Introduction
Water makes up approximately 60% of the adult human body and is involved in virtually every physiological process — from transporting nutrients and regulating temperature to lubricating joints and enabling nerve transmission. Yet studies consistently show that a significant proportion of the population is chronically mildly dehydrated.
How Much Water Do You Actually Need?
The popular “8 glasses a day” rule is an oversimplification. Individual needs vary significantly based on body weight, activity level, climate, diet, and health status. A practical starting point is 30–35ml per kilogram of bodyweight per day — adjusted upward for exercise, heat, or high caffeine and alcohol consumption.
What Counts as Hydration?
All beverages contribute to fluid intake, including tea, coffee, and milk — though caffeinated drinks have a mild diuretic effect at very high intake. Approximately 20% of daily fluid intake comes from food, particularly fruits and vegetables (cucumbers, watermelon, and celery are 90%+ water by weight).
Signs of Dehydration
The most reliable indicator is urine colour — aim for pale straw yellow. Dark yellow or amber urine indicates underhydration. Other signs include dry mouth, fatigue, headache, dizziness, and difficulty concentrating. Thirst is already a sign of mild dehydration (roughly 1–2% fluid deficit).
Hydration and Performance
Even mild dehydration of 2% of bodyweight impairs aerobic performance, strength, and cognitive function. For a 70kg person, this is just 1.4 litres of fluid deficit — easily reached through a morning of work without regular drinking.
Special Considerations
Caffeine: Moderate caffeine intake (up to 400mg/day for healthy adults) does not cause net dehydration, but very high intake may increase fluid losses. Add an extra 150–200ml of water per cup of coffee.
Alcohol: Alcohol is a significant diuretic. For each unit of alcohol, drink an additional 200–250ml of water.