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.
- 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:
|Average daily fluid intake
|From 525 ml for a 3.5-kilo newborn to 1,200 ml for an 8-kilo infant per day, as breast or bottled milk
|Adults aged 19 to 30 years
|Average of 3.7 liters a day for men and 2.7 liters for women, depending on climate, activity, pregnancy status, and health
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:
|Amount of fluid needed
|Up to 12 months
|1 to 3 years
|4 to 8 years
|6 to 13 years
|14 years and over
|11 to 13 cups for males and 8 to 9 cups for females
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:
- fewer falls
- less constipation
- in men, a lower risk of bladder cancer
Dehydration has been linked to a higher frequency of:
- urinary tract infections
- kidney failure
- 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:
- environmental conditions
- individual factors
- body mass
- 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:
|Water content as a percentage (%)
|Food or drink
|Fat-free milk, tea, coffee, juicy fruits such as strawberries and cantaloupes, vegetables such as lettuce, celery, and spinach
|Fruit juice, yogurt, fruits such as apples, pears and oranges, vegetables such as carrots, and cooked broccoli
|Bananas, avocados, baked potatoes, cottage and ricotta cheeses
|Pasta, beans, and peas, fish such as salmon, chicken breasts, and ice cream
|Bread, bagels, and cheddar cheese
|Nuts, chocolate cookies, crackers, cereals
|Oils and sugars
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.”
Benefits of drinking water
Water is necessary for the body. It:
- regulates temperature
- lubricates the joints and bones
- protects the spinal cord and other sensitive tissues
- removes waste from the body
A healthy fluid intake, including water, prevents dehydration.
The short-term symptoms of significant dehydration include:
- unclear thinking
- mood change
- feeling thirsty and having a dry mouth
- dry mouth, eyes, and lips
- urinating less than usual
Overheating can lead to organ damage, coma, and death.
The CDC urge people to make sure they drink enough water before, during, and after physical work, especially if this involves activity in a hot climate. This can help maintain alertness and effectiveness.
In a hot environment, you may need one 8-ounce cup every 15-20 minutes, but not more than 48 ounces in one hour, as this can lead to hyponatremia, a low level of sodium.
Plain water provides hydration without adding calories or jeopardizing dental health. Sports drinks can be useful in moderation, but too many will add unnecessary sugar to the body.
Studies suggest that long-term benefits of drinking water might include a lower risk of:
- colorectal cancer and cancers of the urinary system
- heart disease
- urinary tract infections
- kidney stones
- high blood pressure
However, these possible long-term benefits also depend on a wide variety of other factors.
In addition, study participants with the lower risk of these conditions still drank fewer than eight 8-ounce glasses of water a day.
Drinking water may also help with weight loss, if a person “preloads” with water before a meal. This may help them feel fuller faster during meals. If they choose water over sweetened juice or soda, they consume fewer calories.
In a study where 318 people with obesity or overweight switched from sugary drinks to plain water, an average loss of 2 to 2.5 percent in weight was noted. But this could be easily explained by a decrease in total calories rather than water itself.
Other possible benefits
Other supposed benefits of drinking more water include prevention of:
- high blood pressure
- dry cough
- dry skin
However, there is little or no scientific evidence to confirm these claims.
In children, a higher water consumption has been linked to better diets, behavior and overall health.
When is more water needed?
Extra water may be needed when a person:
- is in a hot climate
- is doing physical activity
- has diarrhea or vomiting
- has a fever
These can lead to additional loss of water.
What about a water detox?
There have also been claims that water can “detox” the body.
These claims are not based on scientific facts.
The liver, kidneys, and human body normally break down toxic substances into less harmful ones or expel them from the body through urine. Water does not have a unique role in these processes.
Too much water can lead to hyponatremia, also known as low sodium. Low sodium levels can be life-threatening causing confusion, seizures, coma and death.
Hydration during exercise
During exercise, factors affecting the amount of fluid lost and the need for extra intake include:
- the type and intensity of the activity
- environmental factors, such as climate
- the size and muscle mass of the athlete
Guidance from the American College of Sports Medicine (ACSM), published in 2007, recommends making an individual estimation of the fluid replacement needed by people taking part in exercise, to avoid dehydration.
Larger athletes may require higher fluid intake, for example, compared with smaller, leaner athletes, due to a larger body surface and higher body weight.
