Running 30 + miles on technical terrain is a marathon‑plus of endurance, heat management, and mental grit. While pacing, footing, and nutrition get most of the spotlight, hydration is the linchpin that holds the whole effort together . Forgetting to drink---or drinking the wrong thing---can turn a triumphant ultra into a nightmarish slog with cramps, dizziness, or worse. Below is a step‑by‑step guide to mastering fluid intake before, during, and after ultra‑trail runs.
Know Your Sweat Profile
| Why it matters | How to test it |
|---|---|
| Determines how much fluid and electrolytes you lose per hour. | Weigh yourself nude before and after a 1‑hour run in race‑like conditions (no water intake). 1 lb lost ≈ 0.45 L of sweat. |
| Guides the sodium, potassium, and carbohydrate mix you'll need. | Use a sweat patch (or a simple skin test) to see salt crystals after a hard effort. |
Takeaway: If you lose 2 lb/h, aim for ~ 0.9 L of fluid + appropriate electrolytes each hour. Adjust for temperature, altitude, and personal tolerance.
Pre‑Race Hydration Blueprint
-
Two‑Day Loading
- Goal: Start the race slightly above normal hydration (≈ 2 % body weight).
- How: Drink 0.5--1 L of water plus a sports drink (300--500 mg Na⁺) per day, spread across meals.
-
Morning of the Race
- 30 min before start: 500 mL of a 6 % carbohydrate electrolyte drink (≈ 20 g carbs, 300 mg Na⁺).
- 15 min before start: 200 mL of plain water or low‑calorie electrolyte solution.
-
Urine Check
In‑Race Fluid Delivery System
| Option | Pros | Cons | Typical Capacity |
|---|---|---|---|
| Hand‑held bottle (e.g., CamelBak Alpine) | Easy to sip; low weight | Limited volume (≤ 1 L) | 0.8 L |
| Hydration pack (bladder + pockets) | Large capacity; storage for food/electrolytes | Slightly heavier; can restrict movement | 2--3 L |
| Drop‑bag system | Customizable electrolytes/food per checkpoint | Requires planning; relies on aid stations | N/A |
| Portable water filter | Unlimited water source on remote trails | Slower flow; adds weight | N/A |
Recommendation for 30‑+ mile races:
- Primary: Hydration pack (2 L).
- Backup: Hand‑held bottle (0.8 L) for quick sips.
- Aid‑station "top‑up" plan: Refill pack at each checkpoint; never run dry.
Hourly Fluid & Electrolyte Targets
| Temperature | Fluid Goal | Sodium Goal | Carbohydrate Goal |
|---|---|---|---|
| < 50 °F (10 °C) | 500--750 mL/h | 300--500 mg/h | 30--45 g/h (optional) |
| 50--70 °F (10‑21 °C) | 750--1000 mL/h | 500--700 mg/h | 45--60 g/h |
| > 70 °F (21 °C) | 1000--1500 mL/h | 700--1000 mg/h | 45--75 g/h (split between fluids & gels) |
Adjust on the fly: If you notice a salty taste, cramping, or a rapid rise in heart rate, increase sodium and reduce pure water to avoid hyponatremia.
Managing Electrolytes on the Fly
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Sodium Capsules (e.g., SaltStick, Endurolytes) -- 200 mg per capsule.
- Protocol: 1 capsule every 45 min in warm weather; 1 every 90 min in cool weather.
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DIY Salt Mix -- ¼ tsp sea salt + ¼ tsp potassium chloride (a pinch of "lite‑salt") dissolved in 500 mL water.
-
Electrolyte Gels -- Combine carbs and electrolytes in a single packet (e.g., GU Energy Gel with 150 mg Na⁺). Use 1--2 per hour depending on intake.
Carbohydrate‑Fluid Synergy
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Goal: 30--60 g carbs per hour to preserve glycogen stores without stomach upset.
-
Method: Alternate sips of a 6 % carb drink with solid gels or chews.
-
Sample schedule:
Time Action 0:00‑0:20 Sip 100 mL of 6 % carb drink every 5 min 0:20‑0:30 1 gel (≈ 25 g carbs) + water splash Repeat Every hour
Adapting to Terrain & Altitude
- Steep climbs: Heart rate spikes → increase fluid and sodium by ~10 %.
- Technical descents: Short bursts of high effort → sip small amounts (30--50 mL) to avoid a sloshy stomach.
- High altitude (> 2,500 m): Respiratory water loss rises → add 250 mL extra fluid per 1,000 m of elevation gain.
Recognizing & Responding to Problems
| Symptom | Likely Issue | Immediate Action |
|---|---|---|
| Light‑headed, nausea, excessive sweating | Dehydration | Drink 250--500 mL water + 300 mg Na⁺ within 5 min |
| Headache, bloating, confusion | Hyponatremia (low Na⁺) | Stop, drink 300--500 mL of 6 % carb drink with 700 mg Na⁺, seek medical help if severe |
| Muscle cramps, tingling | Sodium deficiency | Take 1--2 sodium capsules, sip salty electrolyte drink |
| Stomach cramp after gulping | Over‑hydration / too fast | Slow sip rate, switch to isotonic solution, reduce volume for next hour |
Post‑Race Re‑Hydration Plan
- Re‑weigh (naked) within 30 min. Aim to replace 150 % of fluid lost (e.g., lost 2 kg → drink 3 L).
- Electrolyte Recovery Drink -- 0.5 L of a 2 % carbohydrate, 600 mg Na⁺, 200 mg K⁺ solution.
- Protein‑Carb Meal -- 20--30 g protein + 40--60 g carbs within 2 h to replenish glycogen and aid muscle repair.
- Monitor urine for the next 24 h; stay above 1 L extra fluid per day for the first 2 days.
Quick‑Reference Hydration Cheat Sheet
| Situation | Fluid (mL/h) | Sodium (mg/h) | Carbs (g/h) | Gear |
|---|---|---|---|---|
| Cool < 50 °F | 600 | 300 | 30 | Hand‑held bottle |
| Moderate 50‑70 °F | 900 | 600 | 45 | 2‑L pack + 1‑L bottle |
| Warm > 70 °F | 1,300 | 800 | 60 | 3‑L pack + electrolytes |
| High Altitude | +250 mL per 1,000 m gain | +200 mg per 1,000 m gain | Same | Extra salt capsules |
Print this table, tape it to your vest, and adjust on the fly based on how you feel.
Final Thoughts
Hydration for ultra‑trail running is dynamic, not static . Your body's needs shift with temperature, elevation, effort, and even the day's humidity. By:
- Profiling your sweat before race day,
- Loading wisely in the 48 h leading up,
- Using a reliable delivery system (pack + bottle),
- Hitting hourly fluid, sodium, and carb targets, and
- Listening to your body for early warning signs,
you'll dramatically reduce the risk of cramping, hyponatremia, and premature fatigue. The reward? A clear mind, steady legs, and the confidence to push those 30‑plus miles to the finish line.
Run smart, stay hydrated, and enjoy the trail.