The eight glasses of water a day rule is one of the most persistent pieces of health advice in circulation, and one of the least well-supported. It does not appear in any major clinical guideline with that specific number. It does not account for body size, activity level, diet, climate, or health status. It is, in the most literal sense, made up. Which is not to say hydration does not matter. It matters enormously. But the rule is not the right way to think about it.
What the Evidence Actually Says
The European Food Safety Authority recommends 2 litres per day for women and 2.5 litres for men from all sources, including food. The Institute of Medicine in the United States gives figures of 2.7 litres for women and 3.7 litres for men. Both of these are total fluid intake figures, not just water consumed from a glass. Around 20 percent of daily fluid intake typically comes from food, particularly fruits, vegetables, and cooked grains. Drinking eight 250ml glasses of plain water gets you to two litres of fluid from beverages, which for most people is close to adequate, but is not a clinically precise target.
A More Useful Calculation
A practical starting point used in clinical settings is 30 to 35ml of fluid per kilogram of body weight per day for sedentary adults. A 70kg adult therefore needs roughly 2.1 to 2.45 litres of fluid from all sources daily. This should increase by approximately 500ml for every 30 minutes of moderate exercise, more in hot weather. Adults who eat substantial quantities of fruit and vegetables can reduce their fluid consumption from beverages accordingly, as cucumber, lettuce, watermelon, and cooked oats contain 85 to 97 percent water by weight.
The Problem of Overhydration
Hyponatraemia, abnormally low blood sodium caused by excessive water intake, is a genuine clinical risk, particularly in endurance athletes and in older adults who have been told repeatedly to drink more water. Symptoms include headache, nausea, confusion, and in severe cases seizures. The advice to drink before you are thirsty and to continue drinking throughout exercise is not appropriate for all populations. In healthy adults not engaged in prolonged endurance exercise, thirst is a reasonably reliable guide to fluid needs.
The Urine Colour Test
The most practical daily hydration check is urine colour. Pale straw yellow indicates adequate hydration. Dark yellow or amber suggests inadequate intake. Colourless urine suggests the possibility of overhydration. This is not a perfect measure, as B vitamins turn urine bright yellow regardless of hydration status, but it is more useful than counting glasses.
- Target 30 to 35ml of fluid per kilogram of body weight from all sources
- Add 500ml per 30 minutes of moderate exercise
- Account for fluid from food, particularly fruit and vegetables
- Use urine colour as your daily check: pale straw is the target
- Thirst is a reliable guide in healthy non-exercising adults
Sources
- European Food Safety Authority. (2010). Scientific opinion on dietary reference values for water. EFSA Journal, 8(3), 1459.
- Institute of Medicine. (2004). Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press.
- Sawka, M.N., et al. (2005). American College of Sports Medicine position stand: exercise and fluid replacement. Medicine and Science in Sports and Exercise, 39(2), 377-390.