The ACSM advises drinking water during activity to prevent:
- dehydration that leads to a fall in body weight of more than 2 percent
- changes in the electrolyte balance
These changes, they say, can lead to reduced performance.
However, a study published in the British Journal of Sports Medicine concludes that a loss of up to 3 percent of water has “no effect” on real-world sports performance.
The conclusions were supported by findings from a cycling time-trial over 25 kilometers in hot conditions of 91.4° Fahrenheit and 40 percent relative humidity.
Results showed no difference in performance, physiological, and perceptual variables between participants who received hydration and those who did not.
The reason for this, say the authors, is “the body’s rapid defense of its plasma and blood volume following dehydration.”
In other words, the body can regulate water balance with high sensitivity.
Authors of a study focusing on runners in the 2002 Boston Marathon, published in the New England Journal of Medicine (NEJM), concluded:
“Because runners vary considerably in size and in rates of perspiration, general recommendations regarding specific volumes of fluids and frequencies of intake are probably unsafe and have been superseded by recommendations favoring thirst or individual perspiration rates as a primary guide.”
A review of 15 studies found that exercise-induced dehydration did not reduce performance. The authors encouraged athletes to “drink according to their thirst.”
Water and dehydration in the human body
Dehydration can be measured in terms of blood and urine osmolality.
Osmolality is an estimation of the osmolar concentration of plasma. It reflects the concentration of particles in a solution.
As regards dehydration:
- high osmolality indicates a greater need for hydration
- low osmolality suggests there are too few particles, specifically sodium and electrolytes, a sign of overhydration
How much water is in the human body?
The main chemical making up the human body is water. It accounts for between 55 percent and 75 percent of the body’s composition, and it varies between individuals.
- in the average young man, between 50 and 70 percent of the body weight is water
- in infants, 75 percent of body weight is water
- in older people, it is 55 percent
Differences of age, sex, and aerobic fitness affect an individual’s ratio of lean to fat body mass and therefore how much water they contain.
Calculating human water needs
The amount of water we need to take in to maintain a healthy balance is decided by how much water we use and lose that must be replaced.
Over the course of 24 hours, healthy resting adults regulate their water balance to within around 0.2 percent of body weight.
In older children and adults, if a person’s body weight falls by 3 percent due to fluid loss, this is considered dehydration. Moderate dehydration is when weight falls by 6 percent, and severe dehydration is when it falls by 9 percent.
It is difficult to measure the amount of water used or lost by the body. Measures taken across groups of people in studies have shown wide variation.
However, if people show symptoms of dehydration, such as confusion or decreased urine output, they need medical attention.
What about dark urine?
There is a popular concept that darker urine means a person is dehydrated, while pale urine shows they are adequately hydrated.
However, it is difficult to assess the significance of urine color precisely. It depends on the time of day, medications, and other health problems.
Tea- or cola-colored urine, particularly after exercise, can indicate serious muscle injury and severe dehydration and requires urgent medical attention.
However, in normal circumstances, urine that is well within the limits of normal osmolality in laboratory tests can appear moderately yellow. This does not indicate that a person is dehydrated.
In addition, other factors such as diet, medications and health conditions can cause individuals to differ in their urine color.
How does the body regulate water?
Without water, there is no life. For this reason, all living organisms have adapted to avoid dehydration.
Humans can only survive for a few days without water. Infants and older people who lose water through illness and do not replace it can experience life-threatening complications.
Most of the time, however, our bodies’ sensitive natural mechanisms maintain appropriate fluid levels, or homeostasis, as long as we continue to consume food and water.
The two main ways the body does this are:
Thirst: This which tells us when we need to take in more fluid.
Urine output: The kidneys regulate any excess or lack of the water we consume by either emptying it into the urinary bladder or holding onto it in the blood plasma.
The body expels 0.5 to 1 liter (around 17 to 34 fl oz) per day in the form of urine.
The kidneys also:
- regulate the balance of electrolytes, such as sodium and potassium, in the body fluids
- receive hormonal signals to conserve or release water into the urine if the brain detects changes in the concentration of the solutes in the blood
The brain also responds to these changes in solutes, known as plasma osmolality. This is one factor that triggers the thirst for water.
Other ways in which water is lost or expelled from the body are:
Breathing: About 250-350 ml (8.5 to 11.8 fl oz) per day exit from the lungs during. exhalation
Feces: Around 100-200 ml (3.4 to 6.8 fl oz) per day passes out of the body in feces.
