Hydration advice stresses personal water loss over fixed daily totals

Scientific experts say water needs are determined by how much fluid an individual loses each day, not by a single universal figure. The amount varies with climate, physical activity and personal biology; for example, an active adult in hot conditions may require far more than the generic “2 liters” rule. European food‑safety guidelines set reference intakes of about 2 L for adult women, 2.5 L for men, and up to 3 L in very hot or high‑exercise situations, recognizing that roughly 20 % of daily fluid comes from food.

Electrolytes are essential for fluid balance, but most people obtain sufficient amounts from their diet; sports drinks often contain too little sodium for a meaningful effect. Drinking large quantities of water before bedtime can disturb the body’s natural antidiuretic hormone cycle, causing nocturnal awakenings and, in extreme cases, hyponatremia—a dangerous drop in blood sodium that may lead to headache, nausea, dizziness or cerebral edema. Researchers note that a loss of just 2 % body mass (about 1–1.5 L) can already impair mood, energy and cognition. A 2023 U.S. NIH study linked poor hydration to faster aging, higher chronic‑disease risk and increased mortality, though causation has not been proven.