Sports Nutrition
Dehydration and performance: what matters
Hydration advice swings between "you're chronically dehydrated" and "just drink to thirst." The research supports something closer to the second, with conditions.
Where performance actually declines
Meaningful decrements in endurance performance generally appear once fluid loss exceeds roughly 2% of body mass — about 1.5 kg for a 75 kg athlete. Below that threshold, effects on performance are small and inconsistent. Above it, and particularly in heat, cardiovascular strain rises and performance falls measurably.[1]
Estimating your sweat rate
Weigh yourself before and after an hour of training, in minimal clothing, accounting for any fluid consumed. The difference approximates your hourly sweat loss. Rates vary enormously between individuals and conditions — from under 0.5 L/hour to over 2 L/hour — which is exactly why blanket fluid recommendations fit almost nobody well.
What to drink, and when
- Under 60 minutes — water is sufficient for the overwhelming majority of sessions. See sports drinks vs. water.
- Over 60-90 minutes, or heavy sweating in heat — a drink containing sodium and carbohydrate becomes useful for both fluid retention and fuel.
- Drinking to thirst — for most recreational athletes this self-regulates adequately without any calculation.
Overdrinking is a real risk too
Drinking far beyond sweat losses during long events can dilute blood sodium, causing exercise-associated hyponatremia — a genuinely dangerous condition. More fluid is not automatically better; matching intake roughly to losses is.[1]
Frequently asked questions
At what point does dehydration hurt performance?
Endurance performance generally declines once fluid loss exceeds roughly 2% of body mass, which is about 1.5 kg for a 75 kg athlete. Below that level, effects are small and inconsistent.
How do I work out my sweat rate?
Weigh yourself before and after an hour of training in minimal clothing, adjusting for any fluid you drank. The weight difference approximates your hourly sweat loss.
Can you drink too much water during exercise?
Yes. Drinking well beyond sweat losses during prolonged events can dilute blood sodium and cause exercise-associated hyponatremia, which is dangerous. Matching intake roughly to losses is safer than maximising intake.
Keep reading: Sports drinks vs. water · Electrolytes explained · How much water per day