Too little fluid adds to heat strain; too much causes exercise-associated hyponatremia (EAH), a potentially fatal dilution of blood sodium. The numbers below keep you between the two. 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.
How Much to Drink per Hour
Drink to thirst. For most runners that comes to about 400–750 mL per hour. The ISSN ultramarathon position stand gives 450–750 mL per hour, about 150–250 mL every 20 minutes. In the only 24-hour race intake study, runners drinking freely averaged 378 mL per hour and their plasma volume still rose by about 10%, so they were not dehydrated (Costa 2014). Gaining weight during the race means you are overdrinking.
The 2019 Wilderness Medical Society guideline makes drinking to thirst the primary way to prevent EAH and says overhydration should be avoided. No fixed fluid volume has been shown to prevent EAH, which is why the table below sets ranges and a ceiling rather than a quota.
| Conditions | Per hour | Notes |
|---|---|---|
| Cool, or walking at night | 300–500 mL | Below the ISSN range is normal here; the 24-hour field averaged 378 mL/h |
| Mild, running | 450–600 mL | The lower half of the ISSN range |
| Hot or humid | 600–750 mL | Go above 750 mL only if a sweat-rate test in similar heat shows losses above about 1 L/h |
| Ceiling in any conditions | About 80% of your measured sweat rate | Replacing more than 80% of losses is where modelling shows sodium balance starts to matter (McCubbin 2023) |
Fluid and fuel together. Drinking 500 mL per hour of a 6% sports drink supplies 30 g of carbohydrate, the floor of our site-wide carbohydrate tiers: at least 30 g per hour, 50 g by default, 60–75 g with a trained gut, and 90 g as a ceiling. Do not raise fluid volume to hit a carbohydrate target; add food or a gel instead. The 24-hour nutrition plan and the backyard ultra nutrition plan show how the two fit together.
Measure Your Sweat Rate
Change in body mass is the simplest accurate way to measure whole-body sweat rate (Baker 2017). Typical rates run from about 0.5 to 2.0 L per hour, and endurance athletes in a 1,303-athlete dataset averaged 1.28 L per hour (Barnes 2019). Ultra pace is usually slower, so expect less.
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. Your ceiling is 80% of that, about 680 mL per hour; thirst decides how much of it you drink.
- 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.
How Much Weight You Should Lose
You should finish a long ultra lighter. Over many hours, body mass also falls because fuel is burned and water stored with glycogen is released. Modelling for an average 68.8 kg runner over 161 km shows a loss of 1.9–5.0%(1.3–3.4 kg) is needed just to keep body water steady (Hoffman 2018). The 24-hour runners in Costa's study lost an average of 1.6%. At Western States, serum sodium was lower in runners who lost less weight, and no finisher with hyponatremia had lost more than 4.3% (Hoffman and Stuempfle 2015).
| Weigh-in | Likely meaning | Action |
|---|---|---|
| Heavier than at the start | Overdrinking | Stop drinking until you are thirsty; no extra salt |
| 1.9–5.0% lighter by the end of a 100-mile or 24-hour race | Expected | Keep drinking to thirst |
| More than 5% lighter, with strong thirst | Under-drinking is more likely | Drink to thirst and get checked if you are dizzy |
How Much Sodium per Hour
Sweat sodium ranges from about 10 to 70 mmol/L, roughly 230–1,610 mg per litre(Baker 2017). Endurance athletes lose about 52 mmol, roughly 1,190 mg, of sodium per hour on average (Barnes 2019). You do not need to replace all of it. McCubbin's 2023 modelling found that targeted sodium replacement only becomes necessary when you replace more than 80% of fluid losses and your sweat sodium is at least 40 mmol/L (about 920 mg/L). Even in a 160 km race, the amount needed stays below half of what you lose unless sweat rate is at least 1.5 L/h, sweat sodium at least 40 mmol/L, and fluid replacement 90% or more. The ISSN sets a floor of more than 575 mg of sodium per litre of fluid; most commercial sports drinks carry 460–575 mg/L.
Our recommendation:
- Default: 300–600 mg per hour. The ISSN's 575 mg/L at 450–750 mL/h works out to about 260–430 mg per hour, and half of the average endurance athlete's loss is about 600 mg.
- Heat or a known salty sweater: 600–1,000 mg per hour. "Known" means a sweat test at 40 mmol/L or above, or white salt crusts on your kit after long runs.
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.
More sodium does not prevent EAH or cramps. 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 supplements neither prevented EAH nor caused hypernatremia (Lipman 2021).
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%).
Heat raised the odds of both problems compared with cold races: an odds ratio of 3.5 for EAH, with a wide confidence interval, and 8.8 for hypernatremia. Slower, heavier, less-trained runners in the heat have the most time and opportunity to overdrink, and the heat also makes dehydration more likely. If that describes you, weigh in at aid stations when you can and stick to the ranges above.
Practice Plan
- Do the sweat-rate test twice, once in cool and once in warm conditions, and write down 80% of each result as your ceiling.
- 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.
- Weigh before and after long runs; if you finish heavier, you drank too much.
- Write the race plan as numbers.
A plan might read: "Drink to thirst within 400–750 mL per hour, never more than 680 mL per hour. Take 300–600 mg of sodium per hour from broth, pretzels and drink, up to 1,000 mg in the afternoon heat. Weigh in at every scale; if I am heavier than at the start, stop drinking until I am thirsty."
Red Flags
| Signs | Possible cause | Action |
|---|---|---|
| Headache, nausea or vomiting, puffy hands or face with weight gain, confusion, unusual behavior, seizure | EAH from overdrinking | Stop drinking, do not take salt to self-treat, get race medical staff to check blood sodium |
| Intense thirst, more than 5% weight loss, dizziness on standing, little dark urine | Dehydration or high blood sodium | Drink to thirst, cool down, and get checked |
| 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. & 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. For most runners that comes to about 400–750 mL per hour; the ISSN position stand gives 450–750 mL per hour, or about 150–250 mL every 20 minutes. Runners drinking freely in a 24-hour race averaged 378 mL per hour without becoming dehydrated. Use the top of the range only in heat, and never plan to drink more than about 80% of your measured sweat rate. Gaining weight during the race means you are overdrinking.
How much sodium do ultrarunners need?
Our default is 300–600 mg per hour from drink and food, rising to 600–1,000 mg per hour in heat or if you are a known salty sweater. The ISSN advises fluid carrying more than 575 mg of sodium per litre. Sweat contains roughly 230–1,610 mg of sodium per litre, but in ultramarathon field studies sodium supplements did not prevent hyponatremia or cramps; avoiding overdrinking does.
How much weight should you lose during an ultramarathon?
Some. Modelling for a 68.8 kg runner over 161 km shows a loss of 1.9–5.0% of body mass is needed just to keep body water steady, because fuel is burned and water stored with glycogen is released. At Western States, no finisher with hyponatremia had lost more than 4.3%. Weight gain is the warning sign; a loss above 5% with strong thirst points toward under-drinking.
Can drinking too much water be dangerous?
Yes. Exercise-associated hyponatremia happens when fluid intake exceeds what the body can excrete, diluting blood sodium. It affected 6.6% of Western States finishers in one study. Headache, vomiting, confusion, or puffy hands with weight gain need race medical staff; more salt or fluid is not the treatment.