Last updated: 3 October 2026
Reviewed by Jordan Morley, FA Level 2 Football Coach
Dehydration reduces plasma volume, increases cardiovascular strain, impairs temperature regulation and reduces both physical and cognitive performance. A 2% bodyweight fluid deficit (1.4kg for a 70kg athlete) produces measurable endurance impairment. At 3-4% deficit, cognitive function declines significantly. Thirst is a reliable hydration indicator for most athletes in training — drink when thirsty, don’t force fluids. Pre-exercise urine colour (pale straw = well hydrated; dark yellow = needs hydration) is the simplest practical assessment. Electrolytes (particularly sodium) are required for events over 60-90 minutes to prevent hyponatraemia.
- 2% bodyweight deficit impairs endurance
- 3-4% deficit impairs cognitive function
- Thirst is reliable for most training situations
- Pale straw urine = adequate hydration
In practice
Even mild dehydration (as little as 2% of bodyweight lost through sweat) has been shown to measurably reduce endurance performance and increase perceived effort — which is why structured hydration strategies, tailored to individual sweat rate and the specific conditions of an event, are now considered a core part of endurance performance planning rather than an afterthought.
FAQ
How much fluid should I drink during exercise?
It depends heavily on individual sweat rate, exercise intensity and environmental conditions, but many guidelines suggest 400-800ml per hour as a starting range, ideally fine-tuned through individual sweat-rate testing.
What percentage of bodyweight loss from sweat starts affecting performance?
Research generally shows measurable performance decline beginning around 2% bodyweight loss from fluid, with the effect becoming more pronounced as dehydration increases further.