Hydration: how much water you need, at work and in training
The European Food Safety Authority sets the adequate intake of water at 2.5 litres a day for men and 2.0 litres for women, from all sources: drinks and the water contained in food. Food supplies roughly a fifth of it, which leaves about 2 litres to drink for a man and 1.6 for a woman in temperate conditions. Add what you lose in sweat when you train or live in a hot climate: between half a litre and two litres per hour of exercise, depending on the person and the conditions. The simplest daily check is the colour of your urine, which should be pale yellow. Thirst is a reliable guide for most healthy adults at rest, less so during long exercise in the heat and in older people. Water, tea, coffee and milk all count.
Your water for the day
The European reference value, plus what you lose in training. Use your measured sweat rate if you know it; one litre an hour is a common figure in warm conditions.
- Adequate intake, food included (EFSA)
- 2.5 litres per day
- Added for training
- 1 litres
- Total water for the day
- 3.5 litres
Total water includes food, which supplies about a fifth. In a hot climate, needs at rest are higher than the European reference.
Where the figures come from
The "eight glasses a day" rule has no scientific origin. The reference values that do exist come from bodies such as the European Food Safety Authority, which in 2010 set adequate intakes for total water at 2.5 litres for men and 2.0 litres for women under conditions of moderate temperature and moderate activity.
Two words in that definition matter. Total means everything: water, other drinks and the water in food. Fruit, vegetables, soups and cooked grains are mostly water, and a normal diet supplies 20 to 30% of the day's intake. Adequate means a level observed to be sufficient in healthy populations, not a minimum below which harm begins.
The British National Health Service puts it more simply: six to eight cups or glasses of fluid a day, more in hot weather or when active.
What changes the figure
- Climate. In a hot environment you lose water through the skin even at rest. Residents of the Gulf need noticeably more than the European reference from May to October.
- Exercise. Sweat losses range from under half a litre to more than two litres an hour. See training in the heat for how to measure your own.
- Air travel. Cabin air is very dry. A long-haul flight costs more water than a day at a desk, and alcohol on board adds to the loss.
- Body size. A person of 100 kilograms needs more than one of 60.
- Illness. Fever, vomiting and diarrhoea raise needs sharply.
The calculator above adds your training losses to the reference value.
How to tell whether you are drinking enough
Three signs are more useful than counting glasses.
- Urine colour. Pale straw is the target. Dark yellow in the middle of the day means you are behind. First thing in the morning it is normally darker.
- Frequency. Passing urine every two to four hours during the day is typical of someone well hydrated.
- Body weight on waking. A drop of a kilogram from one morning to the next is water, not fat, and tells you the previous day's intake fell short.
Thirst works well at rest for healthy adults. It lags during prolonged exercise, and it becomes less sensitive with age.
What dehydration does to a working day
A loss of 1 to 2% of body weight in water, common by late afternoon in someone who has drunk only coffee since morning, is enough to produce headache, fatigue and difficulty concentrating. These are easily blamed on the workload.
For exercise, the American College of Sports Medicine identifies a loss greater than 2% of body weight as the level at which aerobic performance is impaired, particularly in the heat.
What counts, and what does not help
- Water is the default.
- Tea and coffee count. At the amounts most people drink, the fluid they provide outweighs the mild diuretic effect of caffeine. See caffeine.
- Milk hydrates well and brings protein.
- Sparkling water hydrates as well as still.
- Fruit juice and sugary drinks hydrate, with a sugar load that the health service advises limiting to a small glass a day.
- Alcohol does not count. It increases urine output. See alcohol and training.
Sports drinks are useful for exercise lasting more than an hour or in heavy sweat, because they replace sodium and supply carbohydrate. For a 40-minute gym session, water is enough.
Can you drink too much?
Yes, though it takes effort. Drinking far beyond your losses, typically during events lasting several hours, dilutes sodium in the blood. The condition, hyponatraemia, causes nausea, headache and confusion, and can be dangerous. The rule during exercise is to replace what you lose, not more.
At rest, healthy kidneys handle large volumes without difficulty. There is no health benefit in forcing down four litres a day in a cool office.
Habits that work for people who forget
- A full glass on waking, before coffee.
- A carafe on the desk and on the meeting table, in sight. People drink what is in front of them.
- A glass with each coffee.
- A bottle in the car, replaced daily in summer.
- On flights, a glass of water each hour awake.
Who needs individual advice
People with heart failure, kidney disease or taking diuretics may be advised to limit fluids, and should follow their physician's instructions rather than a general reference. Older adults, whose sense of thirst is weaker, do well to drink on a schedule in hot weather.
In Gulf summers, the European reference values are a floor, not a target.
Questions
How much water should I drink a day?
The European Food Safety Authority sets the adequate total intake at 2.5 litres for men and 2.0 litres for women, food included. Since food supplies about a fifth, that leaves roughly 2 litres to drink for a man and 1.6 for a woman in temperate conditions. In a hot climate or when you train, add what you lose in sweat.
Does coffee dehydrate you?
Not at ordinary amounts. Caffeine has a mild diuretic effect, but the water in a cup of coffee or tea more than compensates for it in people who drink them regularly. Coffee and tea therefore count towards your daily fluid intake. Very high doses of caffeine taken at once are a different matter and bring other drawbacks.
How do I know if I am dehydrated?
Look at the colour of your urine: pale yellow indicates adequate hydration, dark yellow indicates you need to drink. Other signs are thirst, headache, tiredness, a dry mouth and passing urine less than four times a day. A drop of a kilogram in body weight from one morning to the next also points to a water deficit.