Safety First
Hydration advice can become dangerous when it turns into a rule that ignores the runner in front of you. Too little fluid can contribute to dehydration and heat illness. Too much fluid can contribute to exercise-associated hyponatremia, a potentially life-threatening low blood sodium condition.
This guide is educational, not medical advice. If you have kidney disease, heart disease, a history of hyponatremia, medications that affect fluid balance, or unusual swelling symptoms, talk with a sports medicine clinician before racing.
Drink to Thirst, Then Adjust
For most runners, thirst should be the starting point. Then adjust based on context: heat, humidity, altitude, body size, pace, food intake, and whether you are urinating normally.
| Signal | What It May Mean | Action |
|---|---|---|
| Thirst, dry mouth, low urine | Fluid intake may be low | Drink steadily and cool down if hot |
| Sloshy stomach | Fluid is not emptying well | Slow down, reduce bolus drinking, consider bland food |
| Swollen hands plus clear frequent urine | Possible overdrinking pattern | Stop forcing fluids and seek medical input if symptoms worsen |
| Confusion, severe headache, vomiting | Potential medical emergency | Use race medical support immediately |
Sodium Is Not Magic
Sodium helps replace some sweat losses and can make fluids and food more tolerable, but it does not cancel out overdrinking. A runner who drinks far beyond thirst can still develop dangerous dilution even while taking salt capsules.
Practice with real foods too: broth, salted potatoes, pretzels, ramen, rice balls, and electrolyte drink can all contribute sodium. Pair this with your broader 24-hour nutrition plan or backyard ultra nutrition plan.
Practice Plan
- During long runs, note temperature, effort, fluids, sodium, urine pattern, and gut response.
- Weigh before and after selected training runs if you can do so calmly and safely.
- Test hot-weather and cool-night strategies separately.
- Practice carrying both plain water and electrolyte/carbohydrate drink.
- Write a range for race day, not one rigid number.
A range might look like this: "Start around 400 to 600 mL per hour in mild weather, drink more when genuinely thirsty, reduce forced drinking if the stomach sloshes, and use race medical staff for confusion or swelling plus worsening symptoms."
Red Flags
Get medical help during a race if you have confusion, severe headache, repeated vomiting, collapse, chest pain, trouble breathing, severe dizziness, inability to walk normally, or rapidly worsening swelling. Do not try to self-diagnose dehydration versus hyponatremia from a blog article while impaired.
For broader context, read Is multiday running safe?.
Sources
- Hew-Butler, T. et al. (2015) — Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference. Clinical Journal of Sport Medicine, 25(4), 303-320.
- Costa, R.J.S. et al. (2019) — Gut-training in endurance athletes. Medicine & Science in Sports & Exercise, 51(10), 2029-2039.
- Knechtle, B. and Nikolaidis, P.T. (2018) — Physiology and pathophysiology in ultra-marathon running. Frontiers in Physiology, 9, 634.
Frequently Asked Questions
How much should you drink during an ultramarathon?
There is no universal number. Many runners fall somewhere around 300 to 750 mL per hour depending on heat, size, intensity, and sweat rate, but thirst, body weight trend, urine pattern, weather, and symptoms matter more than a rigid target.
How much sodium do ultrarunners need?
Needs vary widely. Many runners practice with roughly 300 to 800 mg sodium per hour, but heavy sweaters, heat, pace, food choices, and medical history change the picture. More sodium does not make overdrinking safe.
Can drinking too much water be dangerous?
Yes. Exercise-associated hyponatremia can happen when fluid intake exceeds what the body can excrete, diluting blood sodium. Severe symptoms require urgent medical care.