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, aid-station spacing, prior practice, and symptoms. No single sign—including thirst, urine color, or body mass—can diagnose hydration status on its own.
| Signal | What It May Mean | Action |
|---|---|---|
| Thirst with rising heat strain | Fluid and cooling may be needed | Drink to thirst, reduce heat load, and reassess the whole symptom pattern |
| Sloshy stomach, bloating, or weight gain | Possible delayed emptying or overdrinking | Stop forced intake, slow down, and seek medical input if symptoms persist or worsen |
| Swollen hands or feet | Nonspecific: heat, fluid shifts, fit, or overdrinking may contribute | Do not diagnose from swelling alone; urgent help is needed if headache, vomiting, or confusion develops |
| Confusion, severe headache, vomiting | Potential medical emergency | Use race medical support immediately |
| Tea- or cola-colored urine with marked muscle pain | Possible rhabdomyolysis or kidney injury | Stop and seek urgent medical assessment |
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.
In ultramarathon field studies, supplemental sodium intake has not reliably predicted or prevented sodium disorders. Use sodium as one component of a practiced nutrition plan—not as permission to drink beyond thirst or as a treatment for symptoms that require testing.
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, thirst, fluid and food choices, symptoms, and gut response.
- If you use before-and-after weights in training, interpret them as context—not a requirement to maintain starting weight—because fuel use, glycogen-associated water, food, and swelling affect body mass.
- Test hot-weather and cool-night strategies separately.
- Practice the exact concentrations and combinations of plain water, food, and electrolyte or carbohydrate drink you plan to use.
- Write decision rules for thirst, heat, bloating, nausea, and medical red flags rather than one rigid hourly number.
A plan might read: "Drink to thirst; increase cooling and reassess effort when heat strain rises; never force fluid through a sloshy stomach; count sodium from food and drink; and use race medical staff for confusion, severe headache, repeated vomiting, collapse, or worsening swelling with neurological 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.
- 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." Sports Medicine, 48(2), 243–250.
- Hoffman, M.D. et al. (2015) — "Sodium Intake During an Ultramarathon Does Not Prevent Muscle Cramping, Dehydration, Hyponatremia, or Nausea." Sports Medicine - Open, 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?
There is no universal hourly number. Use thirst as the starting signal and adapt to heat, intensity, body size, aid-station spacing, and your practiced response. Avoid forcing fluid to hit a schedule, and do not use body-mass change or urine color alone to diagnose hydration status during an ultra.
How much sodium do ultrarunners need?
There is no universal hourly dose. Food, sports drink, sweat losses, weather, pace, and medical history all affect a plan. Sodium may support palatability and replacement of some losses, but supplements have not been shown to prevent hyponatremia when a runner overdrinks.
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.