Where "Eight Glasses" Came From
The eight-glasses-a-day rule has no clear scientific origin. The most likely source is a 1945 US Food and Nutrition Board recommendation of about 2.5 litres a day — which included the sentence, usually omitted afterwards, that most of this quantity is contained in prepared foods.
Current guidance from the US National Academies is 3.7 litres of total water a day for men and 2.7 for women, from all sources. Roughly 20% arrives in food, so the amount to actually drink is closer to 3.0 and 2.2 litres.
Water Intake Chart
Litres to drink after allowing for what food supplies:
| Body weight | Sedentary | 30 min exercise | 60 min exercise | Hot climate, 60 min |
|---|---|---|---|---|
| 50 kg | 1.4 | 1.7 | 1.9 | 2.4 |
| 60 kg | 1.7 | 1.9 | 2.2 | 2.7 |
| 70 kg | 2.0 | 2.2 | 2.5 | 3.1 |
| 80 kg | 2.2 | 2.5 | 2.8 | 3.5 |
| 90 kg | 2.5 | 2.8 | 3.0 | 3.8 |
| 100 kg | 2.8 | 3.1 | 3.3 | 4.2 |
Thirst Is a Reasonably Good Guide
For healthy adults with normal kidney function, drinking to thirst maintains hydration adequately. The mechanism is precise: a 1–2% drop in body water triggers thirst well before any performance or cognitive effect appears.
The exceptions matter, though: older adults have a blunted thirst response, endurance athletes lose fluid faster than thirst signals it, and illness with fever, vomiting or diarrhoea raises requirements sharply.
Coffee and Tea Count
The belief that caffeinated drinks dehydrate you is largely wrong. Caffeine has a mild diuretic effect, but the fluid in the drink more than compensates. Studies comparing coffee against water find no meaningful difference in hydration status at ordinary intakes. Alcohol is genuinely dehydrating, suppressing vasopressin and increasing urine output.
Checking Hydration
Urine colour is the practical test: pale straw is well hydrated, dark yellow indicates the need for fluid, and completely clear suggests drinking more than necessary. B vitamins and some medications colour urine independently of hydration.
Too Much Water Is Also a Problem
Hyponatraemia — dangerously diluted blood sodium — occurs when fluid intake outpaces the kidneys' excretion capacity of roughly 0.8 to 1.0 litres an hour. It is rare but has killed marathon runners and fraternity pledges. Drinking to a schedule rather than to thirst during endurance events is the usual cause.
Frequently Asked Questions
Does drinking more water help weight loss?
Modestly. Water before meals reduces intake slightly in some studies, and replacing caloric drinks with water reduces calories directly. Water itself does not burn fat.
How much should I drink during exercise?
About 400–800 ml an hour depending on sweat rate, heat and intensity. Weighing yourself before and after a session gives a personal figure: each kilogram lost is roughly a litre of fluid.
Do I need electrolytes?
Only for sessions over about 90 minutes, in significant heat, or for heavy sweaters. For ordinary exercise, water and a normal diet are sufficient.