Sweating: Sedentary loss ranges from around 1,300-3,450 ml (44 to 117 fl oz) a day, but a physically active person can lose 1,550-6,730 ml (52 to 227.5 fl oz) a day
What about ‘8 by 8’?
It is often said that we needed to drink at least eight glasses of water a day, although there is little scientific evidence to support this.
Prof. Heinz Valtin, of Dartmouth Medical School in Hanover, NH, is one expert who has questioned this advice. He suggests it may be not only unnecessary but maybe even harmful.
In an article published in the American Journal of Physiology in 2002, he concludes:
“Not only is there no scientific evidence that we need to drink that much, but the [8 by 8] recommendation could be harmful, both in precipitating potentially dangerous hyponatremia and exposure to pollutants, and also in making many people feel guilty for not drinking enough.”
Prof. Heinz Valtin
The right types of fluid
Recommendations are now more likely to note that water can come from other beverages and from food. However, these should be chosen carefully.
Beverages that are high in fat or sugar or contain alcohol do not benefit health, and plain water is recommended in preference to these.
More than one alcoholic beverage per day in women and more than two alcoholic beverages per day in men on a regular basis can lead to liver inflammation and injury, an increased cancer risk, as well as negatively affecting every system of the body.
Fatty or sweetened drinks increase calories and lack nutritional value.
Processed foods, such as burgers and chips, high fat, and high sugar diets will contain less water than fresh fruits and vegetables.
Nearly all foods contain water, but fresh and healthful ingredients contain the most compared to processed foods, sugars, and fats.
The CDC note with concern that young people who consume high quantities of junk food are also drinking less water and eating less fresh fruit and vegetables. This can lead to further health problems. They are also often drinking highly sweetened beverages rather than water.
A study published by the CDC in 2013 showed that:
- Seven percent of adults said they did not drink any water
- Thirty-six percent said they drank 1 to 3 cups a day
- Thirty-five percent reported drinking 4 to 7 cups
- Twenty-two percent said they drank 8 cups or more
Those who drank the least water were also less likely to pursue other healthful behaviors, such as consuming fresh fruits and vegetables.
Too much water?
Some have raised concerns that consuming too much water could be dangerous.
Not only the amount of fluid in the body but the balance of minerals that is important for maintaining health and life.
Too much water in the body could lead to hyponatremia, or water intoxication, when sodium levels in the blood plasma become too low.
- lung congestion
- brain swelling
- fatigue and lethargy
Hyponatremia may be a risk for people who use the recreational drug Ecstasy. This is likely due to a variety of reasons including a change in hormone levels, brain chemistry, body function, kidney function, along with increased sweating and thirst.
People who drink too much water while exercising could also be at risk. In the Boston runners survey, for example, almost 2,000 of the participants were thought likely to have some degree of hyponatremia due to excessive fluid consumption, and 90 may have finished with critical hyponatremia.
Another risk factor is having certain diseases or using some medications. Diabetes, for example, can lead to excessive thirst. Conditions in which the kidneys cannot excrete enough water, too, can result in this type of problem.
However, just as the body can adapt to higher or lower levels of water and can remind us, through thirst, when we need to drink more, scientists believe an internal mechanism also stops most people from drinking too much water.
There are few scientific measures of how much water we need.
Most studies have focused on how much people consume and assumed that this figure matches or exceeds our needs. It does not determine whether a person is efficiently hydrated or not.
The amount of water a person uses and loses varies according to conditions and activities. Heat, activity, and illness, such as diarrhea and vomiting, can lead to dehydration.
Most healthy people in a temperate climate consume sufficient fluid to remain healthy while carrying out their daily activities, and around one fifth to a quarter of this comes from food. Where there is a lack, the body will usually regulate these needs.
Meanwhile, those who work outside or exercise in a hot climate will not be able to get all their water needs from food. They will need additional water.
Perhaps, as Prof. Farrell of Monash University says, “If we just do what our body tells us, we will probably get it right.”
Should people aim to consume a particular quantity of water each day?
The most recent research supports the theory that thirst is the best guide for determining fluid intake. For the average individual, even in routine physical activity, allow thirst to determine when and how much you drink.
Those with certain medical conditions, such as kidney disease or heart failure, may need to follow their doctor’s advice regarding daily fluid intake in order to avoid the complications of low sodium levels in their blood.
Judith Marcin, MD
Answers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.