Water makes up roughly 60% of the human body, and even minor losses during exercise can significantly impair physical and cognitive performance. Yet over-hydration carries its own risks. The optimal strategy is personalised and evidence-based.
The 2% Rule
Research consistently shows that losing more than 2% of body weight through sweat degrades aerobic performance, increases perceived effort, and impairs thermoregulation. For a 70kg athlete, that's just 1.4kg — easy to reach in a warm environment within an hour of hard training.
Sweat Rate Varies Widely
Individual sweat rates range from 0.5L to over 2.5L per hour, depending on exercise intensity, ambient temperature, humidity, and genetics. You cannot rely on thirst alone — the thirst mechanism lags behind actual fluid needs by 15–20 minutes.
To calculate your personal sweat rate: weigh yourself (nude) before and after a 1-hour training session with no fluid intake. Every kilogram lost equals approximately 1 litre of sweat. Use the Sweat Rate Calculator to automate this calculation and get your personalised hourly fluid target.
The Role of Electrolytes
Sweat isn't just water — it contains sodium, potassium, magnesium, and chloride. Sodium is lost in the highest concentrations and is most critical to replace. Failing to replenish sodium while drinking large volumes of plain water can lead to hyponatremia — dangerously low blood sodium — which presents as nausea, confusion, and in severe cases, seizures.
NorthLine performance drinks contain 300mg of sodium per 500ml serving, designed to match the electrolyte profile of average sweat composition.
Practical Hydration Guidelines
- Before exercise: Drink 400–600ml of fluid in the 2–3 hours before training
- During exercise: Aim for 400–800ml per hour, adjusted for sweat rate and conditions
- After exercise: Drink 1.5x the fluid volume lost (e.g., if you lost 1kg, drink 1.5L)
Urine Colour as a Guide
Pale yellow urine indicates adequate hydration. Dark yellow suggests mild dehydration. Clear urine can paradoxically indicate over-hydration or recent large fluid intake.
Hot vs Cold Conditions
In hot and humid conditions, fluid losses increase dramatically. Adjust your intake upward by 20–30% and prioritise electrolyte replacement. In cold weather, the thirst mechanism is further blunted — discipline yourself to drink even when you don't feel like it.
Note that sweat sodium concentration varies enormously between individuals — up to fivefold. If you regularly see white salt residue on dark clothing, you may be a high-sodium sweater who needs a more targeted electrolyte strategy. Read our guide on identifying your sweat sodium type and fixing your electrolyte strategy.
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