"Drink eight glasses of water a day" is a handy reminder, but it is not a medical rule. How much you need depends on your size, activity, the climate and your health, and a good share of daily water comes from food and other drinks.
What the guidelines say
The US National Academies set an "adequate intake" of total water, from all drinks and food, of roughly 3.7 litres a day for men and 2.7 litres for women. About a fifth of that typically comes from food. The NHS puts it more simply: aim for 6 to 8 cups or glasses of fluid a day. Water, lower-fat milk and sugar-free drinks all count, and so do tea and coffee: the mild diuretic effect of caffeine does not cancel out the fluid in a normal cup.
When you need more
- Hot weather and dry air, including air-conditioned buildings and aircraft cabins.
- Exercise and long days on your feet.
- Illness with fever, vomiting or diarrhoea. Oral rehydration solutions replace salts as well as water.
- Pregnancy and breastfeeding.
- Older age, because the sense of thirst becomes weaker.
Signs of dehydration
Early signs include thirst, dark yellow and strong-smelling urine, peeing less often, a dry mouth, tiredness, headache and feeling light-headed. Pale straw-coloured urine is a practical everyday sign that you are drinking enough. Confusion, a racing heart, very little urine or fainting need urgent medical help.
Alcohol, casinos and the desert
Alcohol makes the kidneys release more water, so a night of drinking leaves you with less fluid than you started with. Casino resorts add two more factors: floors are heavily air-conditioned, and in many venues drinks are brought to players at the tables and machines, which makes it easy to lose count. A simple habit is to alternate every alcoholic drink with a glass of water. In a desert city such as Las Vegas, where summer afternoons are very hot and the air is dry, needs climb fast; our Las Vegas trip health guide covers heat and hydration in detail.
Can you drink too much?
It is rare, but yes. Drinking very large amounts in a short time, which sometimes happens in endurance events, can dilute the sodium in the blood (hyponatraemia). Drink to thirst during long exercise rather than forcing fluids, and use drinks with electrolytes for long sessions in the heat. People with heart or kidney failure may be told to limit fluids, and should follow that advice.
Based on guidance from: US National Academies of Sciences, Engineering, and Medicine, NHS, CDC.
