Do you really need eight glasses of water a day?

The eight-glasses rule has no identifiable scientific origin, and the most-cited candidate is a 1945 recommendation that gave a total fluid figure and then noted that most of it comes from prepared food — a sentence that dropped out of the retelling. Reference intakes are total water figures of roughly 2.7 litres a day for women and 3.7 for men from all sources combined, of which around 20 per cent typically comes from food. Every drink counts toward that, including tea and coffee, whose mild diuretic effect at ordinary consumption levels does not offset the water they contain. Thirst is a sensitive mechanism and works well for healthy adults under ordinary conditions; the situations that call for deliberate intake are heat, illness, heavy exercise and older age, where thirst is blunted. Drinking far beyond need is not virtuous and, at extremes, dilutes blood sodium dangerously.

How confident this is

This one is settled. The mechanism is understood and the consequences follow from measurements or arithmetic rather than from guesswork.

Your body

• What happens if you stop eating sugar? Blood glucose steadies within days and dental risk drops immediately; most other changes come from eating fewer calories overall.
• What happens if you stop drinking alcohol? Sleep and liver markers improve within weeks, and blood pressure typically falls — but heavy drinkers must not stop abruptly without medical advice.
• What happens if you stop smoking? Carbon monoxide clears within a day, lung function improves within months, and heart disease risk halves within a year.
• What happens if you stop eating meat? Fibre intake usually rises and saturated fat falls — but vitamin B12 has to come from somewhere, and it is not plants.
• What happens if you crack your knuckles every day? Nothing much. It does not cause arthritis — a doctor tested it by cracking one hand only for 60 years.
• What happens if you swallow gum? It passes through in a day or two. The seven-years story is entirely false.
• What happens if you stop drinking coffee? A withdrawal headache for two to nine days, then nothing — caffeine tolerance resets in about a week or two.
• What happens if you only drink water? Mostly it removes liquid calories — the average person drinking only water cuts several hundred calories a day without noticing.
• What happens if you stop exercising? Aerobic fitness starts falling within about two weeks; strength holds up considerably longer.
• What happens if you hold your breath for too long? You cannot hold it until you die — the reflex takes over, and consciousness ends any voluntary effort long before that.
• What happens if you never brush your teeth? Plaque hardens into tartar within days, gums inflame within weeks, and teeth are lost over years.
• What happens if you only eat one food? You develop whatever deficiency that food lacks — and every single food lacks something.
• What happens if you stop eating salt? Blood pressure usually falls, which is the point — but sodium is essential, and zero is not the target.
• Does shaving make hair grow back thicker? No. Shaving cuts the shaft at its widest point, which makes regrowth feel coarser without changing anything.