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Needs vary by profile: 30 to 35 ml per kg of body weight for an adult in normal conditions, i.e. about 2.1 to 2.5 liters for a 70 kg person. This baseline should be adjusted for activity (+0.5 to 1 L/h of exercise), climate (+0.5 to 1 L above 25 °C), diuretic drinks, medications, and certain conditions (pregnancy, breastfeeding, fever).

No, it is a very rough average that suits neither larger people, nor athletes, nor hot weather. EFSA recommends 2 L for women and 2.5 L for men (adults) in normal conditions, to be adjusted to your personal context.

Partially. Coffee and tea contribute to hydration at about 70-80% of their volume, but their diuretic effect means you eliminate about 20-30% more water than with plain water. For each cup of coffee (200 ml), plan on ~50-60 ml of extra water to compensate. Alcohol has an even stronger diuretic effect: each drink requires 100-120 ml of compensating water.

The first signs appear from 1-2% dehydration: thirst, dark urine, mild fatigue, reduced concentration. At 3-5%, headaches, dizziness, dry skin, and reduced physical and cognitive performance occur. Above 5%, dehydration becomes severe: confusion, tachycardia, hypotension. Pale yellow urine is the most reliable indicator of good hydration.

Yes, it is rare but possible. Above 4-5 L/day without physical activity, you can develop hyponatremia (a drop in blood sodium through dilution): nausea, headaches, confusion, even seizures in severe cases. The risk mainly concerns endurance athletes who drink plain water without electrolytes for several hours. For everyday use, stay below 4 L/day unless advised otherwise by a doctor.

It is best to spread it evenly over the day, in small amounts (200-300 ml at a time). Do not wait until you are thirsty (already a 1% deficit). 8 key moments: waking up (300-500 ml), breakfast (200 ml), mid-morning (300 ml), lunch (200 ml), mid-afternoon (300 ml), late afternoon (200 ml), dinner (200 ml), before bed (100-200 ml).

Electrolytes (sodium, potassium, magnesium) are needed for water absorption and retention in cells. With heavy sweating (exercise, heat), drinking only plain water can dilute these minerals and make hydration less effective. That is why people who train intensely benefit from combining magnesium supplementation with sufficient potassium (bananas, dried fruit) and sodium intake.

Seniors have similar needs to younger adults (~30 ml/kg) but are at high risk of dehydration because the sensation of thirst decreases with age. In addition, many take diuretic medications. Recommendation: drink regularly even when not thirsty, aim for at least 1.5-2 L/day, and monitor urine color (goal: pale yellow).

Yes, significantly more. Protein metabolism produces urea, which the kidneys eliminate in urine. The more protein you eat, the more water your kidneys need to eliminate this urea. Plan on +30-50% water on a high-protein diet (athletes, ketogenic-style weight loss). Too little: risk of kidney stones and chronic kidney strain.

Tap water in France is perfectly safe to drink and the most economical. Mineral waters also provide certain minerals (Hépar for magnesium, Contrex for calcium, Vichy for bicarbonate) — useful occasionally. Filtration improves the taste, but effectiveness varies by technology. The main criterion remains drinking enough, whatever the source.