"Stay hydrated" is the most repeated piece of endurance advice in the world.
It is also, without context, partially wrong — and for some athletes, dangerously wrong.
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The Hyponatraemia Problem Nobody Talks About
Hyponatraemia is a condition where blood sodium concentration falls below 135 mmol/L. In its severe form, it causes confusion, seizures, pulmonary oedema, and in rare cases, death.
In endurance sports, it is almost always caused not by sodium depletion alone — but by excessive plain water intake that dilutes blood sodium faster than the kidneys can excrete it.
The 2002 Boston Marathon study found that 13% of finishers had hyponatraemia, with 0.6% in a critical range. The athletes who developed it had consumed significantly more fluid than they lost — they were overhydrated, not dehydrated.
The athletes who got into the most trouble were the slower, more cautious runners — those who had been told to "drink before you're thirsty" and followed the instruction faithfully.
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Sweat Is Not Water — It Contains Electrolytes
Human sweat contains primarily:
- Sodium (the dominant electrolyte): 300–1,000 mg per litre of sweat
- Chloride: roughly proportional to sodium
- Potassium: 150–400 mg per litre
- Magnesium: small amounts (~15 mg/L)
The problem is not hydration. The problem is electrolyte-free hydration in high-sweat conditions.
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Why "Drink to Thirst" Is Now the Evidence-Based Recommendation
The scientific consensus on hydration in endurance sports has shifted considerably over the past two decades.
The 2015 British Journal of Sports Medicine review and subsequent consensus statements now recommend drinking to thirst as the primary hydration cue for most endurance athletes — rather than scheduled drinking or drinking to prevent any weight loss.
The reasoning:
- Thirst is a reliable signal that activates at approximately 1–2% body mass loss — the range associated with meaningful performance impact in most athletes
- Drinking beyond thirst, particularly with plain water, creates the dilutional hyponatraemia risk
- Minor dehydration (1–2%) is well-tolerated and does not significantly impair endurance performance in most trained athletes
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The Sodium Calculation That Changes Race Strategy
For events over 90 minutes, the practical framework:
Step 1: Estimate sweat rate Weigh yourself before and after a representative training session of known duration without fluid intake. Every kilogram lost ≈ approximately 1 litre of sweat.
Step 2: Estimate sodium loss Average sodium concentration in sweat: ~500–700 mg/L. High-sweat athletes or those who see significant white residue on skin/kit: ~800–1,000 mg/L.
Step 3: Target electrolyte replacement For efforts >90 minutes:
- Sodium: ~400–800 mg/hour depending on sweat rate and conditions
- This requires electrolyte tabs, sports drinks with sodium content, or salted food — not plain water alone
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What This Looks Like Practically
For a 3-hour marathon runner in warm conditions:
- Sweat rate: ~1.2 L/hour
- Sodium loss: ~700 mg/hour (840 mg if salty sweater)
- Total fluid need: 2.5–3.0L over the race
- Total sodium need: 1,500–2,100 mg over the race
The runner who finishes feeling great after drinking 4L of plain water is either lucky, a slow sweater, or running in cold conditions. The one who finishes confused with puffiness around the ankles has almost certainly diluted their sodium.
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The Practical Protocol
- < 60 min sessions: Water is sufficient. Electrolyte replacement is unnecessary.
- 60–90 min moderate intensity: Water. Optional electrolytes in heat.
- > 90 min or high intensity in heat: Sodium-containing fluids are not optional — they are part of fuelling strategy.
- Never drink beyond thirst in events without clear signs of significant dehydration.
- Test your hydration strategy in training, not on race day.