Why water is essential
The human body is composed of 60–70% water. It participates in almost all biological processes: regulating body temperature, transporting nutrients and oxygen, eliminating metabolic waste, lubricating joints and organs. A water loss of just 2% of body weight is enough to reduce cognitive and physical performance.
The 35 ml per kg rule
The most widely used estimate is about 30–35 ml of water per kilogram of body weight per day. A person weighing 70 kg should take in between 2.1 and 2.45 litres of fluids per day (including beverages and water in food, which covers about 20–30% of needs).
Factors that increase hydration needs
- Physical activity: add 500–750 ml per hour of moderate exercise
- Hot and humid climate: sweating significantly increases fluid loss
- Fever: each degree above 37°C increases needs by 10–15%
- Pregnancy and breastfeeding: needs increase by 300–700 ml per day
- High-protein, high-salt diet: requires more water for renal elimination
Signs of dehydration
Urine colour is the simplest way to monitor hydration: pale straw yellow = good; dark yellow or amber = drink more; nearly colourless may indicate overhydration.
- Mild (1–2%): thirst, concentrated urine, slight performance drop
- Moderate (3–5%): headache, difficulty concentrating, fatigue
- Severe (>6%): confusion, cramps, risk of heat stroke
Practical tips
Don't wait until you're thirsty: thirst is already a sign of dehydration. Spread your intake throughout the day, starting with a glass of water on an empty stomach. Fresh fruit and vegetables naturally cover about 20% of your daily fluid needs.