Hydration & nutrition
Estimate fluid needs by weight, duration and weather.
Hydration guide
- Before (2h before): Drink 500ml about 2 hours before to top up (allows excess to clear).
- 15min before: 200ml to avoid bloating at the start.
- During: Sip ~200ml every 15–20min rather than large volumes spaced out; use isotonic drinks for efforts >60min.
- After: Replace ~150% of weight lost (e.g., lost 1kg → drink 1.5L) and include sodium + carbs for faster recovery.
💧 Good hydration prevents cramps and fatigue and optimizes performance. Recommendations use a scientifically validated sweat-rate model.
💪 Sport
📏 Distance
⏱️ Time target
⚖️ Weight
Why hydration is critical in running
Hydration directly impacts performance. A 2% loss of body mass in water ≈ -10% performance. Above 3% you risk cramps, dizziness and heat illness. Sweating loses both water and electrolytes (sodium, potassium) which must be replaced.
The 3 pillars of sports hydration
- 💧 Water: Rehydrates and maintains blood volume (500–800ml/hour depending on intensity)
- ⚡ Electrolytes (sodium): Prevent hyponatremia and aid water absorption (700–1000mg/h)
- 🍬 Carbohydrates: Immediate energy for efforts >90min (60–90g/h depending on duration)
Signs of dehydration during exercise
Recognize these symptoms to act before performance drops:
- 🚨 Intense thirst: Late signal (already ~1.5% body mass lost)
- 🚨 Dark urine: Dark yellow/orange = serious dehydration
- 🚨 Muscle cramps: Lack of sodium + fluid
- 🚨 Dizziness/nausea: Immediate danger — stop exercise
- 🚨 High heart rate: Heart compensating for low blood volume
Hydration strategy: before / during / after
Before (2h prior):
- Drink ~500ml 2h before (allows time to eliminate excess)
- Drink ~200ml 15min before start to avoid bloating
During:
- Sip every 15–20min (~200ml) rather than large sips
- Use isotonic drinks for efforts >60min (water + sodium + carbs)
- Increase ~+30% if >25°C, ~+50% if >30°C
After (recovery):
- Replace 150% of weight lost (weigh before/after if possible)
- Prefer a drink with sodium (helps retain water) + carbs (replenish glycogen)
- Clear urine within 2h = hydration restored
❓ Hydration FAQ
How much water should I drink during a marathon?
Aim for ~500–800ml/hour depending on intensity and temperature. For a 4h marathon that typically equals 2–3L spread across aid stations (every ~5km). Never drink >1L/hour — risk of hyponatremia. Use the calculator and adjust for your weight and estimated duration.
Water or isotonic drink: which should I choose?
Plain water is fine for efforts <60min. For >90min, use an isotonic (water + ~700mg/L sodium + ~60g/L carbs). DIY mix: 1L water + 1g salt + 60g sugar (or honey). Commercial sports drinks like Gatorade/Powerade provide the right ratios.
How do I know if I'm well hydrated?
Urine color test: pale yellow = well hydrated; dark = mild dehydration; orange = severe. Weighing before/after a session helps: >2% body mass loss = underhydrated. Example: 70kg → loss >1.4kg is problematic. Replace ~1.5L per kg lost.
Can you drink too much during a race?
YES — hyponatremia (water intoxication). Drinking >1L/h can dilute blood sodium → nausea, confusion, cramps, even coma. This is common in slow runners (race >5h) who drink every station. Rule: drink to thirst; elite marathoners often drink 400–600ml/h max.