We often hear that we should drink eight 8-ounce glasses of water a day. However, there is some controversy about this figure and what it really means.
Water is an essential nutrient. It is necessary to sustain all forms of life, and humans can only live a few days without it. It is also a healthful drink.
Health authorities and others often encourage people to consume 2 or more liters of water a day, but is this only plain water or does water from other sources count?
Some sources have described these recommendations as a “myth,” and professionals have questioned the guidelines.
Some point to a lack of scientific evidence to support the claims, while others note that promoters of the concept have included a major mineral water producer.
How much plain water do we really need?
Fast facts on water intake
Here are some key points about daily water consumption. More detail and supporting information is in the article.
- Foods and fluids, including water, are the main source of water in our bodies.
- The advice to drink eight 8-ounce glasses of water a day is not based on evidence.
- The amount of water we need depends on individual needs and circumstances, including activity and climate.
- The healthy body naturally maintains a well-tuned balance of fluid, and the thirst mechanism tells us when we need more.
Recommended daily water intake
In 1945, the U.S. Food and Nutrition Board advised people to consume 2.5 liters (84.5 fluid ounces (fl oz) of water a day, including fluid from prepared foods.
Today, the Centers for Disease Control and Prevention (CDC) say, “There is no recommendation for how much plain water adults and youth should drink daily.” However, there are recommendations about total fluid intake from all sources.
In 2004, the Institute of Medicine set the amount at around 2.7 liters, or 91 fluid ounces (fl oz) of total water a day for women and an average of around 3.7 liters (125 fl oz) daily for men.
This refers to the total daily fluid intake from all sources, defined as “the amount of water consumed from foods, plain drinking water, and other beverages.”
The U.S. Dietary Guidelines 2015-2020 do not recommend a specific daily water or fluid intake, but they do recommend choosing plain rather than flavored water and juices with added sugar.
There is currently no set upper level for water intake, although excessive quantities have been known to have adverse effects.
In the United Kingdom, the National Health Service (NHS) recommend consuming 6 to 8 glasses a day, or 1.9 liters (almost 34 fl0.oz), including water that is in food. They note this amount is suitable for a temperate climate. In hotter climates, they say, more will be needed.
Recommended intake by age
There is no fixed amount of fluid recommended by age, but some patterns emerge among healthy individuals doing a moderate amount of activity in a temperate climate.
The following shows average water intake for infants and adults:
The amount of formula or breast milk an infant takes in averages 780 milliliters (ml), or just over 26 fl oz, of breast milk or formula milk each day until the age of around 6 months. Before the age of 6 months, plain water is not recommended.
This ranges from around 525 ml (just under 18 fl oz) a day for 3.5-kilo newborn to 1,200 ml per day (45 fl oz) for an 8-kilo infant at 6 months, or around 150 ml (5 fl oz) per kilo of weight per day.
This is proportionally far more than an adult needs. After infants start consuming solid foods, they need less fluid from breast milk and formula.
Children aged over 12 months
Children should be encouraged to drink water:
as part of the daily routine, for example, after brushing teeth and before, during and after playtime at school
when the weather is warm
as an alternative to sweetened drinks and juices
Juice consumption should be limited to one glass a day.
Parents are advised to keep a pitcher handy to encourage healthful water-drinking habits, and schools should have water fountains or equivalent facilities.
Children who are sick with a fever
For children who are at risk of dehyration, for example, with a fever, the CDC recommend the following:
If a child is sick with a fever, it is important to seek medical help. A doctor may also advise oral an rehydration solution to ensure an adequate electrolyte balance.
Adults aged 19 to 30 years
The CDC cites figures showing that in 2005-2010 in the U.S., young people were drinking an average of 0.45 liters or 15 fl oz of water on a given day, while adults drank an average of 1.2 liters, or 39 fl oz.
The adequate intakes recommended for total water from all sources each day for most adults between 19 and 30 years of age are:
3.7 liters (or about 130 fl oz) for men
2.7 liters (about 95 fl oz) for women
One source suggests a man’s requirements might range from 2.5 liters (84.5 fl oz) if sedentary to up to 6 liters (203 fl oz) if active and living in a warm climate.
For women, the requirements will probably be 0.5 to 1 liter (17 to 34 fl oz) lower than those for men because of typically smaller body mass.
However, during pregnancy, women are likely to need an extra 0.3 liters (10 fl oz), and an additional 0.7 to 1.1 liters (23 to 37 fl oz) while breast-feeding.
Older adults may be at risk of both dehydration and overhydration, as a result of health conditions, medications, loss of muscle mass, reduction in kidney function, and other factors.
Older adults who are well hydrated have been found to have:
in men, a lower risk of bladder cancer
Dehydration has been linked to a higher frequency of:
urinary tract infections
slower wound healing
Needs for fluid intake will depend on the individual.
Few studies have looked at fluid input and output in older people, but at least one has concluded that it is not significantly different from that of younger people.
Those caring for older people are encouraged to provide fluids regularly and assist with ambulation, especially if a reduction in mobility makes it harder to visit the bathroom.
Where do the figures come from?
While water is known to be crucial for life and for preventing dehydration, recommendations for intake are based mainly on survey results showing the average amounts that people consume.
Conclusions are based on the assumption that these amounts must be about right for optimal hydration.
There is little evidence showing that specific quantities have a particular effect on health.
It is impossible to define an optimal intake, because these vary greatly according to:
sex and age
health status, for example, poor kidney function
medications, such as diuretics
whether or not a person is pregnant or breast-feeding
Recommendations that a person should drink eight glasses of water a day also fail to take into account the fact that much of our fluid intake comes from food and other drinks.
Sources of water
Water in the body comes not only from drinking water.
Estimations vary, but according to one source:
around 20 to 30 percent comes from food
some 60 to 70 percent comes from drinking water and other fluids
a small percentage, about 10 percent is “metabolic water,” produced by cells during normal cell function
The more active the body is, the more metabolic water is produced.
Some surveys suggest that around 20 percent of water intake comes from foodstuffs and the rest is from fluids. This depends on diet. A higher intake of fresh fruit and vegetables will mean a higher intake of water from foods.
Metabolic water accounts for around 250 to 350 ml (8.4 to 11.8 fl oz) a day.
Water content of foods
Here are some examples of the water content of different foods and fluids:
Tap or bottled water?
Bottled or tap water are equally effective at hydrating the body. In terms of hydration, studies in the UK have not found any significant difference between drinking the two.
Mineral waters contain different amounts of minerals, depending on where they come from, but this, too is not significant, as most minerals come from other dietary sources.
What about coffee?
Caffeinated drinks are thought to be dehydrating as opposed to hydrating because of a belief that they have a diuretic effect on our water balance.
A number of studies to test how caffeinated fluids affect hydration have shown that tea and coffee are in fact good sources of water and do not lead to dehydration.
One study of 18 healthy male adults found that no significant difference in impact on hydration, measured in body weight, urine and blood tests after a variety of caffeinated, non-caffeinated, caloric, and non-caloric drinks.
“Advising people to disregard caffeinated beverages as part of the daily fluid intake is not substantiated,” say the researchers.
Another concludes that there is “no evidence of dehydration with moderate daily coffee intake.”