Water
A vital nutrient making up ~60% of the human body, essential to all physiological functions, whose requirements and role are often misunderstood.
Water accounts for ~60% of adult body weight (more in infants, less in older people). It is the solvent of life: transporting nutrients, biochemical reactions, temperature regulation, waste removal (urine), lubrication (joints, eyes), cell structure.
A fresh angle (the « 8 glasses a day » myth): the « 1.5 to 2 litres » or « 8 glasses of water a day » rule is a myth with no solid scientific basis. Its origin is unclear (probably a misreading of an old recommendation that included the water in food).
The reality: fluid needs are highly variable (size, activity, climate, diet) and a large share of your water comes from food (fruit, vegetables and soups contain 80-95% water). The best guide is not a fixed number but listening to thirst (a reliable physiological mechanism in healthy adults) and urine colour (pale = well hydrated, dark = drink more).
The opposite trap (hyponatraemia): counter-intuitively, drinking too much is also dangerous. Drinking excessive amounts of water (beyond thirst, particularly during endurance exercise) can dilute the sodium in your blood (hyponatraemia), a potentially fatal emergency. More is not better; the obsession with « drinking lots » has its limits.
Hydration and mixed-up signals: thirst is sometimes mistaken for hunger (the hypothalamic centres are close together). A « hollow » feeling can sometimes be thirst, so drinking a glass of water before concluding you are hungry is a useful habit.
Water, fullness and weight: drinking water before or during meals slightly increases fullness (stomach volume). Replacing sugary drinks with water is one of the most beneficial changes for health and weight.
Which water? (a practical view): tap water is generally safe and tightly monitored (sometimes more so than bottled water). Mineral waters provide varying minerals (some are rich in magnesium or calcium). There is no need to overpay for « premium » hydration: the essential thing is to drink enough, according to thirst.
Signs of dehydration: thirst, dark urine, fatigue, headaches, reduced concentration, dry mouth. Older people (reduced thirst) and children are at greater risk.