the backyard ultra nutrition plan show how the two fit together.
Measure Your Sweat Rate
A short training-session mass-balance estimate can help you understand losses in those conditions. Baker's review describes the method and its limitations. It becomes less reliable over long races because changes in body mass include more than sweat loss.
Sweat rate (L/h) = (kg before − kg after + litres drunk − litres of urine) ÷ hours
Worked example: you weigh 70.0 kg, run for 2 hours at race effort, drink 0.8 L, pass 0.2 L of urine, and weigh 68.9 kg afterwards. (70.0 − 68.9 + 0.8 − 0.2) ÷ 2 = 1.7 ÷ 2 = 0.85 L per hour. This estimates losses in that session; it does not prescribe replacing all of them or stopping at a fixed percentage. Do not extrapolate a two-hour test unchanged across a full day.
- Weigh without clothes and towel off first; sweat trapped in clothing counts as body mass.
- Keep the test to 1–2 hours and skip food, or weigh the food and count it like fluid.
- Test once in cool conditions and once in warm conditions, because sweat rate rises with heat.
The sweat rate calculator does this arithmetic, estimates sodium lost per hour and checks a race drinking plan against the result.
Interpreting Body-Mass Changes
Body mass reflects fuel use, food and fluid intake, and changes in water balance. It is not a direct measurement of dehydration. The 2018 correction to Hoffman and colleagues' model estimated 3.0% mass loss for its average runner and 1.2–3.5% across alternative assumptions while maintaining body-water balance in a roughly 25–30-hour, 161-km mountain ultra. These are model outputs, not a weight-loss goal, a safe range for every race or a reason to restrict drinking to make a scale reading fit.
| Weigh-in | Interpretation | Action |
|---|---|---|
| Heavier than at the start | May indicate fluid intake exceeding losses; check measurement conditions and intake | Reassess forced drinking; seek medical help if symptoms accompany the change |
| Lighter than at the start | May reflect fuel use as well as fluid losses; does not rule out EAH | Consider thirst, conditions, intake and symptoms together; do not aim for a percentage |
| Any weight change with confusion, repeated vomiting or collapse | The scale cannot identify the cause | Stop and get urgent medical assessment |
How Much Sodium per Hour
Sweat sodium ranges from about 10 to 70 mmol/L, roughly 230–1,610 mg per litre (Baker 2017). Losses vary greatly; a population average or salt marks on clothing cannot determine your dose. The ISSN racing guidance discusses 500–700 mg sodium per litre of fluid and roughly 300–600 mg per hour in hot or humid conditions. These are reference ranges to interpret alongside intake, tolerance and conditions, not mandatory doses.
McCubbin's model estimates sodium requirements from fluid replacement and sweat composition. Its prolonged-exercise scenarios suggest targeted replacement becomes more relevant above 80% fluid replacement with sweat sodium of at least 40 mmol/L. That is a result about sodium in a model, not a universal prohibition on drinking above 80% of sweat losses. Individual planning may need a sports dietitian or clinician, especially with medical risk factors.
Typical sources, from labels; check your own: a regular 250 mL cup of broth carries about 800 mg, 30 g of salted pretzels about 400 mg, 500 mL of sports drink 230–290 mg, and electrolyte capsules anywhere from about 40 to 250 mg.
Sodium does not make excessive drinking safe. At Western States, 93.9% of runners used sodium supplements, and the rate did not differ between finishers with and without hyponatremia (Hoffman and Stuempfle 2015). In a 161 km race in temperatures up to 39 °C, total sodium intake ranged from 2,541 to 38,338 mg and was unrelated to cramping, hyponatremia, or nausea (Hoffman, Stuempfle and Valentino 2015). In desert stage races, sodium supplementation was not associated with protection from EAH or an increased risk of hypernatremia (Lipman 2021). These observational results do not establish a universal dose or rule out every possible benefit.
Who Gets Hyponatremia
EAH affected 6.6% of Western States finishers (Hoffman and Stuempfle 2015). Over an 80 km desert stage, 6.3% of 174 tested runners finished with EAH and 17.2% with hypernatremia, a high blood sodium that usually reflects dehydration (Lipman 2021). The runners with EAH in that study:
- were 14 kg heavier at baseline;
- had trained less;
- took 5–6 hours longer over the 80 km; and
- were more often overhydrated (67%).
These are associations within a particular study, not a checklist that diagnoses an individual runner or shows that everyone else is safe. EAH and dehydration can have overlapping symptoms. Race medical staff need the symptom history and, where indicated, blood testing to distinguish them.
Practice Plan
- Estimate losses in short rehearsals under relevant conditions and record the conditions with each estimate.
- On long runs, record fluid, sodium, food, thirst and gut response each hour.
- Practice the exact drink concentrations and salty foods you will use on race day.
- Interpret comparable weigh-ins alongside intake and symptoms, without setting a weight-loss goal.
- Write down refill locations, tested products, cooling options and when crew should seek medical help.
