The eight-glasses-of-water myth
You've heard it a thousand times: "eight glasses of water a day." The rule comes from a 1945 US National Academy of Sciences recommendation that suggested 2.5 L of water per day, with the precision that most of it came from food. That last part got lost over time.
The truth: your water needs vary based on your weight, activity, climate, and diet. A sedentary 55 kg person who eats lots of fruit doesn't have the same needs as a 90 kg runner in a heat wave.
How Much Water You Actually Need
There is no single number. Your daily water needs depend on your weight, activity, climate, and what you eat. Use this as a starting point and adjust:
| Profile | Recommended intake | When it applies |
|---|---|---|
| Sedentary adult | 30 ml per kg of body weight | Office work, light walking. Example: 70 kg person needs around 2.1 L per day. |
| Active adult | 35 to 40 ml per kg | Regular exercise (3 to 5 times per week). Add 500 ml per hour of training. |
| Athlete or hot climate | 40 to 50 ml per kg + extra | Endurance training, hot weather, or sweating heavily. Add 750 to 1000 ml per hour of intense activity. |
Signs that you're dehydrated
Thirst is a late signal of dehydration. By the time you feel thirsty, you've already lost 1 to 2% of your body weight in water. Learn to spot the earlier signs:
Early signs (mild dehydration)
- Dark yellow urine (ideal: pale yellow)
- Dry mouth mid-day
- Unexplained fatigue around 2 or 3 PM
- Slight headache at the end of the day
- Difficulty concentrating
More serious signs (correct quickly)
- Dizziness when standing up
- Muscle cramps
- Skin that stays pinched after a pinch test
- Rapid heartbeat at rest
The hidden water in your food
20 to 30% of your water intake comes from food, not drinks. The water-content champions:
- Cucumber, lettuce: 96%
- Tomato, watermelon, strawberry: 92 to 95%
- Orange, grapefruit: 88%
- Yogurt, egg, cooked fish: 70 to 80%
A fruit-and-vegetable-rich diet can easily provide 1 L of water per day without you drinking a single extra glass.
Common mistakes
Chugging instead of spreading out
Drinking 1 L in one shot doesn't hydrate you better: your body will just pee out the excess. Spread it through the day.
Coffee and tea don't count?
False. Despite their mild diuretic effect, they contribute to your hydration. 3 or 4 coffees per day = about 600 ml of useful water.
Sparkling or still water?
No hydration difference. Pick what you prefer to drink: the one you drink the most is the best one.
Conclusion: simple but not trivial
Hydration isn't about magic numbers. It's about listening to your body, building regular habits, and eating quality food. Aim for pale urine throughout the day, spread your water out, eat fruits and vegetables. That's it.
If you want to track precisely, the Jeunio app helps you monitor your daily intake and adjust to your activity and climate.