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.
Key points
- 2% bodyweight deficit impairs endurance
- 3-4% deficit impairs cognitive function
- Thirst is reliable for most training situations
- Pale straw urine = adequate hydration