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Hydration · 5 min readMore than water
Electrolytes, timing and why hydration is a performance lever.
Performance falls before thirst becomes urgent
Losing around two per cent of body mass as sweat is where endurance performance starts to measurably decline, and the effect grows steeply after that. For a 65 kg woman that is 1.3 kg — an amount you can lose in an hour of hard work in the heat without noticing anything more than being warm.
The corollary is that thirst is a lagging indicator during exercise. It works well over a day; it works poorly over a hot ninety-minute session.
of body mass lost as sweat
mg per litre, between people
of the deficit, over the following hours
Sodium is doing the real work
Sweat is not water. Sodium concentration varies enormously between people — roughly 200 to 2,000 mg per litre — and the salty sweaters are usually the ones with white marks on a dark top after a session. Replacing volume without replacing sodium dilutes what is left, which is why plain water can leave you feeling worse.
This matters most in long events, in heat, and for anyone who has cramped repeatedly despite drinking plenty. A drink with 500 to 1,000 mg of sodium per litre is a reasonable place to start; commercial 'electrolyte' tablets are often far weaker than the label implies.
Illustrative. The pattern reflects endurance performance studies of progressive dehydration; effects are larger in heat and in longer events, smaller in short or cool ones, and vary between individuals.
Drinking too much is the more dangerous mistake
Exercise-associated hyponatraemia — dangerously low blood sodium from over-drinking — is rare but genuinely serious, and it happens most often to slower participants in long events who drink at every station out of diligence. It is one of the few nutrition errors that can put someone in hospital.
Drinking to thirst, with sodium in the bottle, gets almost everybody to the right place. If you want precision, weigh yourself before and after a session and replace roughly 125 per cent of what you lost, over the following hours rather than all at once.
- 01
Put sodium in the bottle, not just water
The lever - 02
Weigh in and out of one hot session to learn your rate
Measure once - 03
Do not drink at every station out of diligence
The real danger
General nutrition education, not medical advice. If you take medication affecting fluid balance or blood pressure, or have a heart or kidney condition, take individual advice before changing sodium intake. Full health disclaimer →
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