A crew note might read: "Offer the drinks and foods I have rehearsed; do not pressure me to finish a bottle each hour. Record intake and symptoms, reassess when conditions change, and get race medical staff for confusion, repeated vomiting or worsening symptoms."
Red Flags
| Signs | Possible cause | Action |
|---|---|---|
| Worsening headache, repeated vomiting, confusion, unusual behavior or seizure, with or without weight gain | Possible EAH or another serious illness | Stop and get urgent medical help; do not self-treat with forced fluid or salt tablets |
| Persistent intense thirst, dizziness, weakness or markedly reduced urine | Possible dehydration, heat illness or another problem | Stop exertion and get assessed; do not diagnose the problem from weight or urine alone |
| Tea- or cola-colored urine with marked muscle pain | Possible rhabdomyolysis or kidney injury | Stop and get urgent medical assessment |
Symptoms of EAH and dehydration overlap, so a blood test, not a guess, decides the treatment. For warning signs beyond fluid and sodium, see the full medical red-flag list.
Sources
- Hew-Butler, T. et al. (2015) — Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Clinical Journal of Sport Medicine, 25(4), 303–320.
- Bennett, B.L. et al. (2020) — Wilderness Medical Society Clinical Practice Guidelines for the Management of Exercise-Associated Hyponatremia: 2019 Update. Wilderness & Environmental Medicine, 31(1), 50–62.
- Tiller, N.B. et al. (2019) — International Society of Sports Nutrition Position Stand: nutritional considerations for single-stage ultra-marathon training and racing. Journal of the International Society of Sports Nutrition, 16(1), 50.
- Costa, R.J. et al. (2014) — Perturbed energy balance and hydration status in ultra-endurance runners during a 24 h ultra-marathon. British Journal of Nutrition, 112(3), 428–437.
- Baker, L.B. (2017) — Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability. Sports Medicine, 47(Suppl 1), 111–128.
- Barnes, K.A. et al. (2019) — Normative data for sweating rate, sweat sodium concentration, and sweat sodium loss in athletes: An update and analysis by sport. Journal of Sports Sciences, 37(20), 2356–2366.
- McCubbin, A.J. (2023) — Modelling sodium requirements of athletes across a variety of exercise scenarios - Identifying when to test and target, or season to taste. European Journal of Sport Science, 23(6), 992–1000.
- Hoffman, M.D. et al. (2018) — Considerations in the Use of Body Mass Change to Estimate Change in Hydration Status During a 161-Kilometer Ultramarathon Running Competition. Sports Medicine, 48(2), 243–250.
- Hoffman, M.D., Goulet, E.D.B. & Maughan, R.J. (2018) — Correction to: Considerations in the Use of Body Mass Change to Estimate Change in Hydration Status During a 161-Kilometer Ultramarathon Running Competition. Sports Medicine, 48(12), 2893–2894. Corrected model values used above.
- Hoffman, M.D. & Stuempfle, K.J. (2015) — Sodium Supplementation and Exercise-Associated Hyponatremia during Prolonged Exercise. Medicine & Science in Sports & Exercise, 47(9), 1781–1787.
- Hoffman, M.D. et al. (2015) — Sodium Intake During an Ultramarathon Does Not Prevent Muscle Cramping, Dehydration, Hyponatremia, or Nausea. Sports Medicine - Open, 1(1), 39.
- Lipman, G.S. et al. (2021) — Effect of Sodium Supplements and Climate on Dysnatremia During Ultramarathon Running. Clinical Journal of Sport Medicine, 31(6), e327–e334.
Frequently Asked Questions
How much should you drink during an ultramarathon?
Drink to thirst rather than forcing a fixed quota. The ISSN gives a broad racing range of 450–750 mL per hour, but individual needs vary with conditions, effort and physiology. Rehearse your plan and fluid access in comparable conditions. A short sweat-rate estimate is useful context, not a universal intake ceiling; symptoms require assessment rather than another calculation.
How much sodium do ultrarunners need?
There is no single hourly dose for every runner. The ISSN discusses 500–700 mg of sodium per litre of fluid and roughly 300–600 mg per hour in hot or humid conditions. These are planning references, not mandatory doses. Count food and drink together, test tolerance, and seek individual advice when losses or medical factors warrant it. Sodium does not make excessive drinking safe.
How much weight should you lose during an ultramarathon?
Do not aim for a particular percentage of weight loss. Body mass changes with fuel use, food and fluid intake, and water balance. Corrected modelling of a 161-km mountain ultra estimated 1.2–3.5% mass loss with body-water balance maintained under its assumptions; that is not a target or safety threshold. Weight change alone cannot diagnose or exclude dehydration or hyponatremia.
Can drinking too much water be dangerous?
Yes. Excessive fluid intake, together with impaired water excretion, can dilute blood sodium and cause exercise-associated hyponatremia. Headache, repeated vomiting, confusion, seizures or worsening symptoms need race medical staff urgently, whether weight has risen or fallen. Do not try to distinguish or treat serious fluid-balance problems with salt tablets, forced drinking or a scale alone.