Adequate fluid intake supports every major bodily system, from cognition and physical performance to kidney function and skin. Needs vary by body size, activity, climate and diet rather than a single fixed number.
Typical implementation: fluid needs vary by individual, but a practical marker is pale straw-coloured urine and drinking enough that thirst rarely builds up. Food also contributes meaningfully to total fluid intake.
Mild dehydration (as little as 1 to 2 percent of body weight) is linked in research to measurable declines in mood, concentration and physical performance. The often-cited eight glasses a day is a simplification, actual guidance considers food-derived fluid and individual variation.
Excessive water intake, though uncommon, can cause dangerously low sodium levels (hyponatraemia), particularly relevant for endurance athletes. Most people's risk is under-, not over-hydration.
If you have kidney or heart conditions where fluid intake needs specific management, or if you experience symptoms of severe dehydration or overhydration.
Free (tap water).
NHS hydration guidance. Research on mild dehydration and cognitive/physical performance. Educational information only.